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Liat001
Starting Member
17 Posts |
 Posted - 2010-06-06 : 08:57:43
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| Hi all,What is the better preformance to use?Union or join, I need to get data from several tables, sometimes from all of them and sometimes not.What do you recommand? |
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webfred
Master Smack Fu Yak Hacker
8781 Posts |
Posted - 2010-06-06 : 10:15:41
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You can't compare union with join.Give us an example for what you want to do.
No, you're never too old to Yak'n'Roll if you're too young to die. |
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SwePeso
Patron Saint of Lost Yaks
30421 Posts |
Posted - 2010-06-06 : 10:59:29
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UNION adds data (more records, same number of columns) to your existing resultset.JOIN adds more columns (and in some cases, more records) to your existing resultset.UNION is vertical, JOIN is horizontal.A special case is CROSS JOIN which gives you both more records and more columns.
N 56°04'39.26"E 12°55'05.63" |
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Liat001
Starting Member
17 Posts |
Posted - 2010-06-07 : 02:33:10
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| select when case (charindex ('x',@A)>0)then a.AName else when case(charindex ('y',@A)>0) then b.Bname else ... from c join left b on c.id=b.idjoin left a on c.id=a.id...OR--I will create an empty table with the same columns befor union select a.name As name from a join left c on c.id=a.id where (charindex ('x',@A)>0) union select b.name As name from b join left c on c.id=b.id where (charindex ('y',@A)>0) ... |
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Union vs. Join