Question

I was wondering about the difference :

select * 
from Table1 T1 
left join Table1 T2 on T1.id = T2.id + 1 and (T2.id > 3) 

vs

select * 
from Table1 T1 
left join Table1 T2 on T1.id = T2.id + 1 
where (T2.id > 3)

Answer 1

There is a significant difference.

select * from Table1 T1 left join  Table1 T2 on T1.id=T2.id+1     where    (T2>3 

The where clause is changing the left join to an inner join, since it is not allowing for the null value to be returned from the left join. This means all the rows where the left join doesnt find a matching record would be excluded, since the null value returned is compared to 3 and it discards the row (this is the same effect as making it an inner join)

The first statement is applying the filter within the join:

select * from Table1 T1 left join  Table1 T2 on T1.id=T2.id+1     and     (T2>3)  

This means it will take effect and filter the rows that can be joined to using the left join, but this will not cause rows to be discarded when the left join fails to find a matching row.