 <p>The <code>=</code> operator compares a single value against another, so it is assumed that the subquery returns only a single row.</p> <p>To check whether a (column) value is in a set of values, use IN:</p> <pre><code>SELECT * FROM Table2 WHERE fileid IN (SELECT FileID FROM Table1 WHERE F_Property1 &gt; 1) </code></pre>