Is there a logical difference between putting a condition in the ON clause of an inner join versus the where clause of the main query?

Question

Consider these two similar SQLs

(condition in ON clause)

select t1.field1, t2.field1
from
table1 t1 inner join table2 t2 on t1.id = t2.id and t1.bool         


        

Answer 1

Both versions return the same data.

Although this is true for an inner join, it is not true for outer joins.

Stylistically, there is a third possibility. In addition to your two, there is also:

select t1.field1, t2.field1
from (select t1.*
      from table1 t1
      where t1.boolfield = 1
     ) t1 inner join
     table2 t2
    on t1.id = t2.id

Which is preferable all depends on what you want to highlight, so you (or someone else) can later understand and modify the query. I often prefer the third version, because it emphasizes that the query is only using certain rows from the table -- the boolean condition is very close to where the table is specified.

In the other two cases, if you have a long query, it can be problematic to figure out what "t1" really means. I think this is why some people prefer to put the condition in the ON clause. Others prefer the WHERE clause.