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rn5a
Starting Member
25 Posts |
 Posted - 2008-10-27 : 01:46:28
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| Often I see queries using subqueries wherein the subquery comes in the FROM clause &/or in the WHERE clause. How do I understand when to put the subquery in the FROM clause & when to put the subquery in the WHERE clause?Thanks,Ron |
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SwePeso
Patron Saint of Lost Yaks
30421 Posts |
Posted - 2008-10-27 : 02:08:25
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A "subquery" in the FROM part is called a derived table and is evaluated once.A correlated subquery (in the SELECT part) is evaluated for every record.A correlated subquery (in the WHERE part) is evaluated for every record.
E 12°55'05.63"N 56°04'39.26" |
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visakh16
Very Important crosS Applying yaK Herder
52326 Posts |
Posted - 2008-10-27 : 02:09:29
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| depends on your scenario. If you want some grouping or aggregation to be performed on a table and then take the aggregated results to join to another table thats when you use subquery in FROM clause. Its actually called a derived table. Whenever you want to select fields from one table based on existence of a particular field or query condition, that when you use subquery in where clause. |
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Subquery