Editing Select (SQL) (section)

== Query evaluation ANSI ==
The processing of a SELECT statement according to ANSI SQL would be the following:<ref>Inside Microsoft SQL Server 2005: T-SQL Querying by Itzik Ben-Gan, Lubor Kollar, and Dejan Sarka</ref>

{{ordered list
|1=<syntaxhighlight lang="postgresql">
select g.*
from users u inner join groups g on g.Userid = u.Userid
where u.LastName = 'Smith'
and u.FirstName = 'John'
</syntaxhighlight>

|2= the FROM clause is evaluated, a cross join or Cartesian product is produced for the first two tables in the FROM clause resulting in a virtual table as Vtable1
|3= the ON clause is evaluated for vtable1; only records which meet the join condition g.Userid = u.Userid are inserted into Vtable2
|4= If an outer join is specified, records which were dropped from vTable2 are added into VTable 3, for instance if the above query were:
<syntaxhighlight lang="postgresql">
select u.*
from users u left join groups g on g.Userid = u.Userid
where u.LastName = 'Smith'
and u.FirstName = 'John' </syntaxhighlight>
all users who did not belong to any groups would be added back into Vtable3
|5= the WHERE clause is evaluated, in this case only group information for user John Smith would be added to vTable4
|6= the GROUP BY is evaluated; if the above query were:
<syntaxhighlight lang="postgresql">
select g.GroupName, count(g.*) as NumberOfMembers
from users u inner join groups g on g.Userid = u.Userid
group by GroupName
</syntaxhighlight>
vTable5 would consist of members returned from vTable4 arranged by the grouping, in this case the GroupName
|7= the HAVING clause is evaluated for groups for which the HAVING clause is true and inserted into vTable6. For example:
<syntaxhighlight lang="postgresql">
select g.GroupName, count(g.*) as NumberOfMembers
from users u inner join groups g on g.Userid = u.Userid
group by GroupName
having count(g.*) > 5
</syntaxhighlight>
|8= the SELECT list is evaluated and returned as Vtable 7
|9= the DISTINCT clause is evaluated; duplicate rows are removed and returned as Vtable 8
|10= the ORDER BY clause is evaluated, ordering the rows and returning VCursor9. This is a cursor and not a table because ANSI defines a cursor as an ordered set of rows (not relational).
}}