Sql Multiple Join Syntax

Question

So I saw some code that does this...

select *
from A 
left join B
left Join C
on C.column1 = B.column3
on B.column2 = A.column4
and C.column5 > 0
and B.column6 < 0

is this equivalent to

select *
from A 
left join B

on B.column2 = A.column4
and B.column6  < 0

left Join C
on C.column1 = B.column
and C.column5 > 0

Why would anyone ever do that first snippet? was it maybe a typo?

Answer 1

MySQL has a looser syntax for joins than the ANSI standard. In fact, there is a whole section of the documentation devoted just to this topic.

The first query is indeed acceptable syntax, as shown in this SQL Fiddle. This query (same query, different formatting):

select *
from A left join 
     B left Join
     C
     on C.column1 = B.column3
     on B.column2 = A.column4 and C.column5 > 0 and B.column6 < 0

Is interpreted as if there were parentheses:

select *
from A left join 
     (B left Join
      C
      on C.column1 = B.column3
     )
     on B.column2 = A.column4 and C.column5 > 0 and B.column6 < 0;

You can perhaps convince yourself of this by swapping the on clauses. This query:

select *
from A left join 
     B left Join
     C
     on B.column2 = A.column4 and C.column5 > 0 and B.column6 < 0
     on C.column1 = B.column3;

produces an error. The A table reference is not known for the first on clause, because that join is just between B and C.

I'm not sure why someone would do it. It could be that the parentheses were dropped in some "clean-up" phase. It could simply be that when writing the query, this syntax made sense and the person never noticed how awkward the query is (as a note: I've never done such a thing ;). It could be intentional obfuscation.