Question

The following command selects all the items from ‘stock’ that appear in ‘items’ with the correct orderRef.

SELECT a.* FROM stock a LEFT JOIN items b ON a.id = b.stockId WHERE b.orderRef='orderRef'

This works, but I need all the items that aren’t listed in table ‘items’ with the correct orderRef.

I thought I should change ‘ON’ to its opposite, but ‘OFF’ didn’t work.

Answer 1

The best way to do a “not in” query is MySQL is the following:

select s.*
from stock s
where not exists (select 1 from items i where s.id = i.stockID and i.orderref = 'orderref' limit 1)

The way the MySQL optimizers works, the not exists performs best. This can be further enhanced by having an index on items.stockID.

Note that when you use a left outer join, you may inadvertently be multiplying the nubmer of rows, if there are duplicates in the second table.