SQL order of operations [duplicate]

Answer 1

in SQL Server:

As a general rule of thumb, JOIN clauses are evaluated before WHERE clauses.

In case of complex joins that need filters in the join part, I write them along with my join

SELECT *
FROM A
LEFT JOIN B
    ON A.Foo1 = B.Foo1
    And A.Date = 'Yesterday'
OUTER JOIN C
    ON B.Foo2 = C.Foo2
JOIN D
    ON B.Foo3 = D.Foo3

Answer 2

Semantically: After the JOIN. But in this case, there is no difference in timing, because it's on the LEFT side of the JOIN.

As you already have it, "only rows in A from "Yesterday" are joined to B".

The optimizer is free to reorganize its order of operations depending on the equivalences in the relational algebra.

This returns only A.date="Yesterday" and joins B where it can find a match on foo:

SELECT * FROM A
LEFT JOIN B
    ON A.foo=B.foo
WHERE A.date="Yesterday"

This returns all A regardless of any criteria and joins B where A.date="Yesterday" AND it finds a match on foo:

SELECT * FROM A
LEFT JOIN B
    ON A.foo=B.foo
    AND A.date="Yesterday"

Answer 3

Depends on indexes and statistics.

You should show the execution path of the query to determine where (if any) optimizations should be applied.

Answer 4

It depends on the database.

On SQL Server, run: SET SHOWPLAN_ALL ON then run the query, you will get an idea of what happens when it runs.

Answer 5

SELECT * 
FROM (SELECT * FROM A WHERE Date = 'Yesterday') A
LEFT JOIN B 
    ON A.Foo1 = B.Foo1 
OUTER JOIN C 
    ON B.Foo2 = C.Foo2 
JOIN D 
    ON B.Foo3 = D.Foo3 

Answer 6

Your idea of "evaluation" is not correct as SQL is a declarative language.

BTW you can see the query execution plan. In MySQL prefix your query with keyword describe to see the execution plan.