why there is a left join and right join in MySQl?

Question

I think either left/right join is sufficient, the other will derived if table names are interchanged, Anyone know why there is both left and right join? Thanks

Agree with the above comment, and read Gordon's answer which states that right join isn't really needed at all. — Tim Biegeleisen, Jun 06 '21 at 05:39

score 1 · Accepted Answer · answered Jun 06 '21 at 05:44

The order of join evaluation is generally left-to-right. So a join b join c executes as if you had parenthesized it (a join b) join c.

The optimizer can reorder inner joins because inner joins are commutative. Reordering outer joins is not commutative (i.e. a left join b is not the same as b left join a), so the optimizer can't swap them.

If you want to do a left outer join to the result of a joined pair of tables, use parentheses, like: a left join (b join c).

But you can also take advantage of the left-to-right order of evaluation by switching to a right outer join: b join c right outer join a -- without parentheses!

why there is a left join and right join in MySQl?

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