oracle merge从9i到10g的增强

MERGE语句是Oracle9i新增的语法，用来合并UPDATE和INSERT语句。连接条件匹配上的进行UPDATE，无法匹配的执行INSERT。这个语法仅需要一次全表扫描就完成了全部工作，执行效率要高于INSERT＋UPDATE。

下面看个具体例子：

hr@ORCL> select * from p;        ID NAME---------- ----------         1 d         3 e         8 fhr@ORCL> select * from t;        ID NAME---------- ----------         1 a         2 b         3 chr@ORCL> merge into t using p  2         on (t.id=p.id)  3       when matched then  4     update set  5       t.name=p.name  6       when not matched then  7     insert values  8       (p.id,p.name)  9  ;3 rows merged.hr@ORCL> select * from t;        ID NAME---------- ----------         1 d         2 b         3 e         8 fhr@ORCL> commit;Commit complete.

下面稍微修改一下例子：

hr@ORCL> insert into p values(1,'h');1 row created.hr@ORCL> commit;Commit complete.hr@ORCL> merge into t using p  2         on (t.id=p.id)  3       when matched then  4     update set  5       t.name=p.name  6       when not matched then  7     insert values  8       (p.id,p.name)  9  ; merge into t using p                    *ERROR at line 1:ORA-30926: unable to get a stable set of rows in the source tables

这个错误是使用MERGE最常见的错误，造成这个错误的原因是由于通过连接条件得到的源表的记录不唯一。在merge into时需要设定一个key值，会根据这个key值来决定merge into的操作（update还是insert into），所以要求在merge时这个key值是唯一的。所以要先从源表中选出全部数据而且key值是唯一的。

还有一个错误也是比较常见的：

hr@ORCL> create table mm (id number,name varchar2(10));Table created.hr@ORCL> create table mn (id number,name varchar2(10));Table created.hr@ORCL> insert into mm values(1,'A');1 row created.hr@ORCL> insert into mn values(1,'B');1 row created.hr@ORCL> merge into mn using mm  2         on (mn.id=mm.id)  3       when matched then  4     update set mn.id=mm.id  5       when not matched then  6     insert values  7      (mm.id,mm.name);       on (mn.id=mm.id)           *ERROR at line 2:ORA-38104: Columns referenced in the ON Clause cannot be updated: "MN"."ID"

10g增强一：where子句

下面看一个小例子：

hr@ORCL> select * from p;        ID NAME---------- ----------         1 d         2 f         7 ghr@ORCL> select * from t;        ID NAME---------- ----------         1 a         2 b         3 chr@ORCL> merge into t using p  2         on (p.id=t.id)  3      when matched then  4        update set  5          t.name=p.name  6        where p.id=2;1 row merged.hr@ORCL> select * from t;        ID NAME---------- ----------         1 a         2 f         3 c

10g增强二：delete子句

hr@ORCL> select * from p;        ID NAME---------- ----------         1 d         2 f         7 ghr@ORCL> select * from t;        ID NAME---------- ----------         1 a         2 f         3 chr@ORCL> merge into t using p  2         on (p.id=t.id)  3       when matched then  4     update set       5         t.name=p.name  6     delete where  7        (t.id=2);2 rows merged.hr@ORCL> select * from t;        ID NAME---------- ----------         1 d         3 c