tsql - How to improve performance for update statement? -

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suppose have lookup table mylookup(lkid, lkname, ...)

then have other 2 tables:

mytab(id, parentid, name, lookname, ...) yourtab(id, parentid, ...)  --id, parentid coming mytab

then have update try lkid , it's parent lkid, sql like:

 update yourtab set columnx =      case      when (select lkid mylookup join mytab b on a.lkname = b.lkname b.id = c.parentid ) > 3         , (select lkid mylookup join mytab b on a.lkname = b.lkname b.id = c.parentid )  >             (select lkid mylookup lkname = c.lkname )     1     else 0     end     yourtab c

this sql performance not good. (select lkid mylookup join mytab b on a.lkname = b.lkname b.id = c.parentid ) called 2 times each row in yourtb.

how rewrite sql improve performance case?

maybe this..

it's untested need sqlfiddle sample data understand inner workings.

update yourtab set columnx =  case when  not null (   select lkid    mylookup    inner join mytab b      on a.lkname = b.lkname    b.id = c.parentid      , a.lkid > 3      , a.lkname >  c.lkname) 1    else 0   end  yourtab c

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