WHERE followed by ON clause - mysql

Question

What is the semantic difference between the following queries? To me both seemed similar until I executed them on presto. Is this something specific to presto or am I missing something in the SQL standard?
Form 1: All conditions specified in the ON clause.
SELECT
t1.colA,
t1.colB,
t1.colC,
t2.colD
FROM t1
LEFT OUTER JOIN t2
ON t1.colA = t2.colA
AND t1.colB = t2.colB
AND t1.colE = 1
AND t2.colF = 2;
Form 2: Some conditions specified in the WHERE clause instead.
SELECT
t1.colA,
t1.colB,
t1.colC,
t2.colD
FROM t1
LEFT OUTER JOIN t2
ON t1.colA = t2.colA
AND t1.colB = t2.colB
WHERE
t1.colE = 1
AND t2.colF = 2;
Form 1 results in some rows but form 2 doesn't but shouldn't they be equivalent?

Answer 1

There are two important differences.
First the condition t1.colE = 1. A LEFT JOIN keeps all rows in the first table, regardless of what the ON clause evaluates to. So, t1.colE does not change the number of rows in the result set. However, it does have the strange effect that any columns from t2 when this condition is not true are NULL.
Second the condition t2.colF = 2 has a different effect. This turns the LEFT JOIN into an INNER JOIN, because NULL values do not match the WHERE clause.

Answer 2

When you have a LEFT [OUTER] JOIN and WHERE clause that references the right_table, it is equivalent to an [INNER] JOIN. That is, the following two are equivalent:
-- [inner] join
select ...
from left_table
join right_table
on left_table.cola = right_table.cola
and right_table.colb = 2
-- left [outer] join + where
select ...
from left_table
left join right_table
on left_table.cola = right_table.cola
where right_table.colb = 2
Your 2nd example query has a left join and this condition t2.colF = 2 in the where clause, making it different from the first query.
where the the first includes all rows from t1, the second includes only rows where t1.colE = 1 and t2.colF = 2

Answer 3

The way to think about the difference between the WHERE clause and the ON clause is:
The WHERE clause determines which of the rows produced by the FROM clause should be considered in the query, so it's role is to filter rows before they get processed by a GROUP BY, WINDOW or the SELECT clause.
The ON clause determines whether a pair of rows from the left and right side of a JOIN should be considered for the join operation.
For an INNER JOIN, if the ON clause evaluates to false, the rows are not joined with each other and no result is emitted.
For a LEFT JOIN, if, for a given row on the left side the ON clause evaluates to false for all rows on the right side, a row containing the values from the left side and NULLs for the columns on the right side is emitted. The same logic applies for a RIGHT JOIN, but with the sides reversed.
A FULL JOIN is a combination of LEFT and RIGHT joins.
In your example, the second query, any row produced by the LEFT JOIN that contains NULLs in columns from the right side due to the ON clause evaluating to false will be filtered out by the t2.colF = 2 expression WHERE clause.