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diff --git a/www/gencol.html b/www/gencol.html new file mode 100644 index 0000000..8bdae0d --- /dev/null +++ b/www/gencol.html @@ -0,0 +1,308 @@ +<!DOCTYPE html> +<html><head> +<meta name="viewport" content="width=device-width, initial-scale=1.0"> +<meta http-equiv="content-type" content="text/html; charset=UTF-8"> +<link href="sqlite.css" rel="stylesheet"> +<title>Generated Columns</title> +<!-- path= --> +</head> +<body> +<div class=nosearch> +<a href="index.html"> +<img class="logo" src="images/sqlite370_banner.gif" alt="SQLite" border="0"> +</a> +<div><!-- IE hack to prevent disappearing logo --></div> +<div class="tagline desktoponly"> +Small. Fast. 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Introduction</a></div> +<div class="fancy-toc1"><a href="#syntax">2. Syntax</a></div> +<div class="fancy-toc2"><a href="#virtual_versus_stored_columns">2.1. VIRTUAL versus STORED columns</a></div> +<div class="fancy-toc2"><a href="#capabilities">2.2. Capabilities</a></div> +<div class="fancy-toc2"><a href="#limitations">2.3. Limitations</a></div> +<div class="fancy-toc1"><a href="#compatibility">3. Compatibility</a></div> +</div> +</div> +<script> +function toggle_toc(){ +var sub = document.getElementById("toc_sub") +var mk = document.getElementById("toc_mk") +if( sub.style.display!="block" ){ +sub.style.display = "block"; +mk.innerHTML = "▼"; +} else { +sub.style.display = "none"; +mk.innerHTML = "►"; +} +} +</script> +</div> + + + + +<h1 id="introduction"><span>1. </span>Introduction</h1> + +<p>Generated columns (also sometimes called "computed columns") +are columns of a table whose values are a function of other columns +in the same row. +Generated columns can be read, but their values can not be directly +written. The only way to change the value of a generated column is to +modify the values of the other columns used to calculate +the generated column. + +</p><h1 id="syntax"><span>2. </span>Syntax</h1> + +<p>Syntactically, generated columns are designated using a +"GENERATED ALWAYS" <a href="syntax/column-constraint.html">column-constraint</a>. For example: + +</p><div class="codeblock"><pre>CREATE TABLE t1( + a INTEGER PRIMARY KEY, + b INT, + c TEXT, + d INT GENERATED ALWAYS AS (a*abs(b)) VIRTUAL, + e TEXT GENERATED ALWAYS AS (substr(c,b,b+1)) STORED +); +</pre></div> + +<p>The statement above has three ordinary columns, "a" (the PRIMARY KEY), +"b", and "c", and two generated columns "d" and "e". + +</p><p>The "GENERATED ALWAYS" keywords at the beginning of the constraint +and the "VIRTUAL" or "STORED" keyword at the end are all optional. +Only the "AS" keyword and the parenthesized expression are required. +If the trailing "VIRTUAL" or "STORED" keyword is omitted, then +VIRTUAL is the default. Hence, the example statement above could +be simplified to just: + +</p><div class="codeblock"><pre>CREATE TABLE t1( + a INTEGER PRIMARY KEY, + b INT, + c TEXT, + d INT AS (a*abs(b)), + e TEXT AS (substr(c,b,b+1)) STORED +); +</pre></div> + +<h2 id="virtual_versus_stored_columns"><span>2.1. </span>VIRTUAL versus STORED columns</h2> + +<p>Generated columns can be either VIRTUAL or STORED. The value of +a VIRTUAL column is computed when read, whereas the value of a STORED +column is computed when the row is written. STORED columns take up space +in the database file, whereas VIRTUAL columns use more CPU cycles when +being read. + +</p><p>From the point of view of SQL, STORED and VIRTUAL columns are almost +exactly the same. Queries against either class of generated column +produce the same results. The only functional difference is that +one cannot add new STORED columns using the +<a href="lang_altertable.html#altertabaddcol">ALTER TABLE ADD COLUMN</a> command. Only VIRTUAL columns can be added +using ALTER TABLE. + +</p><h2 id="capabilities"><span>2.2. </span>Capabilities</h2> + +<ol> +<li><p> +Generated columns can have a datatype. SQLite attempts to transform +the result of the generating expression into that datatype using the +same <a href="datatype3.html#affinity">affinity</a> rules as for ordinary columns. + +</p></li><li><p> +Generated columns may have NOT NULL, CHECK, and UNIQUE constraints, +and foreign key constraints, just like ordinary columns. + +</p></li><li><p> +Generated columns can participate in indexes, just like ordinary +columns. + +</p></li><li><p> +The expression of a generated column can refer to any of the +other declared columns in the table, including other generated columns, +as long as the expression does not directly or indirectly refer back +to itself. + +</p></li><li><p> +Generated columns can occur anywhere in the table definition. Generated +columns can be interspersed among ordinary columns. It is not necessary +to put generated columns at the end of the list of columns in the +table definition, as is shown in the examples above. +</p></li></ol> + + +<h2 id="limitations"><span>2.3. </span>Limitations</h2> + +<ol> +<li><p> +Generated columns may not have a <a href="lang_createtable.html#dfltval">default value</a> (they may not use the +"DEFAULT" clause). The value of a generated column is always the value +specified by the expression that follows the "AS" keyword. + +</p></li><li><p> +Generated columns may not be used as part of the <a href="lang_createtable.html#primkeyconst">PRIMARY KEY</a>. +(Future versions of SQLite might relax this constraint for STORED columns.) + +</p></li><li><p> +The expression of a generated column may only reference +constant literals and columns within the same row, and may only use +scalar <a href="deterministic.html">deterministic functions</a>. The expression may not use subqueries, +aggregate functions, window functions, or table-valued functions. + +</p></li><li><p> +The expression of a generated column may refer to other generated columns +in the same row, but no generated column can depend upon itself, either +directly or indirectly. + +</p></li><li><p>The expression of a generated column may not directly reference +the <a href="lang_createtable.html#rowid">ROWID</a>, though it can reference the <a href="lang_createtable.html#rowid">INTEGER PRIMARY KEY</a> column, +which is often the same thing. + +</p></li><li><p> +Every table must have at least one non-generated column. + +</p></li><li><p> +It is not possible to <a href="lang_altertable.html#altertabaddcol">ALTER TABLE ADD COLUMN</a> a STORED column. +One can add a VIRTUAL column, however. + +</p></li><li><p> +The datatype and <a href="datatype3.html#collation">collating sequence</a> of the generated column are determined +only by the datatype and <a href="lang_createtable.html#collateclause">COLLATE clause</a> on the column definition. +The datatype and collating sequence of the GENERATED ALWAYS AS expression +have no affect on the datatype and collating sequence of the column itself. + +</p></li><li><p> +Generated columns are not included in the list of columns provided by +the <a href="pragma.html#pragma_table_info">PRAGMA table_info</a> statement. But they are included in the output of +the newer <a href="pragma.html#pragma_table_xinfo">PRAGMA table_xinfo</a> statement. +</p></li></ol> + +<h1 id="compatibility"><span>3. </span>Compatibility</h1> + +<p>Generated column support was added with SQLite version 3.31.0 +(2020-01-22). If an earlier version of SQLite attempts to read +a database file that contains a generated column in its schema, then +that earlier version will perceive the generated column syntax as an +error and will report that the database schema is corrupt. + +</p><p>To clarify: SQLite version 3.31.0 can read and write any database +created by any prior version of SQLite going back to +SQLite 3.0.0 (2004-06-18). And, earlier versions of SQLite, +prior to 3.31.0, can read and write databases created by SQLite +version 3.31.0 and later as long +as the database schema does not contain features, such as +generated columns, that are not understood by the earlier version. +Problems only arise if you create a new database that contains +generated columns, using SQLite version 3.31.0 or later, and then +try to read or write that database file using an earlier version of +SQLite that does not understand generated columns. +</p><p align="center"><small><i>This page last modified on <a href="https://sqlite.org/docsrc/honeypot" id="mtimelink" data-href="https://sqlite.org/docsrc/finfo/pages/gencol.in?m=1b689d97b3">2022-11-09 20:11:26</a> UTC </small></i></p> + |