What is a LEFT JOIN in PostgreSQL

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

What is a LEFT JOIN exactly?

Answers


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  

It is the same as LEFT OUTER (The OUTER is implied because an INNER JOIN requires bilateral matching so a LEFT INNER JOIN would make no sense). The same applies for RIGHT JOIN and FULL JOIN these are equivalent to RIGHT OUTER JOIN and FULL OUTER JOIN respectively


Need Your Help

Format Date as "yyyy-MM-dd'T'HH:mm:ss.SSS'Z'"

javascript facebook date date-format parse-platform

I need to format a date as yyyy-MM-dd'T'HH:mm:ss.SSS'Z' as specified by Parse's REST API for Facebook. I was wondering what the most lightweight solution to this would be.