[5.6] Add helpers for subquery join clauses

[staudenmeir]

This PR adds joinSub(), leftJoinSub() and rightJoinSub() to the query builder. The goal is to make subquery joins easier.

At the moment subquery joins are possible using raw SQL:

$query = DB::table('subtable');
$sql = '(' . $query->toSql() . ') as "sub"';
DB::table('table')->join(DB::raw($sql), ...);

That's ok for simple subqueries but gets a bit tedious if the subquery has bindings:

$query = DB::table('subtable')->where('foo', 'bar');
$sql = '(' . $query->toSql() . ') as "sub"';
DB::table('table')->addBinding($query->getBindings(), 'join')->join(DB::raw($sql), ...);

With the new helpers users can create subquery joins using raw SQL, a closure or a query builder:

DB::table('table')->joinSub('select * from "subtable"', 'sub', ...);
DB::table('table')->joinSub(function ($q) { $q->from('subtable'); }, 'sub', ...);
DB::table('table')->joinSub(DB::table('subtable')->where('foo', 'bar'), 'sub', ...);

[sisve]

There are no tests that verify that it works to joinSub several times in one query. Can you add that? Something like DB::table('table')->joinSub('...', 'sub1')->joinSub('...', 'sub2')?

[staudenmeir]

@sisve I added a test with multiple subquery joins.

[BrandonShar]

How do you feel about using joinSubQuery instead of joinSub? Feels like it's a lot easier to read/understand at a glance and is only 5 more characters. I think it's good to avoid abbreviations in code unless they have some clear benefit.

[staudenmeir]

@BrandonShar I chose the name to match the existing selectSub() and fromSub().

[BrandonShar]

ah, that makes sense; wasn't familiar with those.