32 lines
1.2 KiB
PHP
32 lines
1.2 KiB
PHP
A subquery can contain any of the keywords or clauses that an ordinary SELECT query can contain.
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<br \><br \>
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Some advantages of the subqueries:
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<ul>
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<li \>They allow queries that are structured so that it is possible to isolate each part of a statement.
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<li \>They provide alternative ways to perform operations that would otherwise require complex joins and unions.
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<li \>They are, in many people's opinion, readable. Indeed, it was the innovation of subqueries that gave people the original idea of calling the early SQL “Structured Query Language.”
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</ul>
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<code type="php">
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// finding all users which don't belong to any group 1
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$query = "FROM User WHERE User.id NOT IN
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(SELECT u.id FROM User u
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INNER JOIN u.Group g WHERE g.id = ?";
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$users = $conn->query($query, array(1));
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// finding all users which don't belong to any groups
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// Notice:
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// the usage of INNER JOIN
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// the usage of empty brackets preceding the Group component
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$query = "FROM User WHERE User.id NOT IN
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(SELECT u.id FROM User u
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INNER JOIN u.Group g)";
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$users = $conn->query($query);
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</code>
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