由于工作中使用，研究了merge into 语句是insert 与update语句的结合，可以同时实现update和insert的功能。
一、merge into语句的语法。
MERGE INTO schema. table alias
USING { schema. table | views | query} alias
ON {(condition) }
WHEN MATCHED THEN
UPDATE SET {clause}
WHEN NOT MATCHED THEN
INSERT VALUES {clause};
--解析
INTO 子句
用于指定你所update或者Insert目的表。
USING 子句
用于指定你要update或者Insert的记录的来源，它可能是一个表，视图，子查询。
ON Clause
用于目的表和源表（视图，子查询）的关联，如果匹配（或存在），则更新，否则插入。
merge_update_clause
用于写update语句
merge_insert_clause
用于写insert语句
二、merge 语句的各种用法练习
创建表及插入记录
create table t_B_info_aa
( id varchar2(),
name varchar2(),
type varchar2(),
price number
);
insert into t_B_info_aa values('','冰箱','家电',);
insert into t_B_info_aa values('','洗衣机','家电',);
insert into t_B_info_aa values('','热水器','家电',);
insert into t_B_info_aa values('','净水机','家电',);
insert into t_B_info_aa values('','燃气灶','家电',);
insert into t_B_info_aa values('','太阳能','家电',);
insert into t_B_info_aa values('','西红柿','食品',);
insert into t_B_info_aa values('','黄瓜','食品',3);
insert into t_B_info_aa values('','菠菜','食品',4);
insert into t_B_info_aa values('','香菇','食品',9);
insert into t_B_info_aa values('','韭菜','食品',2);
insert into t_B_info_aa values('','白菜','食品',);
insert into t_B_info_aa values('','芹菜','食品',);
create table t_B_info_bb
( id varchar2(),
type varchar2(),
price number
);
insert into t_B_info_bb values('','家电',);
insert into t_B_info_bb values('','家电',);
1) update和insert同时使用
merge into t_B_info_bb b
using t_B_info_aa a --如果是子查询要用括号括起来
on (a.id = b.id and a.type = b.type) --关联条件要用括号括起来
when matched then
update set b.price = a.price --update 后面直接跟set语句
when not matched then
insert (id, type, price) values (a.id, a.type, a.price) --insert 后面不加into
---这条语句根据t_B_info_aa 更新了t_B_info_bb中的一条记录，插入了条记录
2)只插入不更新
--处理表中数据，使表中的一条数据发生变化，删掉一部分数据。用来验证只插入不更新的功能
update t_B_info_bb b set b.price= where b.id='';
delete from t_B_info_bb b where b.type='食品';
--只是去掉了when matched then update语句
merge into t_B_info_bb b
using t_B_info_aa a
on (a.id = b.id and a.type = b.type)
when not matched then
insert (id, type, price) values (a.id, a.type, a.price)
3)只更新不插入
--处理表中数据，删掉一部分数据，用来验证只更新不插入
delete from t_B_info_bb b where b.type='食品';
--只更新不插入，只是去掉了when not matched then insert语句
merge into t_B_info_bb b
using t_B_info_aa a
on (a.id = b.id and a.type = b.type)
when matched then
update set b.price = a.price
三、加入限制条件的操作
变更表中数据，便于练习
update t_B_info_bb b
set b.price =
where b.id=''
delete from t_b_info_bb b where b.id not in ('','') and b.type='家电'
update t_B_info_bb b
set b.price = 8
where b.id=''
delete from t_b_info_bb b where b.id in ('','','') and b.type='食品'
表中数据
执行merge语句：脚本一和脚本二执行结果相同
脚本一： merge into t_B_info_bb b
using t_B_info_aa a
on (a.id = b.id and a.type = b.type)
when matched then
update set b.price = a.price where a.type = '家电'
when not matched then
insert
(id, type, price)
values
(a.id, a.type, a.price) where a.type = '家电';
----------------------
脚本二： merge into t_B_info_bb b
using (select a.id, a.type, a.price
from t_B_info_aa a
where a.type = '家电') a
on (a.id = b.id and a.type = b.type)
when matched then
update set b.price = a.price
when not matched then
insert (id, type, price) values (a.id, a.type, a.price);
结果：
上面两个语句是只对类型是家电的语句进行插入和更新
而脚本三是只对家电进行更新，其余的全部进行插入
merge into t_B_info_bb b
using t_B_info_aa a
on (a.id = b.id and a.type = b.type)
when matched then
update set b.price = a.price
where a.type = '家电'
when not matched then
insert
(id, type, price)
values
(a.id, a.type, a.price);
四、加删除操作
update子句后面可以跟delete子句来去掉一些不需要的行
delete只能和update配合，从而达到删除满足where条件的子句的记录
例句：
merge into t_B_info_bb b
using t_B_info_aa a
on (a.id=b.id and a.type = b.type)
when matched then
update set b.price=a.price
delete where (a.type='食品')
when not matched then
insert values (a.id, a.type, a.price) where a.id = ''
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