Question

These are the (example) tables:

first:  id      abc

parent: id  childid  foo
         1        0       -- no  child
         2        1       -- one child

child:  id  abc  bar
         1
         2

last:   id  foo  bar

I would like to execute the following query:

SELECT
  first.*
 ,parent.*
 ,child.*
 ,last.*

FROM        first  -- actually some joins

-- select exactly 1 `parent` row for each `first` row
-- with an optional `child` row (or NULLs if child.id=0)
LEFT JOIN ( parent
LEFT JOIN   child ON child.id = parent.childid
                 AND child.abc = first.abc <== ERROR HERE
          )       ON parent.childid = 0
                  OR child.id


-- conditions referring to the parent,child rows
LEFT JOIN   last  ON last.foo = parent.foo
                 AND last.bar = child.bar

Unfortunately MySQL doesn’t like external references in the nested join:
Unknown column 'first.abc' in 'on clause'

I would be happy if someone could help me on fixing this type of query.

Answer 1

1. Lose those brackets!
2. ON parent.childid = 0 OR child.id is invalid. Use ON parent.childid in (0, child.id)

It should look like this:

...
LEFT JOIN  parent  -- removed opening bracket
LEFT JOIN   child ON child.id = parent.childid
                 AND child.abc = first.abc 
                 AND parent.childid = in (0, child.Id) -- removed closing bracket and used 'IN'
...