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copied!<p>An empty set <code>INTERSECT</code>ed with any other set will always be empty. It's like multiplying by 0. You always get 0.</p> <p>Conditional <code>INTERSECT</code>ing will require either a dynamic query, or a staging table, like this:</p> <p><strong>Initializations</strong></p> <pre><code>DECLARE @CAN_USE_TABLE1 BIT DECLARE @CAN_USE_TABLE2 BIT DECLARE @CAN_USE_TABLE3 BIT DECLARE @CAN_USE_TABLE4 BIT DECLARE @TABLE1 AS TABLE ( ABC INT ) DECLARE @TABLE2 AS TABLE ( ABC INT ) DECLARE @TABLE3 AS TABLE ( ABC INT ) DECLARE @TABLE4 AS TABLE ( ABC INT ) DECLARE @RESULT AS TABLE ( ABC INT ) --Adding this result table INSERT INTO @TABLE1 VALUES (1) INSERT INTO @TABLE1 VALUES (2) INSERT INTO @TABLE1 VALUES (3) INSERT INTO @TABLE2 VALUES (1) INSERT INTO @TABLE2 VALUES (2) SET @CAN_USE_TABLE1 = 1 SET @CAN_USE_TABLE2 = 1 SET @CAN_USE_TABLE3 = 0 SET @CAN_USE_TABLE4 = 0 </code></pre> <p><strong>Processing</strong></p> <pre><code>INSERT INTO @RESULT SELECT ABC FROM @TABLE1 WHERE @CAN_USE_TABLE1=1 UNION SELECT ABC FROM @TABLE2 WHERE @CAN_USE_TABLE2=1 UNION SELECT ABC FROM @TABLE3 WHERE @CAN_USE_TABLE3=1 UNION SELECT ABC FROM @TABLE4 WHERE @CAN_USE_TABLE4=1 DELETE r FROM @RESULT r WHERE NOT EXISTS(SELECT 1 FROM @TABLE1 WHERE ABC=r.ABC) AND @CAN_USE_TABLE1=1; DELETE r FROM @RESULT r WHERE NOT EXISTS(SELECT 1 FROM @TABLE2 WHERE ABC=r.ABC) AND @CAN_USE_TABLE2=1; DELETE r FROM @RESULT r WHERE NOT EXISTS(SELECT 1 FROM @TABLE3 WHERE ABC=r.ABC) AND @CAN_USE_TABLE3=1; DELETE r FROM @RESULT r WHERE NOT EXISTS(SELECT 1 FROM @TABLE4 WHERE ABC=r.ABC) AND @CAN_USE_TABLE4=1; SELECT * FROM @RESULT; </code></pre> <p><strong>Result</strong></p> <pre><code>1 2 </code></pre>

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