问题
Possible Duplicate:
Difference between CTE and SubQuery?
I was trying to understand how to use the WITH clause and the purpose of the WITH clause.
All I understood was, the WITH clause was a replacement for normal sub-queries.
Can anyone explain this to me with a small example in detail ?
回答1:
The SQL WITH clause was introduced by Oracle in the Oracle 9i release 2 database. The SQL WITH clause allows you to give a sub-query block a name (a process also called sub-query refactoring), which can be referenced in several places within the main SQL query. The name assigned to the sub-query is treated as though it was an inline view or table. The SQL WITH clause is basically a drop-in replacement to the normal sub-query.
Syntax For The SQL WITH Clause
The following is the syntax of the SQL WITH clause when using a single sub-query alias.
WITH <alias_name> AS (sql_subquery_statement)
SELECT column_list FROM <alias_name>[,table_name]
[WHERE <join_condition>]
When using multiple sub-query aliases, the syntax is as follows.
WITH <alias_name_A> AS (sql_subquery_statement),
<alias_name_B> AS(sql_subquery_statement_from_alias_name_A
or sql_subquery_statement )
SELECT <column_list>
FROM <alias_name_A>, <alias_name_B> [,table_names]
[WHERE <join_condition>]
In the syntax documentation above, the occurrences of alias_name is a meaningful name you would give to the sub-query after the AS clause. Each sub-query should be separated with a comma Example for WITH statement. The rest of the queries follow the standard formats for simple and complex SQL SELECT queries.
For more information: http://www.brighthub.com/internet/web-development/articles/91893.aspx
回答2:
This has been fully answered here.
See Oracle's docs on SELECT to see how subquery factoring works, and Mark's example:
WITH employee AS (SELECT * FROM Employees)
SELECT * FROM employee WHERE ID < 20
UNION ALL
SELECT * FROM employee WHERE Sex = 'M'
来源：https://stackoverflow.com/questions/12552288/sql-with-clause-example