CREATE TABLE Users( id SERIAL PRIMARY KEY,name TEXT UNIQUE );
以下操作的正确单一查询插入是什么:
给定用户名,插入新记录并返回新ID.但如果名称已存在,则只返回id.
我知道Postgresql 9.5中的新语法对于ON CONFLICT(列)DO UPDATE / nothing,但我无法弄清楚如果有的话,它可以提供帮助,因为我需要返回id.
似乎RETURNING id和ON CONFLICT不属于一起.
解决方法
这是手册中解释您情况的关键声明:
The Syntax of the
RETURNING
list is identical to that of the output
list ofSELECT
. Only rows that were successfully inserted or updated
will be returned. For example,if a row was locked but not updated
because anON CONFLICT DO UPDATE ... WHERE
clause condition was not
satisfied,the row will not be returned.
大胆强调我的.
要插入一行:
WITH ins AS ( INSERT INTO users(name) VALUES ('new_usr_name') -- input value ON CONFLICT(name) DO UPDATE SET name = name WHERE FALSE -- never executed,just to lock row RETURNING users.id ) SELECT id FROM ins UNION ALL SELECT id FROM users -- 2nd SELECT never executed if INSERT successful WHERE name = 'new_usr_name' -- input value a 2nd time LIMIT 1;
或者包装成一个函数,只提供一次新名称.就像在这里演示的那样(也考虑LIMIT 1的解释):
> Is SELECT or INSERT in a function prone to race conditions?
可能的竞争:并发事务可能会更改/删除INSERT尝试和SELECT之间的现有行.极不可能,但可能.
如果您没有(可能的)并发写访问权限(或者只是不关心),请简化:
... ON CONFLICT(name) DO nothing ...
要插入一组行:
> How to include excluded rows in RETURNING from INSERT … ON CONFLICT
原文地址:https://www.jb51.cc/mssql/78512.html
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