SQL 查詢中的並(Union)、交(Intersect)、差(Minus)

在資料庫的資料集操作中經常碰到大量的資料進行集合操作，這裡面有幾個比較重要的集合操作，對SQL的效能有較大的協助。下面就是這幾個動作陳述式的介紹。

UNION Statement

Combine data from one or more SELECT statements.

In Oracle 8i (and above) the UNION command has been largely replaced by the new Analytic Features

Syntax:

SELECT command  {UNION | UNION ALL | INTERSECT | MINUS} SELECT command

key:

UNION - Combine the unique rows returned by 2 SELECT statements

UNION ALL - Combine the rows returned by 2 SELECT statements (including all duplicates)

INTERSECT - Return only those rows that are in *both* SELECT statements

MINUS - Return the rows that are in the first SELECT but not the second

To combine more than two SELECTs simply nest the expressions

SELECT expr1 UNION (SELECT expr2 UNION SELECT expr3)