


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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