>Hi all !
>in MySQL table index key which one is the right way ?
>key (branch)
>key (reference)
>key (date)
>select * from transaction where branch=?m.branch and reference=?m.ref and date=?m.date order by branch,reference,date
>or
>key (branch,reference,date)
>select * from transaction where branch=?m.branch and reference=?m.ref and date=?m.date order by branch

It depends on your queries. If you always select based on all 3 keys, then the second variation is correct (you need to put the column which is most selective first). If you anticipate queries based on 1 key, then the first variation will be good.