78 lines
5.2 KiB
HTML
78 lines
5.2 KiB
HTML
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<title>Example: Correlated subqueries in an UPDATE statement</title>
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<body id="rbafyexsub4"><a name="rbafyexsub4"><!-- --></a>
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<!-- Java sync-link --><script language="Javascript" src="../rzahg/synch.js" type="text/javascript"></script>
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<h1 class="topictitle1">Example: Correlated subqueries in an UPDATE statement</h1>
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<div><p>When you use a correlated subquery in an UPDATE statement, the
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correlation name refers to the rows you are interested in updating.</p>
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<div class="section"><p>For example, when all activities of a project must be completed
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before September 1983, your department considers that project to be a priority
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project. You can use the SQL statement below to evaluate the projects in
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the CORPDATA.PROJECT table, and write a 1 (a flag to indicate PRIORITY) in
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the PRIORITY column (a column you added to CORPDATA.PROJECT for this purpose)
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for each priority project. </p>
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<pre> <strong>UPDATE</strong> CORPDATA.PROJECT X
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<strong>SET</strong> PRIORITY = 1
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<strong>WHERE</strong> '1983-09-01' >
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(<strong>SELECT MAX</strong>(EMENDATE)
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<strong>FROM</strong> CORPDATA.EMPPROJACT
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<strong>WHERE</strong> PROJNO = X.PROJNO)</pre>
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</div>
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<div class="section"><p>As SQL examines each row in the CORPDATA.EMPPROJACT table, it
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determines the maximum activity end date (EMENDATE) for all activities of
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the project (from the CORPDATA.PROJECT table). If the end date of each activity
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associated with the project is before September 1983, the current row in the
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CORPDATA.PROJECT table qualifies and is updated.</p>
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</div>
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<div class="section"><p>Update the master order table with any changes to the quantity
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ordered. If the quantity in the orders table is not set (the NULL value),
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keep the value that is in the master order table.</p>
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<pre><strong>UPDATE</strong> MASTER_ORDERS X
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<strong>SET</strong> QTY=(<strong>SELECT COALESCE</strong> (Y.QTY, X.QTY)
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<strong>FROM</strong> ORDERS Y
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<strong>WHERE</strong> X.ORDER_NUM = Y.ORDER_NUM)
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<strong>WHERE</strong> X.ORDER_NUM <strong>IN</strong> (<strong>SELECT</strong> ORDER_NUM
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<strong>FROM</strong> ORDERS)</pre>
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<p>In this example, each row
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of the MASTER_ORDERS table is checked to see if it has a corresponding row
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in the ORDERS table. If it does have a matching row in the ORDERS table, the
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COALESCE function is used to return a value for the QTY column. If QTY in
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the ORDERS table has a non-null value, that value is used to update the QTY
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column in the MASTER_ORDERS table. If the QTY value in the ORDERS table is
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NULL, the MASTER_ORDERS QTY column is updated with its own value.</p>
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</div>
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</div>
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<div>
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<div class="familylinks">
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<div class="parentlink"><strong>Parent topic:</strong> <a href="rbafycorrs.htm" title="You can write a subquery that SQL might need to re-evaluate as it examines each new row (WHERE clause) or group of rows (HAVING clause) in the outer-level SELECT. This is called a correlated subquery.">Correlated subqueries</a></div>
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