Oracle遷移PostgreSQL系列文章之二：merge語句

Posted on 2015-03-06 11:12:46 by osdba

我們知道，Oracle中有一個特別的merge語句。而PostgreSQL中不直接支持這個語法，但PostgreSQL可以使用WITH Queries (Common Table Expressions)的方法實現相同的功能。Common Table Expression在PostgreSQL常縮寫為CTE。

假設oracle中有一張表：

CREATE TABLE test01(id number, note varchar2(32));

orale中的merge語句的SQL如下：

-- 第一條merge語句：

merge into test01 a

using (select 1 as id, 'xxxx' as note from dual) b on (a.id=b.id)

when matched then update set a.note=b.note

when not matched then insert (a.id, a.note) values (b.id, b.note);

-- 第二條merge語句

merge into test01 a

using (select 1 as id, 'yyyy' as note from dual) b on (a.id=b.id)

when matched then update set a.note=b.note

when not matched then insert (a.id, a.note) values (b.id, b.note);

轉換成PostgreSQL的語法如下：

CREATE TABL test01(id int, note varchar(32));

-- 第一條merge語句

WITH upsert as

(update test01 m set note='xxxx' where id= 1

RETURNING m.*

), data as (select 1 as id, 'xxxx' as note)

insert into test01 select * from data a where not exists(select 1 from upsert b where a.id=b.id);

-- 第二條merge語句

WITH upsert as

(update test01 m set note='yyyy' where id= 1

RETURNING m.*

), data as (select 1 as id, 'yyyy' as note)

insert into test01 select * from data a where not exists(select 1 from upsert b where a.id=b.id);

總結

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