diff options
Diffstat (limited to 'doc/src/sgml/html/sql-syntax-lexical.html')
| -rw-r--r-- | doc/src/sgml/html/sql-syntax-lexical.html | 647 |
1 files changed, 647 insertions, 0 deletions
diff --git a/doc/src/sgml/html/sql-syntax-lexical.html b/doc/src/sgml/html/sql-syntax-lexical.html new file mode 100644 index 0000000..02ece7a --- /dev/null +++ b/doc/src/sgml/html/sql-syntax-lexical.html @@ -0,0 +1,647 @@ +<?xml version="1.0" encoding="UTF-8" standalone="no"?> +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"><html xmlns="http://www.w3.org/1999/xhtml"><head><meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /><title>4.1. Lexical Structure</title><link rel="stylesheet" type="text/css" href="stylesheet.css" /><link rev="made" href="pgsql-docs@lists.postgresql.org" /><meta name="generator" content="DocBook XSL Stylesheets Vsnapshot" /><link rel="prev" href="sql-syntax.html" title="Chapter 4. SQL Syntax" /><link rel="next" href="sql-expressions.html" title="4.2. Value Expressions" /></head><body id="docContent" class="container-fluid col-10"><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">4.1. Lexical Structure</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="sql-syntax.html" title="Chapter 4. SQL Syntax">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="sql-syntax.html" title="Chapter 4. SQL Syntax">Up</a></td><th width="60%" align="center">Chapter 4. SQL Syntax</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 14.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="sql-expressions.html" title="4.2. Value Expressions">Next</a></td></tr></table><hr></hr></div><div class="sect1" id="SQL-SYNTAX-LEXICAL"><div class="titlepage"><div><div><h2 class="title" style="clear: both">4.1. Lexical Structure</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS">4.1.1. Identifiers and Key Words</a></span></dt><dt><span class="sect2"><a href="sql-syntax-lexical.html#SQL-SYNTAX-CONSTANTS">4.1.2. Constants</a></span></dt><dt><span class="sect2"><a href="sql-syntax-lexical.html#SQL-SYNTAX-OPERATORS">4.1.3. Operators</a></span></dt><dt><span class="sect2"><a href="sql-syntax-lexical.html#SQL-SYNTAX-SPECIAL-CHARS">4.1.4. Special Characters</a></span></dt><dt><span class="sect2"><a href="sql-syntax-lexical.html#SQL-SYNTAX-COMMENTS">4.1.5. Comments</a></span></dt><dt><span class="sect2"><a href="sql-syntax-lexical.html#SQL-PRECEDENCE">4.1.6. Operator Precedence</a></span></dt></dl></div><a id="id-1.5.3.5.2" class="indexterm"></a><p> + SQL input consists of a sequence of + <em class="firstterm">commands</em>. A command is composed of a + sequence of <em class="firstterm">tokens</em>, terminated by a + semicolon (<span class="quote">“<span class="quote">;</span>”</span>). The end of the input stream also + terminates a command. Which tokens are valid depends on the syntax + of the particular command. + </p><p> + A token can be a <em class="firstterm">key word</em>, an + <em class="firstterm">identifier</em>, a <em class="firstterm">quoted + identifier</em>, a <em class="firstterm">literal</em> (or + constant), or a special character symbol. Tokens are normally + separated by whitespace (space, tab, newline), but need not be if + there is no ambiguity (which is generally only the case if a + special character is adjacent to some other token type). + </p><p> + For example, the following is (syntactically) valid SQL input: +</p><pre class="programlisting"> +SELECT * FROM MY_TABLE; +UPDATE MY_TABLE SET A = 5; +INSERT INTO MY_TABLE VALUES (3, 'hi there'); +</pre><p> + This is a sequence of three commands, one per line (although this + is not required; more than one command can be on a line, and + commands can usefully be split across lines). + </p><p> + Additionally, <em class="firstterm">comments</em> can occur in SQL + input. They are not tokens, they are effectively equivalent to + whitespace. + </p><p> + The SQL syntax is not very consistent regarding what tokens + identify commands and which are operands or parameters. The first + few tokens are generally the command name, so in the above example + we would usually speak of a <span class="quote">“<span class="quote">SELECT</span>”</span>, an + <span class="quote">“<span class="quote">UPDATE</span>”</span>, and an <span class="quote">“<span class="quote">INSERT</span>”</span> command. But + for instance the <code class="command">UPDATE</code> command always requires + a <code class="token">SET</code> token to appear in a certain position, and + this particular variation of <code class="command">INSERT</code> also + requires a <code class="token">VALUES</code> in order to be complete. The + precise syntax rules for each command are described in <a class="xref" href="reference.html" title="Part VI. Reference">Part VI</a>. + </p><div class="sect2" id="SQL-SYNTAX-IDENTIFIERS"><div class="titlepage"><div><div><h3 class="title">4.1.1. Identifiers and Key Words</h3></div></div></div><a id="id-1.5.3.5.8.2" class="indexterm"></a><a id="id-1.5.3.5.8.3" class="indexterm"></a><a id="id-1.5.3.5.8.4" class="indexterm"></a><p> + Tokens such as <code class="token">SELECT</code>, <code class="token">UPDATE</code>, or + <code class="token">VALUES</code> in the example above are examples of + <em class="firstterm">key words</em>, that is, words that have a fixed + meaning in the SQL language. The tokens <code class="token">MY_TABLE</code> + and <code class="token">A</code> are examples of + <em class="firstterm">identifiers</em>. They identify names of + tables, columns, or other database objects, depending on the + command they are used in. Therefore they are sometimes simply + called <span class="quote">“<span class="quote">names</span>”</span>. Key words and identifiers have the + same lexical structure, meaning that one cannot know whether a + token is an identifier or a key word without knowing the language. + A complete list of key words can be found in <a class="xref" href="sql-keywords-appendix.html" title="Appendix C. SQL Key Words">Appendix C</a>. + </p><p> + SQL identifiers and key words must begin with a letter + (<code class="literal">a</code>-<code class="literal">z</code>, but also letters with + diacritical marks and non-Latin letters) or an underscore + (<code class="literal">_</code>). Subsequent characters in an identifier or + key word can be letters, underscores, digits + (<code class="literal">0</code>-<code class="literal">9</code>), or dollar signs + (<code class="literal">$</code>). Note that dollar signs are not allowed in identifiers + according to the letter of the SQL standard, so their use might render + applications less portable. + The SQL standard will not define a key word that contains + digits or starts or ends with an underscore, so identifiers of this + form are safe against possible conflict with future extensions of the + standard. + </p><p> + <a id="id-1.5.3.5.8.7.1" class="indexterm"></a> + The system uses no more than <code class="symbol">NAMEDATALEN</code>-1 + bytes of an identifier; longer names can be written in + commands, but they will be truncated. By default, + <code class="symbol">NAMEDATALEN</code> is 64 so the maximum identifier + length is 63 bytes. If this limit is problematic, it can be raised by + changing the <code class="symbol">NAMEDATALEN</code> constant in + <code class="filename">src/include/pg_config_manual.h</code>. + </p><p> + <a id="id-1.5.3.5.8.8.1" class="indexterm"></a> + Key words and unquoted identifiers are case insensitive. Therefore: +</p><pre class="programlisting"> +UPDATE MY_TABLE SET A = 5; +</pre><p> + can equivalently be written as: +</p><pre class="programlisting"> +uPDaTE my_TabLE SeT a = 5; +</pre><p> + A convention often used is to write key words in upper + case and names in lower case, e.g.: +</p><pre class="programlisting"> +UPDATE my_table SET a = 5; +</pre><p> + </p><p> + <a id="id-1.5.3.5.8.9.1" class="indexterm"></a> + There is a second kind of identifier: the <em class="firstterm">delimited + identifier</em> or <em class="firstterm">quoted + identifier</em>. It is formed by enclosing an arbitrary + sequence of characters in double-quotes + (<code class="literal">"</code>). A delimited + identifier is always an identifier, never a key word. So + <code class="literal">"select"</code> could be used to refer to a column or + table named <span class="quote">“<span class="quote">select</span>”</span>, whereas an unquoted + <code class="literal">select</code> would be taken as a key word and + would therefore provoke a parse error when used where a table or + column name is expected. The example can be written with quoted + identifiers like this: +</p><pre class="programlisting"> +UPDATE "my_table" SET "a" = 5; +</pre><p> + </p><p> + Quoted identifiers can contain any character, except the character + with code zero. (To include a double quote, write two double quotes.) + This allows constructing table or column names that would + otherwise not be possible, such as ones containing spaces or + ampersands. The length limitation still applies. + </p><p> + Quoting an identifier also makes it case-sensitive, whereas + unquoted names are always folded to lower case. For example, the + identifiers <code class="literal">FOO</code>, <code class="literal">foo</code>, and + <code class="literal">"foo"</code> are considered the same by + <span class="productname">PostgreSQL</span>, but + <code class="literal">"Foo"</code> and <code class="literal">"FOO"</code> are + different from these three and each other. (The folding of + unquoted names to lower case in <span class="productname">PostgreSQL</span> is + incompatible with the SQL standard, which says that unquoted names + should be folded to upper case. Thus, <code class="literal">foo</code> + should be equivalent to <code class="literal">"FOO"</code> not + <code class="literal">"foo"</code> according to the standard. If you want + to write portable applications you are advised to always quote a + particular name or never quote it.) + </p><a id="id-1.5.3.5.8.12" class="indexterm"></a><p> + A variant of quoted + identifiers allows including escaped Unicode characters identified + by their code points. This variant starts + with <code class="literal">U&</code> (upper or lower case U followed by + ampersand) immediately before the opening double quote, without + any spaces in between, for example <code class="literal">U&"foo"</code>. + (Note that this creates an ambiguity with the + operator <code class="literal">&</code>. Use spaces around the operator to + avoid this problem.) Inside the quotes, Unicode characters can be + specified in escaped form by writing a backslash followed by the + four-digit hexadecimal code point number or alternatively a + backslash followed by a plus sign followed by a six-digit + hexadecimal code point number. For example, the + identifier <code class="literal">"data"</code> could be written as +</p><pre class="programlisting"> +U&"d\0061t\+000061" +</pre><p> + The following less trivial example writes the Russian + word <span class="quote">“<span class="quote">slon</span>”</span> (elephant) in Cyrillic letters: +</p><pre class="programlisting"> +U&"\0441\043B\043E\043D" +</pre><p> + </p><p> + If a different escape character than backslash is desired, it can + be specified using + the <code class="literal">UESCAPE</code><a id="id-1.5.3.5.8.14.2" class="indexterm"></a> + clause after the string, for example: +</p><pre class="programlisting"> +U&"d!0061t!+000061" UESCAPE '!' +</pre><p> + The escape character can be any single character other than a + hexadecimal digit, the plus sign, a single quote, a double quote, + or a whitespace character. Note that the escape character is + written in single quotes, not double quotes, + after <code class="literal">UESCAPE</code>. + </p><p> + To include the escape character in the identifier literally, write + it twice. + </p><p> + Either the 4-digit or the 6-digit escape form can be used to + specify UTF-16 surrogate pairs to compose characters with code + points larger than U+FFFF, although the availability of the + 6-digit form technically makes this unnecessary. (Surrogate + pairs are not stored directly, but are combined into a single + code point.) + </p><p> + If the server encoding is not UTF-8, the Unicode code point identified + by one of these escape sequences is converted to the actual server + encoding; an error is reported if that's not possible. + </p></div><div class="sect2" id="SQL-SYNTAX-CONSTANTS"><div class="titlepage"><div><div><h3 class="title">4.1.2. Constants</h3></div></div></div><a id="id-1.5.3.5.9.2" class="indexterm"></a><p> + There are three kinds of <em class="firstterm">implicitly-typed + constants</em> in <span class="productname">PostgreSQL</span>: + strings, bit strings, and numbers. + Constants can also be specified with explicit types, which can + enable more accurate representation and more efficient handling by + the system. These alternatives are discussed in the following + subsections. + </p><div class="sect3" id="SQL-SYNTAX-STRINGS"><div class="titlepage"><div><div><h4 class="title">4.1.2.1. String Constants</h4></div></div></div><a id="id-1.5.3.5.9.4.2" class="indexterm"></a><p> + <a id="id-1.5.3.5.9.4.3.1" class="indexterm"></a> + A string constant in SQL is an arbitrary sequence of characters + bounded by single quotes (<code class="literal">'</code>), for example + <code class="literal">'This is a string'</code>. To include + a single-quote character within a string constant, + write two adjacent single quotes, e.g., + <code class="literal">'Dianne''s horse'</code>. + Note that this is <span class="emphasis"><em>not</em></span> the same as a double-quote + character (<code class="literal">"</code>). + </p><p> + Two string constants that are only separated by whitespace + <span class="emphasis"><em>with at least one newline</em></span> are concatenated + and effectively treated as if the string had been written as one + constant. For example: +</p><pre class="programlisting"> +SELECT 'foo' +'bar'; +</pre><p> + is equivalent to: +</p><pre class="programlisting"> +SELECT 'foobar'; +</pre><p> + but: +</p><pre class="programlisting"> +SELECT 'foo' 'bar'; +</pre><p> + is not valid syntax. (This slightly bizarre behavior is specified + by <acronym class="acronym">SQL</acronym>; <span class="productname">PostgreSQL</span> is + following the standard.) + </p></div><div class="sect3" id="SQL-SYNTAX-STRINGS-ESCAPE"><div class="titlepage"><div><div><h4 class="title">4.1.2.2. String Constants with C-Style Escapes</h4></div></div></div><a id="id-1.5.3.5.9.5.2" class="indexterm"></a><a id="id-1.5.3.5.9.5.3" class="indexterm"></a><p> + <span class="productname">PostgreSQL</span> also accepts <span class="quote">“<span class="quote">escape</span>”</span> + string constants, which are an extension to the SQL standard. + An escape string constant is specified by writing the letter + <code class="literal">E</code> (upper or lower case) just before the opening single + quote, e.g., <code class="literal">E'foo'</code>. (When continuing an escape string + constant across lines, write <code class="literal">E</code> only before the first opening + quote.) + Within an escape string, a backslash character (<code class="literal">\</code>) begins a + C-like <em class="firstterm">backslash escape</em> sequence, in which the combination + of backslash and following character(s) represent a special byte + value, as shown in <a class="xref" href="sql-syntax-lexical.html#SQL-BACKSLASH-TABLE" title="Table 4.1. Backslash Escape Sequences">Table 4.1</a>. + </p><div class="table" id="SQL-BACKSLASH-TABLE"><p class="title"><strong>Table 4.1. Backslash Escape Sequences</strong></p><div class="table-contents"><table class="table" summary="Backslash Escape Sequences" border="1"><colgroup><col /><col /></colgroup><thead><tr><th>Backslash Escape Sequence</th><th>Interpretation</th></tr></thead><tbody><tr><td><code class="literal">\b</code></td><td>backspace</td></tr><tr><td><code class="literal">\f</code></td><td>form feed</td></tr><tr><td><code class="literal">\n</code></td><td>newline</td></tr><tr><td><code class="literal">\r</code></td><td>carriage return</td></tr><tr><td><code class="literal">\t</code></td><td>tab</td></tr><tr><td> + <code class="literal">\<em class="replaceable"><code>o</code></em></code>, + <code class="literal">\<em class="replaceable"><code>oo</code></em></code>, + <code class="literal">\<em class="replaceable"><code>ooo</code></em></code> + (<em class="replaceable"><code>o</code></em> = 0–7) + </td><td>octal byte value</td></tr><tr><td> + <code class="literal">\x<em class="replaceable"><code>h</code></em></code>, + <code class="literal">\x<em class="replaceable"><code>hh</code></em></code> + (<em class="replaceable"><code>h</code></em> = 0–9, A–F) + </td><td>hexadecimal byte value</td></tr><tr><td> + <code class="literal">\u<em class="replaceable"><code>xxxx</code></em></code>, + <code class="literal">\U<em class="replaceable"><code>xxxxxxxx</code></em></code> + (<em class="replaceable"><code>x</code></em> = 0–9, A–F) + </td><td>16 or 32-bit hexadecimal Unicode character value</td></tr></tbody></table></div></div><br class="table-break" /><p> + Any other + character following a backslash is taken literally. Thus, to + include a backslash character, write two backslashes (<code class="literal">\\</code>). + Also, a single quote can be included in an escape string by writing + <code class="literal">\'</code>, in addition to the normal way of <code class="literal">''</code>. + </p><p> + It is your responsibility that the byte sequences you create, + especially when using the octal or hexadecimal escapes, compose + valid characters in the server character set encoding. + A useful alternative is to use Unicode escapes or the + alternative Unicode escape syntax, explained + in <a class="xref" href="sql-syntax-lexical.html#SQL-SYNTAX-STRINGS-UESCAPE" title="4.1.2.3. String Constants with Unicode Escapes">Section 4.1.2.3</a>; then the server + will check that the character conversion is possible. + </p><div class="caution"><h3 class="title">Caution</h3><p> + If the configuration parameter + <a class="xref" href="runtime-config-compatible.html#GUC-STANDARD-CONFORMING-STRINGS">standard_conforming_strings</a> is <code class="literal">off</code>, + then <span class="productname">PostgreSQL</span> recognizes backslash escapes + in both regular and escape string constants. However, as of + <span class="productname">PostgreSQL</span> 9.1, the default is <code class="literal">on</code>, meaning + that backslash escapes are recognized only in escape string constants. + This behavior is more standards-compliant, but might break applications + which rely on the historical behavior, where backslash escapes + were always recognized. As a workaround, you can set this parameter + to <code class="literal">off</code>, but it is better to migrate away from using backslash + escapes. If you need to use a backslash escape to represent a special + character, write the string constant with an <code class="literal">E</code>. + </p><p> + In addition to <code class="varname">standard_conforming_strings</code>, the configuration + parameters <a class="xref" href="runtime-config-compatible.html#GUC-ESCAPE-STRING-WARNING">escape_string_warning</a> and + <a class="xref" href="runtime-config-compatible.html#GUC-BACKSLASH-QUOTE">backslash_quote</a> govern treatment of backslashes + in string constants. + </p></div><p> + The character with the code zero cannot be in a string constant. + </p></div><div class="sect3" id="SQL-SYNTAX-STRINGS-UESCAPE"><div class="titlepage"><div><div><h4 class="title">4.1.2.3. String Constants with Unicode Escapes</h4></div></div></div><a id="id-1.5.3.5.9.6.2" class="indexterm"></a><p> + <span class="productname">PostgreSQL</span> also supports another type + of escape syntax for strings that allows specifying arbitrary + Unicode characters by code point. A Unicode escape string + constant starts with <code class="literal">U&</code> (upper or lower case + letter U followed by ampersand) immediately before the opening + quote, without any spaces in between, for + example <code class="literal">U&'foo'</code>. (Note that this creates an + ambiguity with the operator <code class="literal">&</code>. Use spaces + around the operator to avoid this problem.) Inside the quotes, + Unicode characters can be specified in escaped form by writing a + backslash followed by the four-digit hexadecimal code point + number or alternatively a backslash followed by a plus sign + followed by a six-digit hexadecimal code point number. For + example, the string <code class="literal">'data'</code> could be written as +</p><pre class="programlisting"> +U&'d\0061t\+000061' +</pre><p> + The following less trivial example writes the Russian + word <span class="quote">“<span class="quote">slon</span>”</span> (elephant) in Cyrillic letters: +</p><pre class="programlisting"> +U&'\0441\043B\043E\043D' +</pre><p> + </p><p> + If a different escape character than backslash is desired, it can + be specified using + the <code class="literal">UESCAPE</code><a id="id-1.5.3.5.9.6.4.2" class="indexterm"></a> + clause after the string, for example: +</p><pre class="programlisting"> +U&'d!0061t!+000061' UESCAPE '!' +</pre><p> + The escape character can be any single character other than a + hexadecimal digit, the plus sign, a single quote, a double quote, + or a whitespace character. + </p><p> + To include the escape character in the string literally, write + it twice. + </p><p> + Either the 4-digit or the 6-digit escape form can be used to + specify UTF-16 surrogate pairs to compose characters with code + points larger than U+FFFF, although the availability of the + 6-digit form technically makes this unnecessary. (Surrogate + pairs are not stored directly, but are combined into a single + code point.) + </p><p> + If the server encoding is not UTF-8, the Unicode code point identified + by one of these escape sequences is converted to the actual server + encoding; an error is reported if that's not possible. + </p><p> + Also, the Unicode escape syntax for string constants only works + when the configuration + parameter <a class="xref" href="runtime-config-compatible.html#GUC-STANDARD-CONFORMING-STRINGS">standard_conforming_strings</a> is + turned on. This is because otherwise this syntax could confuse + clients that parse the SQL statements to the point that it could + lead to SQL injections and similar security issues. If the + parameter is set to off, this syntax will be rejected with an + error message. + </p></div><div class="sect3" id="SQL-SYNTAX-DOLLAR-QUOTING"><div class="titlepage"><div><div><h4 class="title">4.1.2.4. Dollar-Quoted String Constants</h4></div></div></div><a id="id-1.5.3.5.9.7.2" class="indexterm"></a><p> + While the standard syntax for specifying string constants is usually + convenient, it can be difficult to understand when the desired string + contains many single quotes or backslashes, since each of those must + be doubled. To allow more readable queries in such situations, + <span class="productname">PostgreSQL</span> provides another way, called + <span class="quote">“<span class="quote">dollar quoting</span>”</span>, to write string constants. + A dollar-quoted string constant + consists of a dollar sign (<code class="literal">$</code>), an optional + <span class="quote">“<span class="quote">tag</span>”</span> of zero or more characters, another dollar + sign, an arbitrary sequence of characters that makes up the + string content, a dollar sign, the same tag that began this + dollar quote, and a dollar sign. For example, here are two + different ways to specify the string <span class="quote">“<span class="quote">Dianne's horse</span>”</span> + using dollar quoting: +</p><pre class="programlisting"> +$$Dianne's horse$$ +$SomeTag$Dianne's horse$SomeTag$ +</pre><p> + Notice that inside the dollar-quoted string, single quotes can be + used without needing to be escaped. Indeed, no characters inside + a dollar-quoted string are ever escaped: the string content is always + written literally. Backslashes are not special, and neither are + dollar signs, unless they are part of a sequence matching the opening + tag. + </p><p> + It is possible to nest dollar-quoted string constants by choosing + different tags at each nesting level. This is most commonly used in + writing function definitions. For example: +</p><pre class="programlisting"> +$function$ +BEGIN + RETURN ($1 ~ $q$[\t\r\n\v\\]$q$); +END; +$function$ +</pre><p> + Here, the sequence <code class="literal">$q$[\t\r\n\v\\]$q$</code> represents a + dollar-quoted literal string <code class="literal">[\t\r\n\v\\]</code>, which will + be recognized when the function body is executed by + <span class="productname">PostgreSQL</span>. But since the sequence does not match + the outer dollar quoting delimiter <code class="literal">$function$</code>, it is + just some more characters within the constant so far as the outer + string is concerned. + </p><p> + The tag, if any, of a dollar-quoted string follows the same rules + as an unquoted identifier, except that it cannot contain a dollar sign. + Tags are case sensitive, so <code class="literal">$tag$String content$tag$</code> + is correct, but <code class="literal">$TAG$String content$tag$</code> is not. + </p><p> + A dollar-quoted string that follows a keyword or identifier must + be separated from it by whitespace; otherwise the dollar quoting + delimiter would be taken as part of the preceding identifier. + </p><p> + Dollar quoting is not part of the SQL standard, but it is often a more + convenient way to write complicated string literals than the + standard-compliant single quote syntax. It is particularly useful when + representing string constants inside other constants, as is often needed + in procedural function definitions. With single-quote syntax, each + backslash in the above example would have to be written as four + backslashes, which would be reduced to two backslashes in parsing the + original string constant, and then to one when the inner string constant + is re-parsed during function execution. + </p></div><div class="sect3" id="SQL-SYNTAX-BIT-STRINGS"><div class="titlepage"><div><div><h4 class="title">4.1.2.5. Bit-String Constants</h4></div></div></div><a id="id-1.5.3.5.9.8.2" class="indexterm"></a><p> + Bit-string constants look like regular string constants with a + <code class="literal">B</code> (upper or lower case) immediately before the + opening quote (no intervening whitespace), e.g., + <code class="literal">B'1001'</code>. The only characters allowed within + bit-string constants are <code class="literal">0</code> and + <code class="literal">1</code>. + </p><p> + Alternatively, bit-string constants can be specified in hexadecimal + notation, using a leading <code class="literal">X</code> (upper or lower case), + e.g., <code class="literal">X'1FF'</code>. This notation is equivalent to + a bit-string constant with four binary digits for each hexadecimal digit. + </p><p> + Both forms of bit-string constant can be continued + across lines in the same way as regular string constants. + Dollar quoting cannot be used in a bit-string constant. + </p></div><div class="sect3" id="SQL-SYNTAX-CONSTANTS-NUMERIC"><div class="titlepage"><div><div><h4 class="title">4.1.2.6. Numeric Constants</h4></div></div></div><a id="id-1.5.3.5.9.9.2" class="indexterm"></a><p> + Numeric constants are accepted in these general forms: +</p><pre class="synopsis"> +<em class="replaceable"><code>digits</code></em> +<em class="replaceable"><code>digits</code></em>.[<span class="optional"><em class="replaceable"><code>digits</code></em></span>][<span class="optional">e[<span class="optional">+-</span>]<em class="replaceable"><code>digits</code></em></span>] +[<span class="optional"><em class="replaceable"><code>digits</code></em></span>].<em class="replaceable"><code>digits</code></em>[<span class="optional">e[<span class="optional">+-</span>]<em class="replaceable"><code>digits</code></em></span>] +<em class="replaceable"><code>digits</code></em>e[<span class="optional">+-</span>]<em class="replaceable"><code>digits</code></em> +</pre><p> + where <em class="replaceable"><code>digits</code></em> is one or more decimal + digits (0 through 9). At least one digit must be before or after the + decimal point, if one is used. At least one digit must follow the + exponent marker (<code class="literal">e</code>), if one is present. + There cannot be any spaces or other characters embedded in the + constant. Note that any leading plus or minus sign is not actually + considered part of the constant; it is an operator applied to the + constant. + </p><p> + These are some examples of valid numeric constants: +</p><div class="literallayout"><p><br /> +42<br /> +3.5<br /> +4.<br /> +.001<br /> +5e2<br /> +1.925e-3<br /> +</p></div><p> + </p><p> + <a id="id-1.5.3.5.9.9.5.1" class="indexterm"></a> + <a id="id-1.5.3.5.9.9.5.2" class="indexterm"></a> + <a id="id-1.5.3.5.9.9.5.3" class="indexterm"></a> + A numeric constant that contains neither a decimal point nor an + exponent is initially presumed to be type <code class="type">integer</code> if its + value fits in type <code class="type">integer</code> (32 bits); otherwise it is + presumed to be type <code class="type">bigint</code> if its + value fits in type <code class="type">bigint</code> (64 bits); otherwise it is + taken to be type <code class="type">numeric</code>. Constants that contain decimal + points and/or exponents are always initially presumed to be type + <code class="type">numeric</code>. + </p><p> + The initially assigned data type of a numeric constant is just a + starting point for the type resolution algorithms. In most cases + the constant will be automatically coerced to the most + appropriate type depending on context. When necessary, you can + force a numeric value to be interpreted as a specific data type + by casting it.<a id="id-1.5.3.5.9.9.6.1" class="indexterm"></a> + For example, you can force a numeric value to be treated as type + <code class="type">real</code> (<code class="type">float4</code>) by writing: + +</p><pre class="programlisting"> +REAL '1.23' -- string style +1.23::REAL -- PostgreSQL (historical) style +</pre><p> + + These are actually just special cases of the general casting + notations discussed next. + </p></div><div class="sect3" id="SQL-SYNTAX-CONSTANTS-GENERIC"><div class="titlepage"><div><div><h4 class="title">4.1.2.7. Constants of Other Types</h4></div></div></div><a id="id-1.5.3.5.9.10.2" class="indexterm"></a><p> + A constant of an <span class="emphasis"><em>arbitrary</em></span> type can be + entered using any one of the following notations: +</p><pre class="synopsis"> +<em class="replaceable"><code>type</code></em> '<em class="replaceable"><code>string</code></em>' +'<em class="replaceable"><code>string</code></em>'::<em class="replaceable"><code>type</code></em> +CAST ( '<em class="replaceable"><code>string</code></em>' AS <em class="replaceable"><code>type</code></em> ) +</pre><p> + The string constant's text is passed to the input conversion + routine for the type called <em class="replaceable"><code>type</code></em>. The + result is a constant of the indicated type. The explicit type + cast can be omitted if there is no ambiguity as to the type the + constant must be (for example, when it is assigned directly to a + table column), in which case it is automatically coerced. + </p><p> + The string constant can be written using either regular SQL + notation or dollar-quoting. + </p><p> + It is also possible to specify a type coercion using a function-like + syntax: +</p><pre class="synopsis"> +<em class="replaceable"><code>typename</code></em> ( '<em class="replaceable"><code>string</code></em>' ) +</pre><p> + but not all type names can be used in this way; see <a class="xref" href="sql-expressions.html#SQL-SYNTAX-TYPE-CASTS" title="4.2.9. Type Casts">Section 4.2.9</a> for details. + </p><p> + The <code class="literal">::</code>, <code class="literal">CAST()</code>, and + function-call syntaxes can also be used to specify run-time type + conversions of arbitrary expressions, as discussed in <a class="xref" href="sql-expressions.html#SQL-SYNTAX-TYPE-CASTS" title="4.2.9. Type Casts">Section 4.2.9</a>. To avoid syntactic ambiguity, the + <code class="literal"><em class="replaceable"><code>type</code></em> '<em class="replaceable"><code>string</code></em>'</code> + syntax can only be used to specify the type of a simple literal constant. + Another restriction on the + <code class="literal"><em class="replaceable"><code>type</code></em> '<em class="replaceable"><code>string</code></em>'</code> + syntax is that it does not work for array types; use <code class="literal">::</code> + or <code class="literal">CAST()</code> to specify the type of an array constant. + </p><p> + The <code class="literal">CAST()</code> syntax conforms to SQL. The + <code class="literal"><em class="replaceable"><code>type</code></em> '<em class="replaceable"><code>string</code></em>'</code> + syntax is a generalization of the standard: SQL specifies this syntax only + for a few data types, but <span class="productname">PostgreSQL</span> allows it + for all types. The syntax with + <code class="literal">::</code> is historical <span class="productname">PostgreSQL</span> + usage, as is the function-call syntax. + </p></div></div><div class="sect2" id="SQL-SYNTAX-OPERATORS"><div class="titlepage"><div><div><h3 class="title">4.1.3. Operators</h3></div></div></div><a id="id-1.5.3.5.10.2" class="indexterm"></a><p> + An operator name is a sequence of up to <code class="symbol">NAMEDATALEN</code>-1 + (63 by default) characters from the following list: +</p><div class="literallayout"><p><br /> ++ - * / < > = ~ ! @ # % ^ & | ` ?<br /> +</p></div><p> + + There are a few restrictions on operator names, however: + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + <code class="literal">--</code> and <code class="literal">/*</code> cannot appear + anywhere in an operator name, since they will be taken as the + start of a comment. + </p></li><li class="listitem"><p> + A multiple-character operator name cannot end in <code class="literal">+</code> or <code class="literal">-</code>, + unless the name also contains at least one of these characters: +</p><div class="literallayout"><p><br /> +~ ! @ # % ^ & | ` ?<br /> +</p></div><p> + For example, <code class="literal">@-</code> is an allowed operator name, + but <code class="literal">*-</code> is not. This restriction allows + <span class="productname">PostgreSQL</span> to parse SQL-compliant + queries without requiring spaces between tokens. + </p></li></ul></div><p> + </p><p> + When working with non-SQL-standard operator names, you will usually + need to separate adjacent operators with spaces to avoid ambiguity. + For example, if you have defined a prefix operator named <code class="literal">@</code>, + you cannot write <code class="literal">X*@Y</code>; you must write + <code class="literal">X* @Y</code> to ensure that + <span class="productname">PostgreSQL</span> reads it as two operator names + not one. + </p></div><div class="sect2" id="SQL-SYNTAX-SPECIAL-CHARS"><div class="titlepage"><div><div><h3 class="title">4.1.4. Special Characters</h3></div></div></div><p> + Some characters that are not alphanumeric have a special meaning + that is different from being an operator. Details on the usage can + be found at the location where the respective syntax element is + described. This section only exists to advise the existence and + summarize the purposes of these characters. + + </p><div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem"><p> + A dollar sign (<code class="literal">$</code>) followed by digits is used + to represent a positional parameter in the body of a function + definition or a prepared statement. In other contexts the + dollar sign can be part of an identifier or a dollar-quoted string + constant. + </p></li><li class="listitem"><p> + Parentheses (<code class="literal">()</code>) have their usual meaning to + group expressions and enforce precedence. In some cases + parentheses are required as part of the fixed syntax of a + particular SQL command. + </p></li><li class="listitem"><p> + Brackets (<code class="literal">[]</code>) are used to select the elements + of an array. See <a class="xref" href="arrays.html" title="8.15. Arrays">Section 8.15</a> for more information + on arrays. + </p></li><li class="listitem"><p> + Commas (<code class="literal">,</code>) are used in some syntactical + constructs to separate the elements of a list. + </p></li><li class="listitem"><p> + The semicolon (<code class="literal">;</code>) terminates an SQL command. + It cannot appear anywhere within a command, except within a + string constant or quoted identifier. + </p></li><li class="listitem"><p> + The colon (<code class="literal">:</code>) is used to select + <span class="quote">“<span class="quote">slices</span>”</span> from arrays. (See <a class="xref" href="arrays.html" title="8.15. Arrays">Section 8.15</a>.) In certain SQL dialects (such as Embedded + SQL), the colon is used to prefix variable names. + </p></li><li class="listitem"><p> + The asterisk (<code class="literal">*</code>) is used in some contexts to denote + all the fields of a table row or composite value. It also + has a special meaning when used as the argument of an + aggregate function, namely that the aggregate does not require + any explicit parameter. + </p></li><li class="listitem"><p> + The period (<code class="literal">.</code>) is used in numeric + constants, and to separate schema, table, and column names. + </p></li></ul></div><p> + + </p></div><div class="sect2" id="SQL-SYNTAX-COMMENTS"><div class="titlepage"><div><div><h3 class="title">4.1.5. Comments</h3></div></div></div><a id="id-1.5.3.5.12.2" class="indexterm"></a><p> + A comment is a sequence of characters beginning with + double dashes and extending to the end of the line, e.g.: +</p><pre class="programlisting"> +-- This is a standard SQL comment +</pre><p> + </p><p> + Alternatively, C-style block comments can be used: +</p><pre class="programlisting"> +/* multiline comment + * with nesting: /* nested block comment */ + */ +</pre><p> + where the comment begins with <code class="literal">/*</code> and extends to + the matching occurrence of <code class="literal">*/</code>. These block + comments nest, as specified in the SQL standard but unlike C, so that one can + comment out larger blocks of code that might contain existing block + comments. + </p><p> + A comment is removed from the input stream before further syntax + analysis and is effectively replaced by whitespace. + </p></div><div class="sect2" id="SQL-PRECEDENCE"><div class="titlepage"><div><div><h3 class="title">4.1.6. Operator Precedence</h3></div></div></div><a id="id-1.5.3.5.13.2" class="indexterm"></a><p> + <a class="xref" href="sql-syntax-lexical.html#SQL-PRECEDENCE-TABLE" title="Table 4.2. Operator Precedence (highest to lowest)">Table 4.2</a> shows the precedence and + associativity of the operators in <span class="productname">PostgreSQL</span>. + Most operators have the same precedence and are left-associative. + The precedence and associativity of the operators is hard-wired + into the parser. + Add parentheses if you want an expression with multiple operators + to be parsed in some other way than what the precedence rules imply. + </p><div class="table" id="SQL-PRECEDENCE-TABLE"><p class="title"><strong>Table 4.2. Operator Precedence (highest to lowest)</strong></p><div class="table-contents"><table class="table" summary="Operator Precedence (highest to lowest)" border="1"><colgroup><col class="col1" /><col class="col2" /><col class="col3" /></colgroup><thead><tr><th>Operator/Element</th><th>Associativity</th><th>Description</th></tr></thead><tbody><tr><td><code class="token">.</code></td><td>left</td><td>table/column name separator</td></tr><tr><td><code class="token">::</code></td><td>left</td><td><span class="productname">PostgreSQL</span>-style typecast</td></tr><tr><td><code class="token">[</code> <code class="token">]</code></td><td>left</td><td>array element selection</td></tr><tr><td><code class="token">+</code> <code class="token">-</code></td><td>right</td><td>unary plus, unary minus</td></tr><tr><td><code class="token">^</code></td><td>left</td><td>exponentiation</td></tr><tr><td><code class="token">*</code> <code class="token">/</code> <code class="token">%</code></td><td>left</td><td>multiplication, division, modulo</td></tr><tr><td><code class="token">+</code> <code class="token">-</code></td><td>left</td><td>addition, subtraction</td></tr><tr><td>(any other operator)</td><td>left</td><td>all other native and user-defined operators</td></tr><tr><td><code class="token">BETWEEN</code> <code class="token">IN</code> <code class="token">LIKE</code> <code class="token">ILIKE</code> <code class="token">SIMILAR</code></td><td> </td><td>range containment, set membership, string matching</td></tr><tr><td><code class="token"><</code> <code class="token">></code> <code class="token">=</code> <code class="token"><=</code> <code class="token">>=</code> <code class="token"><></code> +</td><td> </td><td>comparison operators</td></tr><tr><td><code class="token">IS</code> <code class="token">ISNULL</code> <code class="token">NOTNULL</code></td><td> </td><td><code class="literal">IS TRUE</code>, <code class="literal">IS FALSE</code>, <code class="literal">IS + NULL</code>, <code class="literal">IS DISTINCT FROM</code>, etc</td></tr><tr><td><code class="token">NOT</code></td><td>right</td><td>logical negation</td></tr><tr><td><code class="token">AND</code></td><td>left</td><td>logical conjunction</td></tr><tr><td><code class="token">OR</code></td><td>left</td><td>logical disjunction</td></tr></tbody></table></div></div><br class="table-break" /><p> + Note that the operator precedence rules also apply to user-defined + operators that have the same names as the built-in operators + mentioned above. For example, if you define a + <span class="quote">“<span class="quote">+</span>”</span> operator for some custom data type it will have + the same precedence as the built-in <span class="quote">“<span class="quote">+</span>”</span> operator, no + matter what yours does. + </p><p> + When a schema-qualified operator name is used in the + <code class="literal">OPERATOR</code> syntax, as for example in: +</p><pre class="programlisting"> +SELECT 3 OPERATOR(pg_catalog.+) 4; +</pre><p> + the <code class="literal">OPERATOR</code> construct is taken to have the default precedence + shown in <a class="xref" href="sql-syntax-lexical.html#SQL-PRECEDENCE-TABLE" title="Table 4.2. Operator Precedence (highest to lowest)">Table 4.2</a> for + <span class="quote">“<span class="quote">any other operator</span>”</span>. This is true no matter + which specific operator appears inside <code class="literal">OPERATOR()</code>. + </p><div class="note"><h3 class="title">Note</h3><p> + <span class="productname">PostgreSQL</span> versions before 9.5 used slightly different + operator precedence rules. In particular, <code class="token"><=</code> + <code class="token">>=</code> and <code class="token"><></code> used to be treated as + generic operators; <code class="literal">IS</code> tests used to have higher priority; + and <code class="literal">NOT BETWEEN</code> and related constructs acted inconsistently, + being taken in some cases as having the precedence of <code class="literal">NOT</code> + rather than <code class="literal">BETWEEN</code>. These rules were changed for better + compliance with the SQL standard and to reduce confusion from + inconsistent treatment of logically equivalent constructs. In most + cases, these changes will result in no behavioral change, or perhaps + in <span class="quote">“<span class="quote">no such operator</span>”</span> failures which can be resolved by adding + parentheses. However there are corner cases in which a query might + change behavior without any parsing error being reported. + </p></div></div></div><div xmlns="http://www.w3.org/TR/xhtml1/transitional" class="navfooter"><hr></hr><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="sql-syntax.html" title="Chapter 4. SQL Syntax">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="sql-syntax.html" title="Chapter 4. SQL Syntax">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="sql-expressions.html" title="4.2. Value Expressions">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 4. SQL Syntax </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 14.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 4.2. Value Expressions</td></tr></table></div></body></html>
\ No newline at end of file |