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m1k3ch3ck
Starting Member
7 Posts |
 Posted - 2012-06-06 : 08:48:09
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hey guys,i have been into sql for a while, but havent used it in maybe one or two years.nevertheless i do not understand the following simple introduction example: |
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nigelrivett
Master Smack Fu Yak Hacker
3385 Posts |
Posted - 2012-06-06 : 09:01:24
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| It's a correlated subquery. It runs for each row in t1So - take a row in t1, that has a value for year.count all the rows from t2 where year is less than that year, add 1 to it (in this case could be <= without the +).The resultset is each year in t1 plus the above calculation for each of those rows.==========================================Cursors are useful if you don't know sql.SSIS can be used in a similar way.Beer is not cold and it isn't fizzy. |
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m1k3ch3ck
Starting Member
7 Posts |
Posted - 2012-06-06 : 11:14:55
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| k. got that.already understood that its a subquery, and also that its correlated.didnt (and kinda still dont) get the sence why someone would run a query like this, even though i do now understand the mechanic behind it and what it does. |
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nigelrivett
Master Smack Fu Yak Hacker
3385 Posts |
Posted - 2012-06-06 : 11:21:51
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| It's quite common.Find how many entries there are prior to the end of each year.Probably more sensible to aggregate per year then sum the prior years but if there's not much data this is simpler.==========================================Cursors are useful if you don't know sql.SSIS can be used in a similar way.Beer is not cold and it isn't fizzy. |
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Select Count(*) Where T2.year > T1.year