1.merge into 的語法
MERGE INTO table_name alias1
USING (table | view | sub_query) alias2
ON (join condition)
WHEN MATCHED THEN
UPDATE table_name SET col1 = col_val1, col2 = col2_val
WHEN NOT MATCHED THEN
INSERT (column_list) VALUES (column_values);
通過MERGE語句,根據一張表或子查詢的連接條件對另外一張表進行查詢
條件匹配的進行UPDATE,無法匹配的執行INSERT。
這個語法僅需要一次全表掃描就完成了全部工作,執行效率要高於INSERT+UPDATE。
舉例:
需求:判斷表中是否存在ID爲666的數據,如果存在更新名字爲Lily,如果不存在,插入一條Lily的數據。
merge into student a
using (select '666' as id from dual) s
on (a.id = s.id)
when matched then
update set a.name = 'Lily'
when not matched then
insert (id, name, class) values ('666', 'Lily', '1603');
需要注意的地方(一些坑):
1.如果using中的語句查詢不出來數據,是不會執行insert方法的,因爲這個語法是根據using 中的查詢數據進行判斷 例如這個語句是不會進行insert的
merge into student a
using (select id from student where id = '666') s
on (a.id = s.id )
when matched then
update set a.name = 'Lily'
when not matched then
insert (id, name, class) values ('666', 'Lily', '1603');
2. on 中的矯健要準確的過濾,不然可能會執行全表更新
如下這條語句會使得所有的student_name都改爲Lily
merge into student a
using (select count(1) count, id from student group by id ) s
on (a.id = s.id and count> 0)
when matched then
update set a.name = 'Lily'
when not matched then
insert (id, name, class) values ('666', 'Lily', '1603')
或者這種,同上
merge into student a
using (select id from student where id='666') s
on (1>0)
when matched then
update set a.name = 'Lily'
when not matched then
insert (id, name, class) values ('666', 'Lily', '1603')
3.on中的條件和update中set的字段不能是同一個字段,否則會報錯
下例中 on的條件和update中set的字段同爲id,會導致報錯
merge into student a
using (select '666' as id from dual) s
on (a.id = s.id)
when matched then
update set a.id = '666'
when not matched then
insert (id, name, class) values ('7', 'Lily', '1603');
4.using 中查詢出來的數據不能有重複,否則會報錯