Exist

Question

Answer 1

In Exists subquery count(*) always results to be true so that all the tuples of table T is displayed

Comments

How Exists subquery returns true ??

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if its just * in subquery it returns false but its count(*) because of this exists returns true

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but b does not has a 2 value..how come it is true then??

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ur correct @Akriti sood but the thing is mainly in count(*) if count(*) is present in subquery of exists irrespective of query i.e even b=2 tuple doesn't present but the count(*) evaluates the query is true so if exists subquery is true then all tuples of table are resulted

Answer 2

Here in the inner query we have EXISTS clause

Exists return true when it get value or the inner query return at least 1 record

Here Select count (*) From T where B =2 return a row 2 10  (COUNT =1 )

HENCE NO OF ROWS RETURNED = 1

Comments

B value in any row not 2.so,how inner query return row 2 10

I think inner query return 0.but after this what wil happen?

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so 0 (zero) is a returned value that means...the inner query has returned something.. EXISTS will treat it as TRUE as some value has been returned, and so the number of rows returned will be 3

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@namanom1 So if exists return NULL than only no row will be selected ryt??

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it wont return NULL...either TRUE or FALSE...if no data is found in the EXIST part then it will return a FALSE..