Re: Nested queries V/S Joins

> What is more efficient nested subqueries or joins? Depends. >Query 1: > > select item from a, b > where a.item = b.item and> b.num = 50; > >Query 2: > > select item from a > where item in (select item from b where b.num = 50);> >If I "set explain on" on the above two queries, Query 1 has a higher >estimated cost than Query 2, but the execution times (reported by time) >are less for Query 1. What you will find if you put a lot of data into tables a and b is that Q2 will run faster and generally return more consistent response times. Q1 will degrade consistently over time. Depending on indexes, YMMV, etc. >What does the "estimated cost of a query" mean anyway? It's a pretty arbitrary indication of how long the query should take. In general, the bigger the number, the longer a query will take (allegedly), but there doesn't seem to be any way of translating the number into time. HTH. -- Cheers, Billy "Have you got it yet?" (c) Dazza, 1995