Oracle With 語法 示例

lishiran發表於2007-03-13
Oracle

以下相當於一句獨立的sql，可以在sql window 中獨立執行：
With
ab As (Select * From scott.dept),
ba As (Select * From scott.emp)
Select ab.*,ba.*
From ab,ba
Where ab.deptno = ba.deptno

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另外，遊標示例如下：
Begin
For iLoop In (
With
ab As (Select * From scott.dept),
ba As (Select * From scott.emp)
Select ab.*
From ab,ba
Where ab.deptno = ba.deptno
) Loop
dbms_output.put_line('>' || iLoop.Deptno || iLoop.Dname);
End Loop;
End;

Declare
Cursor cur_main Is
With
ab As (Select * From scott.dept),
ba As (Select * From scott.emp)
Select ab.*
From ab,ba
Where ab.deptno = ba.deptno;
Begin
For iLoop In cur_main Loop
dbms_output.put_line('>' || iLoop.Deptno || iLoop.Dname);
End Loop;
End;

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About Oracle WITH clause

Starting in Oracle9i release 2 we see an incorporation of the SQL-99 “WITH clause”, a tool for materializing subqueries to save Oracle from having to re-compute them multiple times.

The SQL “WITH clause” is very similar to the use of Global temporary tables (GTT), a technique that is often used to improve query speed for complex subqueries. Here are some important notes about the Oracle “WITH clause”:

• The SQL “WITH clause” only works on Oracle 9i release 2 and beyond.
• Formally, the “WITH clause” is called subquery factoring
• The SQL “WITH clause” is used when a subquery is executed multiple times
• Also useful for recursive queries (SQL-99, but not Oracle SQL)

To keep it simple, the following example only references the aggregations once, where the SQL “WITH clause” is normally used when an aggregation is referenced multiple times in a query.
We can also use the SQL-99 “WITH clause” instead of temporary tables. The Oracle SQL “WITH clause” will compute the aggregation once, give it a name, and allow us to reference it (maybe multiple times), later in the query.

The SQL-99 “WITH clause” is very confusing at first because the SQL statement does not begin with the word SELECT. Instead, we use the “WITH clause” to start our SQL query, defining the aggregations, which can then be named in the main query as if they were “real” tables

來自 “ ITPUB部落格 ” ，連結：http://blog.itpub.net/9099175/viewspace-904552/，如需轉載，請註明出處，否則將追究法律責任。

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