Merge Into 语法支持

KINGBASE 兼容Oracle 语法，实现了merge into 的功能。以下以例子的形式，介绍merge into语法的使用。以下例子在V8R6 ，且 database_mode=oracle 环境下验证过，database_mode=pg 不支持merge into 语法。

一、创建测试数据

create table source_table(s_id integer,s_name char(9));
create table target_table(t_id integer,t_name char(9));

insert into source_table values(1,'s_a'),(2,'s_b');
insert into target_table values(1,'t_a'),(3,'t_c');

test=# select * from source_table ;
s_id | s_name
------+-----------
1 | s_a
2 | s_b
(2 rows)

test=# select * from target_table ;
t_id | t_name
------+-----------
1 | t_a
3 | t_c
(2 rows)

二、测试例子

1、例子1

test=# begin;
BEGIN
test=# merge into target_table as t
test-# using source_table as s
test-# on (t.t_id=s.s_id)
test-# when matched then update set t.t_name=s.s_name
test-# when not matched then insert values(s.s_id,s.s_name);
MERGE 2

test=# select * from target_table order by t_id;
t_id | t_name
------+-----------
1 | s_a
2 | s_b
3 | t_c
(3 rows)

test=# rollback;
ROLLBACK

注意：更新的列不能是ON 条件中被引用的列。

2、例子2

更新时可以通过WHERE 条件指明要更新的行，条件中既可以包含源表的列，也可以包含目标表的列，当指明WHERE 条件且条件为假时，则不更新。

test=# begin;
BEGIN
test=# merge into target_table as t
test-# using source_table as s
test-# on (t.t_id=s.s_id)
test-# when matched then update set t.t_name=s.s_name where t.t_id=3
test-# when not matched then insert values(s.s_id,s.s_name);
MERGE 1
test=# 

test=# select * from target_table order by t_id;
 t_id |  t_name
------+-----------
    1 | t_a        --没有被更新
    2 | s_b
    3 | t_c
(3 rows)

test=# rollback;
ROLLBACK

3、例子3

test=# begin;
BEGIN
test=# merge into target_table as t
test-# using source_table as s
test-# on (t.t_id=s.s_id)
test-# when matched then update set t.t_name=s.s_name
test-#    delete where t.t_name='s_a'
test-# when not matched then insert values(s.s_id,s.s_name);
MERGE 3
test=# select * from target_table order by t_id;
 t_id |  t_name
------+-----------
    2 | s_b
    3 | t_c
(2 rows)

test=# rollback;
ROLLBACK

DELETE 子句只删除目标表和源表的ON 条件为真、并且是更新后的符合删除条件的记录，DELETE 子句不影响INSERT 项插入的行

三、Postgresql 实现类 merge into 的方法

test=# begin;
BEGIN
test=# with upsert as (
test(#     update target_table
test(#     set t_name = source_table.s_name
test(#     from source_table
test(#     where target_table.t_id = source_table.s_id
test(#     returning target_table.*
test(# )
test-# insert into target_table select * from source_table
test-# where not exists (
test(#     select 1
test(#     from upsert b
test(#     where source_table.s_id = b.t_id
test(# );
INSERT 0 1
test=# select * from target_table order by t_id;
 t_id |  t_name
------+-----------
    1 | s_a
    2 | s_b
    3 | t_c
(3 rows)

test=# rollback;
ROLLBACK