In SQL / MySQL, what is the difference between “ON” and “WHERE” in a join statement?

Question

The following statements give the same result (one is using on, and the other using where):

mysql> select * from gifts INNER JOI         


        

Answer 1

When using INNER JOIN, ON and WHERE will have the same result. So,

select *
from Table1 t1
inner join Table2 t2 on t1.id = t2.id
where t1.Name = 'John'

will have the exact same output as

select *
from Table1 t1
inner join Table2 t2 on t1.id = t2.id
    and t1.Name = 'John'

As you have noted, this is not the case when using OUTER JOIN. What query plan gets built is dependent on the database platform as well as query specifics, and is subject to change, so making decisions on that basis alone is not going to give a guaranteed query plan.

As a rule of thumb, you should use columns that join your tables in ON clauses and columns that are used for filtering in WHERE clauses. This provides the best readability.