PotgresSQL插入选择案例问题

Question

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

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？

Answer 1

union all可以提供帮助：

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';

Answer 2

这需要使用您最喜欢的编程语言（TM）或database server-side procedural language在数据库客户端上的过程代码。普通SQL是说明性语言，没有高级conditionals。 SQL CASE不足以完成任务。

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; 
$definition$;