


How Can I Correctly Use CASE Statements Within WHERE Clauses in SQL Server 2008?
Addressing CASE Statement Issues in SQL Server 2008 WHERE Clauses
Employing CASE statements within WHERE clauses in SQL Server 2008 can present difficulties. The following query exemplifies a common problem:
SELECT tl.storenum 'Store #', co.ccnum 'FuelFirst Card #', co.dtentered 'Date Entered', CASE st.reasonid WHEN 1 THEN 'Active' WHEN 2 THEN 'Not Active' WHEN 0 THEN st.ccstatustypename ELSE 'Unknown' END 'Status', CASE st.ccstatustypename WHEN 'Active' THEN ' ' WHEN 'Not Active' THEN ' ' ELSE st.ccstatustypename END 'Reason', UPPER(REPLACE(REPLACE(co.personentered,'RT\\',''),'RACETRAC\\','')) 'Person Entered', co.comments 'Comments or Notes' FROM comments co INNER JOIN cards cc ON co.ccnum=cc.ccnum INNER JOIN customerinfo ci ON cc.customerinfoid=ci.customerinfoid INNER JOIN ccstatustype st ON st.ccstatustypeid=cc.ccstatustypeid INNER JOIN customerstatus cs ON cs.customerstatuscd=ci.customerstatuscd INNER JOIN transactionlog tl ON tl.transactionlogid=co.transactionlogid LEFT JOIN stores s ON s.StoreNum = tl.StoreNum WHERE CASE LEN('TestPerson') WHEN 0 THEN co.personentered = co.personentered ELSE co.personentered LIKE '%TestPerson' END AND cc.ccnum = CASE LEN('TestFFNum') WHEN 0 THEN cc.ccnum ELSE 'TestFFNum' END AND CASE LEN('2011-01-09 11:56:29.327') WHEN 0 THEN co.DTEntered = co.DTEntered ELSE CASE LEN('2012-01-09 11:56:29.327') WHEN 0 THEN co.DTEntered >= '2011-01-09 11:56:29.327' ELSE co.DTEntered BETWEEN '2011-01-09 11:56:29.327' AND '2012-01-09 11:56:29.327' END END AND tl.storenum < 699 ORDER BY tl.StoreNum
The Solution: Refactoring with Boolean Logic
The core issue lies in the inappropriate use of CASE statements directly within the WHERE clause's conditional logic. CASE expressions should produce a value; they shouldn't be the entire condition.
The solution involves replacing the CASE statements with equivalent boolean logic using AND
and OR
operators:
SELECT tl.storenum 'Store #', co.ccnum 'FuelFirst Card #', co.dtentered 'Date Entered', CASE st.reasonid WHEN 1 THEN 'Active' WHEN 2 THEN 'Not Active' WHEN 0 THEN st.ccstatustypename ELSE 'Unknown' END 'Status', CASE st.ccstatustypename WHEN 'Active' THEN ' ' WHEN 'Not Active' THEN ' ' ELSE st.ccstatustypename END 'Reason', UPPER(REPLACE(REPLACE(co.personentered,'RT\\',''),'RACETRAC\\','')) 'Person Entered', co.comments 'Comments or Notes' FROM comments co INNER JOIN cards cc ON co.ccnum=cc.ccnum INNER JOIN customerinfo ci ON cc.customerinfoid=ci.customerinfoid INNER JOIN ccstatustype st ON st.ccstatustypeid=cc.ccstatustypeid INNER JOIN customerstatus cs ON cs.customerstatuscd=ci.customerstatuscd INNER JOIN transactionlog tl ON tl.transactionlogid=co.transactionlogid LEFT JOIN stores s ON s.StoreNum = tl.StoreNum WHERE (LEN('TestPerson') = 0 AND co.personentered = co.personentered) OR (LEN('TestPerson') > 0 AND co.personentered LIKE '%TestPerson') AND ((LEN('TestFFNum') = 0 AND cc.ccnum = cc.ccnum) OR (LEN('TestFFNum') > 0 AND cc.ccnum = 'TestFFNum')) AND ((LEN('2011-01-09 11:56:29.327') = 0 AND co.DTEntered = co.DTEntered) OR (LEN('2012-01-09 11:56:29.327') = 0 AND co.DTEntered >= '2011-01-09 11:56:29.327') OR (co.DTEntered BETWEEN '2011-01-09 11:56:29.327' AND '2012-01-09 11:56:29.327')) AND tl.storenum < 699 ORDER BY tl.StoreNum;
Performance Enhancements
While this corrected query functions correctly, performance can be further improved:
-
Reduce Nesting: The deeply nested
OR
conditions can be less efficient. Consider alternative query structures (e.g., usingUNION ALL
for distinct search criteria). -
Indexing: Ensure indexes exist on columns frequently used in
WHERE
clauses (e.g.,co.personentered
,cc.ccnum
,co.DTEntered
,tl.storenum
). - Parameterization: If these 'Test' values are variables, use parameterized queries to avoid repeated query parsing.
By addressing these points, you can create a more efficient and readable SQL query.
The above is the detailed content of How Can I Correctly Use CASE Statements Within WHERE Clauses in SQL Server 2008?. For more information, please follow other related articles on the PHP Chinese website!

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