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POSQL Natural Join and nested SELECT query
 

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    <p>What might make these two queries different?</p> <p>The post is indeed mostly code.</p> <pre><code>SELECT DISTINCT S.name FROM student S NATURAL JOIN taking NATURAL JOIN (select * from class where classnum ='121') WHERE department='CMPSC' AND semester='Spring 2013'; SELECT DISTINCT S.name FROM student S NATURAL JOIN taking NATURAL JOIN class WHERE department='CMPSC' AND semester='Spring 2013' AND classnum='121'; </code></pre> <p>Thank you!</p> <p>EDIT:</p> <p>As a response to the request for explain command: I had to do it on ORACLE so I'm not sure if this is the result expected:</p> <p>This is the first query:</p> <pre><code>Plan hash value: 3259400360 ------------------------------------------------------ | Id | Operation | Name | ------------------------------------------------------ | 0 | SELECT STATEMENT | | | 1 | HASH UNIQUE | | | 2 | NESTED LOOPS | | | 3 | NESTED LOOPS | | | 4 | TABLE ACCESS FULL | CLASS | | 5 | INDEX FULL SCAN | SYS_C0099014 | | 6 | TABLE ACCESS BY INDEX ROWID| STUDENT | | 7 | INDEX UNIQUE SCAN | SYS_C0098998 | ------------------------------------------------------ Predicate Information (identified by operation id): --------------------------------------------------- 4 - filter("CLASSNUM"=121 AND "CLASS"."SEMESTER"='Spring 2013' AND "CLASS"."DEPARTMENT"='CMPSC') PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------- 5 - access("TAKING"."SCHEDULENUM"="CLASS"."SCHEDULENUM" AND "TAKING"."SEMESTER"='Spring 2013') filter("TAKING"."SEMESTER"='Spring 2013' AND "TAKING"."SCHEDULENUM"="CLASS"."SCHEDULENUM") 7 - access("S"."STUDENTNUM"="TAKING"."STUDENTNUM") </code></pre> <p>Second QUERY:</p> <pre><code>PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------- Plan hash value: 226170808 ------------------------------------------------------- | Id | Operation | Name | ------------------------------------------------------- | 0 | SELECT STATEMENT | | | 1 | HASH UNIQUE | | | 2 | HASH JOIN | | | 3 | MERGE JOIN | | | 4 | TABLE ACCESS BY INDEX ROWID| STUDENT | | 5 | INDEX FULL SCAN | SYS_C0098998 | | 6 | SORT JOIN | | | 7 | INDEX FULL SCAN | SYS_C0099014 | | 8 | TABLE ACCESS FULL | CLASS | ------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------- 2 - access("TAKING"."SEMESTER"="CLASS"."SEMESTER") 6 - access("S"."STUDENTNUM"="TAKING"."STUDENTNUM") filter("S"."STUDENTNUM"="TAKING"."STUDENTNUM") 7 - access("TAKING"."SEMESTER"='Spring 2013') filter("TAKING"."SEMESTER"='Spring 2013') 8 - filter("CLASS"."CLASSNUM"=121 AND "CLASS"."SEMESTER"='Spring 2013' AND "CLASS"."DEPARTMENT"='CMPSC') </code></pre>
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    1. COAre you saying the results are different or the performance? If the results are different, please post examples.
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    2. COTry running `explain` on the two queries to get the query execution plan; compare them to see the differences. Also post them here for others to analyse. [Explain syntax] (http://dev.mysql.com/doc/refman/5.0/en/explain.html)
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    3. CO@Kshitij I hope this is what you asked for, I'm running Oracle.
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