Question

What’s the difference between the following two SQLs:

select ...
from table1 t1
left join table2 t2 on t2.col1=t1.id and t2.col2='xxx'
where t1.col2='xxx'

---

select ...
from table1 t1
left join table2 t2 on t2.col1=t1.id
where t1.col2='xxx' and t2.col2='xxx'

I found when the same clause gets put at different positions the result could sometimes be different, even more the sql performance could vary dramatically. What’s the major differences if I put the extra clause right after a left join or to put it after ‘where’?

Can somebody offer an explanation? Thanks in advance.

Answer 1

Let’s rewrite SQLs:

select t1.id, t2.col1
from table1 t1
left join table2 t2 on t2.col1=t1.id and t2.col2='xxx'
where t1.col2='xxx'

---

select t1.id, t2.col1
from table1 t1
left join table2 t2 on t2.col1=t1.id
where t1.col2='xxx' and t2.col2='xxx'

Fill the table with data
t1 (id, col2):
1 xxx
2 xxx
3 jjj

t2 (col1, col2):
1 uuu
2 xxx
3 xxx

Result of first query:
1 NULL
2 2
Result of second query:
2 2

So, if you put any conditions in ‘join’, joining line may not be joined and result of join will be null.

See this link to understand