Question

I’ve seen a query using a LEFT JOIN as opposed to an INNER or LEFT OUTER.

What is a LEFT JOIN exactly?

Answer 1

Where an inner join returns only entries that match in both tables, a left join takes all the entries from first table and any that match in the second table. A right join is the reverse of a left join (ie: all from the second table)

So if TableA is

A B
1 a
2 b
3 c

and TableB is

A B
1 d
2 e

Then Select * from TableA inner join TableB on TableA.A = TableB.A returns

1 a 1 d
2 b 2 e

And Select * from TableA left join TableB on TableA.A = TableB.A returns

1 a 1 d
2 b 2 e
3 c null null