CODEWITHSUNDEEP
sqlpostgresqljoin
Sam
Sam

Reputation: 133

SQL: Select records where ALL joined records satisfy some condition

How can I write an SQL query that returns a record only if ALL of the associated records in a joined table satisfy some condition.

For example, if A has many B, I want to SELECT * FROM A WHERE all related B's for a given A have B.some_val > value

I know this is probably a pretty basic question, so thanks for any help. Also, if it makes a difference, I'm using postgres.

Sam

Upvotes: 11

Views: 6760

Answers (6)

OMG Ponies
OMG Ponies

Reputation: 332531

Assuming no need for correlation, use:

SELECT a.*
  FROM A a
 WHERE EXISTS(SELECT NULL
                FROM B b
              HAVING MIN(b.some_val) > a.val)

If you do need correlation:

SELECT a.*
  FROM A a
 WHERE EXISTS(SELECT NULL
                FROM B b
               WHERE b.id = a.id
              HAVING MIN(b.some_val) > a.val)

Explanation

The EXISTS evaluates on a boolean, based on the first match - this makes it faster than say using IN, and -- unlike using a JOIN -- will not duplicate rows. The SELECT portion doesn't matter - you can change it to EXISTS SELECT 1/0 ... and the query will still work though there's an obvious division by zero error.

The subquery within the EXISTS uses the aggregate function MIN to get the smallest B.some_val - if that value is larger than the a.val value, the a.val is smaller than all of the b values. The only need for a WHERE clause is for correlation - aggregate functions can only be used in the HAVING clause.

Upvotes: 8

Ryan Liang
Ryan Liang

Reputation: 668

Use bookstores and books as an example.

Bookstore

bookstoreID, bookID

Book

bookID, price

I suppose you want to return all the bookstores in which all the books have a price greater than X.

select *
from Bookstore bs1
where bs1.bookstoreID not exist
 (
  select bs.bookstoreID
  from Bookstore bs, Book b
  where bs.bookID= b.bookID
  b.price < x;   -- your value                                                     
 )

Upvotes: -1

Jason
Jason

Reputation: 1

select * from a where a.key = b.a_key where b.value > condition

Upvotes: -1

Jordan S. Jones
Jordan S. Jones

Reputation: 13883

You are wanting an INNER JOIN:

SELECT
    A.*
FROM
    A
INNER JOIN B
    ON A.identifier = B.identifier
WHERE
    B.some_val > value

You will want to ensure that there is a foreign key from A to B, or some other common identifier.

Upvotes: -1

Adam Musch
Adam Musch

Reputation: 13583

select * from A
 where -- at least one record in B is greater than some_val
       exists (select null from B
                where B.some_val > :value
                  and A.join_column = B.join_column)
   and -- no records in B are not greater than some_val
       not exists (select null from B
                    where B.some_val <= :value
                      and A.join_column = B.join_column)

Upvotes: 4

sgriffinusa
sgriffinusa

Reputation: 4221

The following should work:

SELECT *
FROM a
JOIN b ON a.key = b.key AND a.value > b.value

Because this does an inner join and not a outer join, records from A will only be included if they have records in B that satisfy the condition.

I don't use PostGRE, so I can't guarantee that the syntax is exactly correct.

Upvotes: -1

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