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HomeDatabaseMysql TutorialOracle及SqlServer的多表关联更新

一条Update更新语句是不能更新多张表的,除非使用触发器隐含更新;而表的更新操作中,在很多情况下需要在表达式中引用要更新的表

一条Update更新语句是不能更新多张表的,除非使用触发器隐含更新;而表的更新操作中,在很多情况下需要在表达式中引用要更新的表以外的数据。

一、MS SQL Server 多表关联更新

sql server提供了update的from 子句,可以将要更新的表与其它的数据源连接起来。虽然只能对一个表进行更新,但是通过将要更新的表与其它的数据源连接起来,就可以在update的表达式中引用要更新的表以外的其它数据。

一般形式:

update A SET 字段1=B表字段表达式, 字段2=B表字段表达式   from B WHERE    逻辑表达式

例如:

UPDATE dbo.Table2

SET dbo.Table2.ColB = dbo.Table2.ColB + dbo.Table1.ColB

FROM dbo.Table2

INNER JOIN dbo.Table1

ON (dbo.Table2.ColA = dbo.Table1.ColA);

实际更新的操作是在要更新的表上进行的,而不是在from子句所形成的新的结果集上进行的。

二、Oracle 多表关联更新

Oracle没有update from语法,可以通过两种实现方式:

1、利用子查询:

update    A

SET    字段1=(select    字段表达式    from    B    WHERE    ...),

字段2=(select    字段表达式    from    B    WHERE    ...)

WHERE    逻辑表达式

UPDATE多个字段两种写法:

写法一:

UPDATE table_1 a

SET col_x1 = (SELECT b.col_y1 FROM table_2 b WHERE b.col_n = a.col_m),

col_x2 = (SELECT b.col_y2 FROM table_2 b WHERE b.col_n = a.col_m)

WHERE EXISTS (SELECT 1 FROM table_2 b WHERE b.col_n = a.col_m)

UPDATE table_1 a

SET col_x1 = (SELECT b.col_y1 FROM table_2 b WHERE b.col_n = a.col_m),

col_x2 = (SELECT b.col_y2 FROM table_2 b WHERE b.col_n = a.col_m)

WHERE a.col_m=(SELECT b.col_n FROM table_2 b WHERE b.col_n = a.col_m)

写法二:

UPDATE table_1 a

SET (col_x1, col_x2) = (SELECT b.col_y1, b.col_y2 FROM table_2 b WHERE b.col_n = a.col_m)

WHERE EXISTS (SELECT 1 FROM table_2 b WHERE b.col_n = a.col_m);

UPDATE table_1 a

SET (col_x1, col_x2) = (SELECT b.col_y1, b.col_y2 FROM table_2 b WHERE b.col_n = a.col_m) WHERE a.col_m=(SELECT b.col_n FROM table_2 b WHERE b.col_n = a.col_m);

注意:

1. 对于子查询的值只能是一个唯一值,不能是多值。

2. 子查询在绝大多数情况下,,最后面的where EXISTS子句是重要的,否则将得到错误的结果;且where EXISTS子句可用另一方法代替,如上。最后的子句是对a表被更新记录的限制,如无此句,对于a表中某记录,如在b表中关联不到对应的记录,则该记录被更新字段将被更新为null;where EXISTS子句就是排除对a表中该情况的记录进行更新。

2、利用视图:

UPDATE (SELECT A.NAME ANAME,B.NAME BNAME FROM A,B WHERE A.ID=B.ID)

SET ANAME=BNAME;

对于视图更新的限制:

如果视图基于多个表的连接,那么用户更新(update)视图记录的能力将受到限制。除非update只涉及一个表且视图列中包含了被更新的表的整个主键,否则不能更新视图的基表。

三、Oracle 多表关联删除

Oracle中的Delete的from子句也没有多表联接的功能,只能通过子查询的方式来做:

DELETE FROM 表A WHERE EXISTS (SELECT 1 FROM 表B WHERE 表A.empid=表B.empid)

DELETE FROM 表A WHERE 表A.empid in (SELECT empid WHERE表B)

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