MogDB/openGauss中merge的语法解析

MogDB/openGauss 中 merge 的语法解析

近期了解学习了 MogDB/openGauss 中 merge 的使用，merge 语法是根据源表对目标表进行匹配查询，匹配成功时更新，不成功时插入。简单来说就是有则更新，无则插入，语句简洁，效率高。

下面展示 MogDB/openGauss 中 merge 的语法

openGauss=# \h merge

Command: MERGE

Description: insert, update, or delete rows of a table based upon source data

Syntax:

MERGE [/*+ plan_hint */] INTO table_name [ [ AS ] alias ]

USING { { table_name | view_name } | subquery } [ [ AS ] alias ]

ON ( condition )

[

WHEN MATCHED THEN

UPDATE SET { column_name = { expression | DEFAULT } |

( column_name [, ...] ) = ( { expression | DEFAULT } [, ...] ) } [, ...]

[ WHERE condition ]

]

[

WHEN NOT MATCHED THEN

INSERT { DEFAULT VALUES |

[ ( column_name [, ...] ) ] VALUES ( { expression | DEFAULT } [, ...] ) [, ...] [ WHERE condition ] }

];

创建测试表

merge 有几种匹配条件可以交叉选择。 作用: 判断源表和目标表是否满足合并的条件 如果满足

用源表去更新目标表

用源表去删除目标表

什么也不干

如果不满足

用源表去插入目标表

什么也不干

创建出满足的表

create table a_merge (

id int not null,

name varchar not null,

year int

);

create table b_merge (

id int not null,

aid int not null,

name varchar not null,

year int,

city varchar

);

create table c_merge (

id int not null,

name varchar not null,

city varchar not null

);

测试一：匹配则修改，无则插入

--插入数据

insert into a_merge values(1,'liuwei',20);

insert into a_merge values(2,'zhangbin',21);

insert into a_merge values(3,'fuguo',20);

insert into b_merge values(1,2,'zhangbin',30,'吉林');

insert into b_merge values(2,4,'yihe',33,'黑龙江');

insert into b_merge (id,aid,name,city) values(3,3,'fuguo','山东');

--数据对比

select * from a_merge; select * from b_merge;

id | name | year

----+----------+------

1 | liuwei | 20

2 | zhangbin | 21

3 | fuguo | 20

(3 rows)

id | aid | name | year | city

----+-----+----------+------+--------

1 | 2 | zhangbin | 30 | 吉林

2 | 4 | yihe | 33 | 黑龙江

3 | 3 | fuguo | | 山东

(3 rows)

--merge语句

merge into a_merge a

using (select b.aid,b.name,b.year from b_merge b) c on (a.id=c.aid)

when matched then

update set year=c.year

when not matched then

insert values(c.aid,c.name,c.year);

--更新后的a_merge表

select * from a_merge;

id | name | year

----+----------+------

1 | liuwei | 20

2 | zhangbin | 30

3 | fuguo |

4 | yihe | 33

(4 rows)

测试二：匹配则修改，无则不操作

--插入数据

insert into b_merge values(4,1,'liuwei',80,'江西');

insert into b_merge values(5,5,'tiantian',23,'河南');

--核对数据

select * from a_merge;select * from b_merge;

id | name | year

----+----------+------

1 | liuwei | 20

2 | zhangbin | 30

3 | fuguo |

4 | yihe | 33

(4 rows)

id | aid | name | year | city

----+-----+----------+------+--------

1 | 2 | zhangbin | 30 | 吉林

2 | 4 | yihe | 33 | 黑龙江

3 | 3 | fuguo | | 山东

4 | 1 | liuwei | 80 | 江西

5 | 5 | tiantian | 23 | 河南

(5 rows)

--merge语句

merge into a_merge a

using (select b.aid,b.name,b.year from b_merge b) c on (a.id=c.aid)

when matched then

update set year=c.year;

--数据对比

select * from a_merge;

id | name | year

----+----------+------

1 | liuwei | 80

2 | zhangbin | 30

3 | fuguo |

4 | yihe | 33

(4 rows)

测试三：匹配无操作，不匹配进行 insert

--修改测试数据

update b_merge set year=70 where aid=2;

--两表对比

select * from a_merge;select * from b_merge;

id | name | year

----+----------+------

1 | liuwei | 80

2 | zhangbin | 30

3 | fuguo |

4 | yihe | 33

(4 rows)

id | aid | name | year | city

----+-----+----------+------+--------

2 | 4 | yihe | 33 | 黑龙江

3 | 3 | fuguo | | 山东

4 | 1 | liuwei | 80 | 江西

5 | 5 | tiantian | 23 | 河南

1 | 2 | zhangbin | 70 | 吉林

(5 rows)

--merge语句

merge into a_merge a

using (select b.aid,b.name,b.year from b_merge b) c on (a.id=c.aid)

when not matched then

insert values(c.aid,c.name,c.year);

--查看a_merge表

select * from a_merge;

id | name | year

----+----------+------

1 | liuwei | 80

2 | zhangbin | 30

3 | fuguo |

4 | yihe | 33

5 | tiantian | 23

(5 rows)

测试四：一律 insert

--merge语句

merge into c_merge c

using (select b.aid,b.name,b.city from b_merge b) b on (1=0)

when not matched then

insert values(b.aid,b.name,b.city);

--查看两表，条数相同

select * from c_merge ;select * from b_merge ;

id | name | city

----+----------+--------

3 | fuguo | 山东

5 | tiantian | 河南

2 | zhangbin | 吉林

4 | yihe++ | 黑龙江

1 | liuwei++ | 江西

6 | ningqin | 江西

7 | bing | 吉安

(7 rows)

id | aid | name | year | city

----+-----+----------+------+--------

3 | 3 | fuguo | | 山东

5 | 5 | tiantian | 23 | 河南

1 | 2 | zhangbin | 70 | 吉林

2 | 4 | yihe++ | 33 | 黑龙江

4 | 1 | liuwei++ | 80 | 江西

6 | 6 | ningqin | 23 | 江西

7 | 7 | bing | 24 | 吉安

(7 rows)