Question

I’ve got one master table, which has items stored in multiple levels, parents and childs, and there is a second table which may or may not have additional data. I need to query two levels from my master table and have a left join on my second table, but because of the ordering within my query this will not work.

SELECT something FROM master as parent, master as child
  LEFT JOIN second as parentdata ON parent.secondary_id = parentdata.id
  LEFT JOIN second as childdata ON child.secondary_id = childdata.id
WHERE parent.id = child.parent_id AND parent.parent_id = 'rootID'

The left join only works with the last table in the from clause, so I am only able to make it work for one of the left joins. In the example above none of the left joins will work because the first left join points towards the first table in the from clause, the second one will never work like this.

How can I make this work?

Answer 1

This kind of query should work – after rewriting with explicit JOIN syntax:

SELECT something
FROM   master      parent
JOIN   master      child ON child.parent_id = parent.id
LEFT   JOIN second parentdata ON parentdata.id = parent.secondary_id
LEFT   JOIN second childdata ON childdata.id = child.secondary_id
WHERE  parent.parent_id = 'rootID';

The tripping wire here is that an explicit JOIN binds before a comma (,), which is otherwise equivalent to CROSS JOIN. The manual here:

In any case JOIN binds more tightly than the commas separating
FROM-list items.

After rewriting the first, all joins are applied left-to-right (logically – Postgres is free to rearrange tables in the query plan otherwise) and it works.

Just to make my point, this would work, too:

SELECT something
FROM   master parent
LEFT   JOIN second parentdata ON parentdata.id = parent.secondary_id
,      master child
LEFT   JOIN second childdata  ON childdata.id = child.secondary_id
WHERE  child.parent_id = parent.id
AND    parent.parent_id = 'rootID';

But explicit JOIN syntax is generally clearer.

And be aware that multiple (LEFT) JOIN can multiply rows:

• Two SQL LEFT JOINS produce incorrect result