thank you for your reply.

but if delete records "0006,5" and "0006,56",

how can select "0006,20"

It's means

if 

values(0006,5)

values(0006,20)

values(0006,21)

values(0006,22)

then

select values(0006,5)

values(0006,20)

if

values(0006,20)

values(0006,21)

values(0006,22)

then

select values(0006,20)