[Oracle] 多表关联的update和delete
[Oracle] 多表关联的update和delete
由于Oracle不支持update或delete from语句,因此,Oracle的多表关联update和delete必须借助于子查询,同理,Oracle也不支持同时update或delete多张表,其典型用法如下:
多表关联update
首先,构造测试表和数据如下:
[sql] SYS@TEST16> create table testa as select owner,table_name,status from dba_tables; Table created. SYS@TEST16> create table testb as select owner,object_name,status from dba_objects; Table created.
1)更新testa表的status=‘VALID’,关联条件为
testa.owner=testb.owner and testa.table_name=testb.table_name [sql] update testa a set status='VALID' where exists (select 1 from testb b where a.owner=b.owner and a.table_name=b.object_name);
2)更新testa表的status等于testb表的status,关联条件同上
[sql] update testa a set a.status=(select b.status from testb b where a.owner=b.owner and a.table_name=b.object_name) where exists (select 1 from testb b where a.owner=b.owner and a.table_name=b.object_name);
这里要注意的是:只有当子查询每次只返回1条或0条数据时,上述语句才能执行成功,如果返回超过1条数据,将会出现如下错误:
[plain]
ORA-01427: single-row subquery returns more than one row
这时候,你就必须得对子查询里的返回条数进行限定,如rownum=1或distinct等方法。
这里的where exists字句是不可以省略的,否则将update全表!这里要千万小心。
如果查看执行计划,你会发现上述的update语句将扫描testb表两次,如果该表比较大的话,对性能会有影响,如果只想扫描一次的话,可以用如下方法代替:
[sql] update testa a set a.status=nvl((select b.status from testb b where a.owner=b.owner and a.table_name=b.object_name),a.status);
多表关联delete
1)利用in或not in删除数据 [sql] delete from testa where table_name in (select object_name from testb); 2)利用exists或not exists删除数据 [sql] delete from testb b where exists (select 1 from testa a where a.owner=b.owner and a.table_name=b.object_name)