INNER JOIN where **every** row must match the WHERE clause? - sql

Question

Here's a simplified example of what I'm trying to do. I have two tables, A and B.
A B
----- -----
id id
name a_id
value
I want to select only the rows from A where ALL the values of the rows from B match a where clause. Something like:
SELECT * from A INNER JOIN B on B.a_id = A.id WHERE B.value > 2
The problem with the above query is that if ANY row from B has a value > 2 I'll get the corresponding row from A, and I only want the row from A if
1.) ALL the rows in B for B.a_id = A.id match the WHERE, OR
2.) There are no rows in B that reference A
B is basically a table of filters.

Answer 1

SELECT *
FROM a
WHERE NOT EXISTS
(
SELECT NULL
FROM b
WHERE b.a_id = a.a_id
AND (b.value <= 2 OR b.value IS NULL)
)

Answer 2

This should solve your problem:
SELECT *
FROM a
WHERE NOT EXISTS (SELECT *
FROM b
WHERE b.a_id = a.id
AND b.value <= 2)
Here is the way in which this is obtained.
Suppose that we have available a universal quantifier (parallel to EXISTS, the existential quantifier), with a syntax like:
FORALL table WHERE condition1 : condition2
(to be read: FORALL the elements of table that satisfy the condition1, then condition2 is true)
So you could write your query in this way:
SELECT *
FROM a
WHERE FORALL b WHERE b.a_id = a.id : b.value > 2
(Note that forall is true even when no element in b exists with a value of a.id)
Then we can transform the universal quantifier in the existential one, with a double negation, as usual:
SELECT *
FROM a
WHERE NOT EXISTS b WHERE b.a_id = a.id : NOT (b.value > 2)
In plain SQL this can be written as:
SELECT *
FROM a
WHERE NOT EXISTS (SELECT *
FROM b
WHERE b.a_id = a.id
AND (b.value > 2) IS NOT TRUE)
This technique is very handy in case of universal quantification.

Answer 3

Answering this question (which it seems you actually meant to ask):
Return all rows from A, where all rows in B with B.a_id = A.id also pass the test B.value > 2.
Which is equivalent to:
Return all rows from A, where no row in B with B.a_id = A.id fails the test B.value > 2.
SELECT a.* -- "rows from A" (so don't include other columns)
FROM a
LEFT JOIN b ON b.a_id = a.id
AND (b.value > 2) IS NOT TRUE -- safe inversion of logic
WHERE b.a_id IS NULL;
When inverting a WHERE condition carefully consider NULL. IS NOT TRUE is the simple and safe way to perfectly invert a WHERE condition. The alternative would be (b.value <= 2 OR b.value IS NULL) which is longer but may be faster (easier to support with index).
Select rows which are not present in other table

Answer 4

Try this
SELECT * FROM A
LEFT JOIN B ON B.a_id = A.id
WHERE B.value > 2 OR B.a_id IS NULL

Answer 5

SELECT * FROM A LEFT JOIN B ON b.a_id = a.id
WHERE B.a_id IS NULL OR NOT EXIST (
SELECT 1
FROM b
WHERE b.value <= 2)

Answer 6

SELECT a.is, a.name, c.id as B_id, c.value from A
INNER JOIN (Select b.id, b.a_id, b.value from B WHERE B.value > 2) C
on C.a_id = A.id
Note it is a poor practice to use select *. You shoudl only specify fields you need. IN this case, I might possibly remove the b.Id refernces becasue they are probably not needed. If you have a join there is a 100% chance you are wasting resouces sending data you don't need becasue the join fields will be repeated. That is why I did nto include a_id in the final result set.

Answer 7

If you prefer not to use EXISTS, you can use an outer join.
SELECT A.*
FROM
A
LEFT JOIN B ON
B.a_id = A.id
AND B.value <= 2 -- note: condition reversed!!
WHERE B.id IS NULL
This works by searching for the existence of a failing record in B. If it finds one, then the join will match, and the final WHERE clause will exclude that record.