Multiple LEFT JOINs - what is the "left" table?

Question

I've been using this for years, so it is high time to understand it fully. Suppose a query like this:

SELECT 
  *
FROM a
LEFT JOIN b ON foo...
LEFT JOIN c ON bar...

The documentation tells us that

T1 { [INNER] | { LEFT | RIGHT | FULL } [OUTER] } JOIN T2 ON boolean_expression

LEFT OUTER JOIN

First, an inner join is performed. Then, for each row in T1 that does not satisfy the join condition with any row in T2, a joined row is added with null values in columns of T2. Thus, the joined table always has at least one row for each row in T1.

The question is simple: what is T1 in this case? Is it a? Or is it a LEFT JOIN b ON foo? (or, is it the same?)

Answer 1

A FROM clause parses the conditions from left to right (unless overridden by parentheses). So:

FROM a 
LEFT JOIN b
     ON foo... 
LEFT JOIN c
     ON bar...

is parsed as:

FROM (
        a 
        LEFT JOIN b
           ON foo...
     ) 
LEFT JOIN c
     ON bar...

This is explained in the documentation under the join-type section of the FROM clause:

Use parentheses if necessary to determine the order of nesting. In the absence of parentheses, JOINs nest left-to-right. In any case JOIN binds more tightly than the commas separating FROM-list items.

As a consequence, a series of LEFT JOINs keeps all records in the first mentioned table. This is a convenience.

Note that the parsing of the FROM clause is the same regardless of the join type.