SQL 查询中的并(Union)、交(Intersect)、差(Minus)

在数据库的数据集操作中经常碰到大量的数据进行集合操作，这里面有几个比较重要的集合操作，对SQL的性能有较大的帮助。下面就是这几个操作语句的介绍。

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) 

最后

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