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authorDaniel Baumann <daniel.baumann@progress-linux.org>2024-05-04 12:17:33 +0000
committerDaniel Baumann <daniel.baumann@progress-linux.org>2024-05-04 12:17:33 +0000
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+<!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>36.5. Dynamic SQL</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="ecpg-variables.html" title="36.4. Using Host Variables" /><link rel="next" href="ecpg-pgtypes.html" title="36.6. pgtypes Library" /></head><body id="docContent" class="container-fluid col-10"><div class="navheader"><table width="100%" summary="Navigation header"><tr><th colspan="5" align="center">36.5. Dynamic SQL</th></tr><tr><td width="10%" align="left"><a accesskey="p" href="ecpg-variables.html" title="36.4. Using Host Variables">Prev</a> </td><td width="10%" align="left"><a accesskey="u" href="ecpg.html" title="Chapter 36. ECPG — Embedded SQL in C">Up</a></td><th width="60%" align="center">Chapter 36. <span class="application">ECPG</span> — Embedded <acronym class="acronym">SQL</acronym> in C</th><td width="10%" align="right"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="10%" align="right"> <a accesskey="n" href="ecpg-pgtypes.html" title="36.6. pgtypes Library">Next</a></td></tr></table><hr /></div><div class="sect1" id="ECPG-DYNAMIC"><div class="titlepage"><div><div><h2 class="title" style="clear: both">36.5. Dynamic SQL</h2></div></div></div><div class="toc"><dl class="toc"><dt><span class="sect2"><a href="ecpg-dynamic.html#ECPG-DYNAMIC-WITHOUT-RESULT">36.5.1. Executing Statements without a Result Set</a></span></dt><dt><span class="sect2"><a href="ecpg-dynamic.html#ECPG-DYNAMIC-INPUT">36.5.2. Executing a Statement with Input Parameters</a></span></dt><dt><span class="sect2"><a href="ecpg-dynamic.html#ECPG-DYNAMIC-WITH-RESULT">36.5.3. Executing a Statement with a Result Set</a></span></dt></dl></div><p>
+ In many cases, the particular SQL statements that an application
+ has to execute are known at the time the application is written.
+ In some cases, however, the SQL statements are composed at run time
+ or provided by an external source. In these cases you cannot embed
+ the SQL statements directly into the C source code, but there is a
+ facility that allows you to call arbitrary SQL statements that you
+ provide in a string variable.
+ </p><div class="sect2" id="ECPG-DYNAMIC-WITHOUT-RESULT"><div class="titlepage"><div><div><h3 class="title">36.5.1. Executing Statements without a Result Set</h3></div></div></div><p>
+ The simplest way to execute an arbitrary SQL statement is to use
+ the command <code class="command">EXECUTE IMMEDIATE</code>. For example:
+</p><pre class="programlisting">
+EXEC SQL BEGIN DECLARE SECTION;
+const char *stmt = "CREATE TABLE test1 (...);";
+EXEC SQL END DECLARE SECTION;
+
+EXEC SQL EXECUTE IMMEDIATE :stmt;
+</pre><p>
+ <code class="command">EXECUTE IMMEDIATE</code> can be used for SQL
+ statements that do not return a result set (e.g.,
+ DDL, <code class="command">INSERT</code>, <code class="command">UPDATE</code>,
+ <code class="command">DELETE</code>). You cannot execute statements that
+ retrieve data (e.g., <code class="command">SELECT</code>) this way. The
+ next section describes how to do that.
+ </p></div><div class="sect2" id="ECPG-DYNAMIC-INPUT"><div class="titlepage"><div><div><h3 class="title">36.5.2. Executing a Statement with Input Parameters</h3></div></div></div><p>
+ A more powerful way to execute arbitrary SQL statements is to
+ prepare them once and execute the prepared statement as often as
+ you like. It is also possible to prepare a generalized version of
+ a statement and then execute specific versions of it by
+ substituting parameters. When preparing the statement, write
+ question marks where you want to substitute parameters later. For
+ example:
+</p><pre class="programlisting">
+EXEC SQL BEGIN DECLARE SECTION;
+const char *stmt = "INSERT INTO test1 VALUES(?, ?);";
+EXEC SQL END DECLARE SECTION;
+
+EXEC SQL PREPARE mystmt FROM :stmt;
+ ...
+EXEC SQL EXECUTE mystmt USING 42, 'foobar';
+</pre><p>
+ </p><p>
+ When you don't need the prepared statement anymore, you should
+ deallocate it:
+</p><pre class="programlisting">
+EXEC SQL DEALLOCATE PREPARE <em class="replaceable"><code>name</code></em>;
+</pre><p>
+ </p></div><div class="sect2" id="ECPG-DYNAMIC-WITH-RESULT"><div class="titlepage"><div><div><h3 class="title">36.5.3. Executing a Statement with a Result Set</h3></div></div></div><p>
+ To execute an SQL statement with a single result row,
+ <code class="command">EXECUTE</code> can be used. To save the result, add
+ an <code class="literal">INTO</code> clause.
+</p><pre class="programlisting">
+EXEC SQL BEGIN DECLARE SECTION;
+const char *stmt = "SELECT a, b, c FROM test1 WHERE a &gt; ?";
+int v1, v2;
+VARCHAR v3[50];
+EXEC SQL END DECLARE SECTION;
+
+EXEC SQL PREPARE mystmt FROM :stmt;
+ ...
+EXEC SQL EXECUTE mystmt INTO :v1, :v2, :v3 USING 37;
+
+</pre><p>
+ An <code class="command">EXECUTE</code> command can have an
+ <code class="literal">INTO</code> clause, a <code class="literal">USING</code> clause,
+ both, or neither.
+ </p><p>
+ If a query is expected to return more than one result row, a
+ cursor should be used, as in the following example.
+ (See <a class="xref" href="ecpg-commands.html#ECPG-CURSORS" title="36.3.2. Using Cursors">Section 36.3.2</a> for more details about the
+ cursor.)
+</p><pre class="programlisting">
+EXEC SQL BEGIN DECLARE SECTION;
+char dbaname[128];
+char datname[128];
+char *stmt = "SELECT u.usename as dbaname, d.datname "
+ " FROM pg_database d, pg_user u "
+ " WHERE d.datdba = u.usesysid";
+EXEC SQL END DECLARE SECTION;
+
+EXEC SQL CONNECT TO testdb AS con1 USER testuser;
+EXEC SQL SELECT pg_catalog.set_config('search_path', '', false); EXEC SQL COMMIT;
+
+EXEC SQL PREPARE stmt1 FROM :stmt;
+
+EXEC SQL DECLARE cursor1 CURSOR FOR stmt1;
+EXEC SQL OPEN cursor1;
+
+EXEC SQL WHENEVER NOT FOUND DO BREAK;
+
+while (1)
+{
+ EXEC SQL FETCH cursor1 INTO :dbaname,:datname;
+ printf("dbaname=%s, datname=%s\n", dbaname, datname);
+}
+
+EXEC SQL CLOSE cursor1;
+
+EXEC SQL COMMIT;
+EXEC SQL DISCONNECT ALL;
+</pre><p>
+ </p></div></div><div class="navfooter"><hr /><table width="100%" summary="Navigation footer"><tr><td width="40%" align="left"><a accesskey="p" href="ecpg-variables.html" title="36.4. Using Host Variables">Prev</a> </td><td width="20%" align="center"><a accesskey="u" href="ecpg.html" title="Chapter 36. ECPG — Embedded SQL in C">Up</a></td><td width="40%" align="right"> <a accesskey="n" href="ecpg-pgtypes.html" title="36.6. pgtypes Library">Next</a></td></tr><tr><td width="40%" align="left" valign="top">36.4. Using Host Variables </td><td width="20%" align="center"><a accesskey="h" href="index.html" title="PostgreSQL 15.5 Documentation">Home</a></td><td width="40%" align="right" valign="top"> 36.6. pgtypes Library</td></tr></table></div></body></html> \ No newline at end of file