create table A(id int);
create table B(id int);
INSERT INTO A VALUES(1);
INSERT INTO A VALUES(2);
INSERT INTO A VALUES(3);
INSERT INTO B VALUES(1);
INSERT INTO B VALUES(2);
INSERT INTO B VALUES(3);
SELECT * FROM A;
SELECT * FROM B;
id
-----------
1
2
3
id
-----------
1
2
3
Filter on the JOIN to prevent rows from being added during the JOIN process.
select a.*,b.*
from A a left join B b
on a.id =b.id and a.id=2;
id id
----------- -----------
1 NULL
2 2
3 NULL
WHERE will filter after the JOIN has occurred.
select a.*,b.*
from A a left join B b
on a.id =b.id
where a.id=2;
id id
----------- -----------
2 2
