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

In your specific case, the t1.boolfield specifies an additional selection condition, not a condition for matching records between the two tables, so the second example is more correct.

If you're speaking about the cases when a condition for matching records is put in the ON clause vs. in the WHERE clause, see this question.

Answer 2

For INNER JOIN, there is no effective difference, although I think the second option is cleaner.

For LEFT JOIN, there is a huge difference. The ON clause specifies which records will be selected from the tables for comparison and the WHERE clause filters the results.

Example 1: returns all the rows from tbl 1 and matches them up with appropriate rows from tbl2 that have boolfield=1

Select *
From tbl1
  LEFT JOIN tbl2 on tbl1.id=tbl2.id and tbl2.boolfield=1

Example 2: will only include rows from tbl1 that have a matching row in tbl2 with boolfield=1. It joins the tables, and then filters out the rows that don't meet the condition.

Select *
From tbl1
  LEFT JOIN tbl2 on tbl1.id=tbl2.id
WHERE tbl2.boolfield=1

Answer 3

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.