Query with join equivalency? - sql

Question

Are these two queries equivalent (assuming varying/any kinds of data in the table)? Are there any scenarios in which they would return different results?
Query 1:
select * from tablea a
left join tableb b on a.keyacol = b.keybcol
inner join tablec c on c.keyccol = b.keybcol;
Query 2:
select * from tablea a
left join (
select b.*, c.* from tableb b
inner join tablec c on c.keyccol = b.keybcol
) sub on a.keyacol = sub.keybcol;

Answer 1

No, they are not equivalent. Example:
CREATE TABLE a
( keya int ) ;
CREATE TABLE b
( keyb int ) ;
CREATE TABLE c
( keyc int ) ;
INSERT INTO a
VALUES
(1) ;
INSERT INTO b
VALUES
(1),(2) ;
INSERT INTO c
VALUES
(2) ;
Results:
SELECT *
FROM a
LEFT JOIN b
ON a.keya = b.keyb
INNER JOIN c
ON c.keyc = b.keyb ;
Result
----------------------
| keya | keyb | keyc |
----------------------
SELECT *
FROM a
LEFT JOIN
( SELECT b.*, c.*
FROM b
INNER JOIN c
ON c.keyc = b.keyb
) sub
ON a.keya = sub.keyb ;
Result
----------------------
| keya | keyb | keyc |
----------------------
| 1 | NULL | NULL |
----------------------
As to why this happens, a LEFT JOIN b INNER JOIN c is parsed as (a LEFT JOIN b) INNER JOIN c which is equivalent to (a INNER JOIN b) INNER JOIN c because the condition on the INNER join cancels the LEFT join.
You can also write the second query in this form - without subquery - which is parsed as a LEFT JOIN (b INNER JOIN c) because of the different placing of the ON clauses:
SELECT *
FROM a
LEFT JOIN
b
INNER JOIN c
ON c.keyc = b.keyb
ON a.keya = b.keyb ;
Result
----------------------
| keya | keyb | keyc |
----------------------
| 1 | NULL | NULL |
----------------------

Answer 2

They're not equivalent.
Essentially, there are four scenarios here, for records on A:
corresponding records on B and C exist;
corresponding records exist on B but not C;
corresponding records exist on C but not B;
corresponding records don't exist on B or C.
Both queries will return the same values for scenario 1.
However, they will return different values for the other scenarios - the inner join on a value of B to a value C in the first query means that you will be attempting to join a null to a value of C in the other scenarios.

Answer 3

The INNER JOIN keyword return rows when there is at least one match in
both tables/selections. If there are rows in tableb that do not have
matches in tablec, those rows will NOT be listed.
The LEFT JOIN keyword returns all the rows from the left table
(tablea), even if there are no matches in the right table (tableb or
sub selection).
Since the left join returns all rows, the two queries might differ if there aren't any correspondent matches in the inner join from the first query. Those rows won't be selected in the first query but will appear in the second since it's used a left join.
Another thing that migh differ was the columns. But since the select * from you will select all colums from all tables/selections and:
Query 1 returns all columns from tablea, tableb and tablec
Query 2 returns all columns from tablea and selection sub (returns all columns from tableb and tablec) = returns all columns from tablea, tableb and tablec
this isn't a problem.
So NO, they are equivalent.