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mathmax
Yak Posting Veteran
95 Posts |
 Posted - 2008-06-15 : 15:25:11
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Hello,Lets consider two tables : CUSTOMERS and ORDERS.I would like to know which of the following query is the fastest:select * from customers Cwhere exists (select 0 from ORDERS O where C.Name like 'A%' and O.Charged = 1) select * from customers Cwhere exists (select 0 from ORDERS O where O.Charged = 1) and C.Name like 'A%' I don't want to use a JOIN clause. I just want to know if there is a difference of efficiency when the condition on C.Name is inside or outside the sub query.Thank you in advance for any advices,regards,mathmax |
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LoztInSpace
Aged Yak Warrior
940 Posts |
Posted - 2008-06-15 : 20:07:38
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| Have you tried looking at the query plans? They look like they would end up the same to me. I suspect this is not actually doing what you intend because there is no correlation between the sub query and the outer one. |
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mathmax
Yak Posting Veteran
95 Posts |
Posted - 2008-06-15 : 23:13:05
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| Sorry, I forgot the condition c.ID = o.Customer_Id in both queries.But don't you think the second query will be fastest ? In the first one, will the condition C.Name like 'A%' be evaluated for each record in Orders as it is in the sub query ? |
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subqueries performances