Question 89595 in PostgreSQL Question posted in
The official documentation can be found here.

Null result from multiple left Join in postgresql

I have 3 tables in a database A, B and C which share the same column "name".
B has an attribute "title" and C has an attribute "age".

I am looking to write a SQL query where I will have to choose all the contents of A based on query input either B.title, C.age or all of them.

What I have tried so far,

SELECT * FROM A
LEFT JOIN B ON A.name = B.name
LEFT JOIN C ON A.name = C.name 
WHERE B.title = COALESCE($1, B.title)
AND C.age = COALESCE($2, C.age)

$1 and $2 are coming from an external program. Some examples would be $1 = "Jones" and $2 = 12.

I am getting a null result from the above query. Also Left Join seems to be very expensive. Are there faster ways to achieve this without joining the tables.

If $1 is null then the query will not have any join from B table. I would like to get back the names from A. If $2 is null then the query will not join C table and so on. If both are null it will return whatever A has.

Tags:

2

Answers


  1. 0

    If no B exists for a given name, the condition B.title = COALESCE($1, B.title) will never match, it evaluates to NULL regardless of the value of $1. Same for C and $2.

    If you want to ignore the condition if the parameter is NULL, you should write

    SELECT * FROM A
WHERE ($1 IS NULL OR (SELECT title FROM B WHERE B.name = A.name) = $1)
  AND ($2 IS NULL OR (SELECT age FROM C WHERE C.name = A.name) = $2)

    You can also try

    SELECT * FROM A
LEFT JOIN B USING (name)
LEFT JOIN C USING (name)
WHERE B.title IS NOT DISTINCT FROM COALESCE($1, B.title)
  AND C.age IS NOT DISTINCT FROM COALESCE($2, C.age)

    or (imo clearer to understand)

    SELECT * FROM A
LEFT JOIN B USING (name)
LEFT JOIN C USING (name)
WHERE ($1 IS NULL OR $1 = B.title)
  AND ($2 IS NULL OR $2 = C.age)

    but you should check the query plans for those, they seem harder to optimise.

    Login or Signup to reply.
  2. 0

    Move the conditions of the WHERE clause to their respective ON clauses:

    SELECT * 
FROM A
LEFT JOIN B ON B.name = A.name AND B.title = $1
LEFT JOIN C ON C.name = A.name AND C.age = $2;

    If $1 is null, the condition:

    B.title = $1

    will return null and the full condition:

    A.name = B.name AND B.title = $1

    will also be null, which will lead to a no-match for all the rows of B.

    The same applies to $2.

    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search