PostgreSQL INSERT INTO SELECT CASE 问题

Posted 2023-03-27

【中文标题】PostgreSQL INSERT INTO SELECT CASE 问题

【英文标题】：PotgresSQL INSERT INTO SELECT CASE problem

【发布时间】：2020-04-25 11:06:44

【问题描述】：

我有以下查询，我想从子查询中插入任意数量的行。当然我得到一个错误SQL Error [21000]: ERROR: more than one row returned by a sub query used as an expression，因为一些子查询返回多行

insert into logs (_timestap, _message, _mode, _user_id)
select :_timestamp, :_message, :_mode,
case :_mode

    -- select all users that have the given grade. NOTE that this sub query returns multiple rows
    when 'byGrade' then (select u.id from users u where grade = :_grade)

    -- The user id value is already passed
    when 'byIndividual' then (select :_user_id limit 1)

    -- This sub query also returns multiple rows (all infact) from the users table
    when 'everyone' then (select id from users)
end

PostgreSQL 版本 10。

我该如何解决这个问题？

【参考方案1】：

union all可以帮忙：

insert into logs (_timestap, _message, _mode, _user_id)

select :_timestamp, :_message, :_mode, id
from users
where :_mode = 'byGrade' and grade = :_grade

union all

select :_timestamp, :_message, :_mode, :_user_id
where :_mode = 'byIndividual'

union all

select :_timestamp, :_message, :_mode, id
from users
where :_mode = 'everyone';

【参考方案2】：

这需要过程代码，可以在数据库客户端使用您最喜欢的编程语言 (TM) 或 database server-side procedural language。普通 SQL 是声明性语言，没有高级 conditionals。 SQL CASE 不足以完成这项工作。

在 Pl/PgSQL 中它看起来像：

CREATE FUNCTION do_the_thing(
  p_timestap timestamptz,
  p_message text,
  p_mode text,
  p_user_id integer)
RETURNS VOID
LANGUAGE plpgsql AS $definition$
BEGIN
if p_mode = 'byGrade' then
  insert into logs (_timestap, _message, _mode, _user_id)
  select p_timestamp, p_message, p_mode, u.id from users u where grade = p_grade;
elsif p_mode = 'byIndividual' then
  insert into logs (_timestap, _message, _mode, _user_id)
  select p_timestamp, p_message, p_mode, p_user_id;
elsif p_mode = 'everyone' then
  insert into logs (_timestap, _message, _mode, _user_id)
  select p_timestamp, p_message, p_mode, id from users;
end if;
END; 
$definition$;