In which sequence are queries and sub-queries executed by the SQL Engine

in sequence queries , sub-queries executed sql engine?

which correct ?

a. primary query -> sub query -> sub sub query , on
b. sub sub query -> sub query -> prime query
c. whole query interpreted @ 1 time
d. there no fixed sequence of interpretation, query parser takes decision on fly

hi accessguru,

the query sequence depends on type of query use. post, confused b option, take nested sub query example. nested sub query can nested inside other sub queries, select statement nested within select statement , return intermediate results. sql executes innermost sub query first, , next level. results of sub query query conditions of primary query. in case, query sequence sub query-> primary query, option b right answer. more information nested query, please refer article: http://www.w3resource.com/sql/subqueries/nested-subqueries.php#sthash.o0wi3mau.dpuf.

if correlated sub query, sub query uses values outer query. sub query evaluated once each row processed outer query. means sub query executed repeatedly, once each row might selected outer query. in case, query sequence primary query -> sub query , option right answer. more information correlated sub query, please refer article: http://technet.microsoft.com/en-us/library/ms187638(v=sql.105).aspx

regards,

michelle li

SQL Server  >  Getting started with SQL Server