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diff --git a/doc/src/sgml/html/ddl-basics.html b/doc/src/sgml/html/ddl-basics.html new file mode 100644 index 0000000..33d00e7 --- /dev/null +++ b/doc/src/sgml/html/ddl-basics.html @@ -0,0 +1,101 @@ +<?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>5.1. Table Basics</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="ddl.html" title="Chapter 5. Data Definition" /><link rel="next" href="ddl-default.html" title="5.2. Default Values" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">5.1. Table Basics</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ddl.html" title="Chapter 5. Data Definition">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><th width="60%" align="center">Chapter 5. Data Definition</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.4 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ddl-default.html" title="5.2. Default Values">Next</a></td></tr></table><hr /></div><div class="sect1" id="DDL-BASICS"><div class="titlepage"><div><div><h2 class="title" style="clear: both">5.1. Table Basics</h2></div></div></div><a id="id-1.5.4.3.2" class="indexterm"></a><a id="id-1.5.4.3.3" class="indexterm"></a><a id="id-1.5.4.3.4" class="indexterm"></a><p> + A table in a relational database is much like a table on paper: It + consists of rows and columns. The number and order of the columns + is fixed, and each column has a name. The number of rows is + variable — it reflects how much data is stored at a given moment. + SQL does not make any guarantees about the order of the rows in a + table. When a table is read, the rows will appear in an unspecified order, + unless sorting is explicitly requested. This is covered in <a class="xref" href="queries.html" title="Chapter 7. Queries">Chapter 7</a>. Furthermore, SQL does not assign unique + identifiers to rows, so it is possible to have several completely + identical rows in a table. This is a consequence of the + mathematical model that underlies SQL but is usually not desirable. + Later in this chapter we will see how to deal with this issue. + </p><p> + Each column has a data type. The data type constrains the set of + possible values that can be assigned to a column and assigns + semantics to the data stored in the column so that it can be used + for computations. For instance, a column declared to be of a + numerical type will not accept arbitrary text strings, and the data + stored in such a column can be used for mathematical computations. + By contrast, a column declared to be of a character string type + will accept almost any kind of data but it does not lend itself to + mathematical calculations, although other operations such as string + concatenation are available. + </p><p> + <span class="productname">PostgreSQL</span> includes a sizable set of + built-in data types that fit many applications. Users can also + define their own data types. Most built-in data types have obvious + names and semantics, so we defer a detailed explanation to <a class="xref" href="datatype.html" title="Chapter 8. Data Types">Chapter 8</a>. Some of the frequently used data types are + <code class="type">integer</code> for whole numbers, <code class="type">numeric</code> for + possibly fractional numbers, <code class="type">text</code> for character + strings, <code class="type">date</code> for dates, <code class="type">time</code> for + time-of-day values, and <code class="type">timestamp</code> for values + containing both date and time. + </p><a id="id-1.5.4.3.8" class="indexterm"></a><p> + To create a table, you use the aptly named <a class="xref" href="sql-createtable.html" title="CREATE TABLE"><span class="refentrytitle">CREATE TABLE</span></a> command. + In this command you specify at least a name for the new table, the + names of the columns and the data type of each column. For + example: +</p><pre class="programlisting"> +CREATE TABLE my_first_table ( + first_column text, + second_column integer +); +</pre><p> + This creates a table named <code class="literal">my_first_table</code> with + two columns. The first column is named + <code class="literal">first_column</code> and has a data type of + <code class="type">text</code>; the second column has the name + <code class="literal">second_column</code> and the type <code class="type">integer</code>. + The table and column names follow the identifier syntax explained + in <a class="xref" href="sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS" title="4.1.1. Identifiers and Key Words">Section 4.1.1</a>. The type names are + usually also identifiers, but there are some exceptions. Note that the + column list is comma-separated and surrounded by parentheses. + </p><p> + Of course, the previous example was heavily contrived. Normally, + you would give names to your tables and columns that convey what + kind of data they store. So let's look at a more realistic + example: +</p><pre class="programlisting"> +CREATE TABLE products ( + product_no integer, + name text, + price numeric +); +</pre><p> + (The <code class="type">numeric</code> type can store fractional components, as + would be typical of monetary amounts.) + </p><div class="tip"><h3 class="title">Tip</h3><p> + When you create many interrelated tables it is wise to choose a + consistent naming pattern for the tables and columns. For + instance, there is a choice of using singular or plural nouns for + table names, both of which are favored by some theorist or other. + </p></div><p> + There is a limit on how many columns a table can contain. + Depending on the column types, it is between 250 and 1600. + However, defining a table with anywhere near this many columns is + highly unusual and often a questionable design. + </p><a id="id-1.5.4.3.13" class="indexterm"></a><p> + If you no longer need a table, you can remove it using the <a class="xref" href="sql-droptable.html" title="DROP TABLE"><span class="refentrytitle">DROP TABLE</span></a> command. + For example: +</p><pre class="programlisting"> +DROP TABLE my_first_table; +DROP TABLE products; +</pre><p> + Attempting to drop a table that does not exist is an error. + Nevertheless, it is common in SQL script files to unconditionally + try to drop each table before creating it, ignoring any error + messages, so that the script works whether or not the table exists. + (If you like, you can use the <code class="literal">DROP TABLE IF EXISTS</code> variant + to avoid the error messages, but this is not standard SQL.) + </p><p> + If you need to modify a table that already exists, see <a class="xref" href="ddl-alter.html" title="5.6. Modifying Tables">Section 5.6</a> later in this chapter. + </p><p> + With the tools discussed so far you can create fully functional + tables. The remainder of this chapter is concerned with adding + features to the table definition to ensure data integrity, + security, or convenience. If you are eager to fill your tables with + data now you can skip ahead to <a class="xref" href="dml.html" title="Chapter 6. Data Manipulation">Chapter 6</a> and read the + rest of this chapter later. + </p></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ddl.html" title="Chapter 5. Data Definition">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ddl.html" title="Chapter 5. Data Definition">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ddl-default.html" title="5.2. Default Values">Next</a></td></tr><tr><td width="40%" align="left" valign="top">Chapter 5. 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