如何合并具有不同列数的两个表？

Question

这是我的查询。

select id,sender_id,receiver_id,date_added,subject,book_id,message from trading_message where (receiver_id=1132 or receiver_id=66) and (sender_id=1132 or sender_id=66)
union
select id,sender_id,receiver_id,date_added,message,'A' as subject,'B' as book_id from personal_message where (receiver_id=1132 or receiver_id=66) and (sender_id=1132 or sender_id=66) order by date_added asc

我得到这样的错误。 “ where子句”中的未知列“ A”

请帮忙。

Answer 1

这应该适用于MYSQL 。

联合中的重要规则之一：字段数必须相等。

select id,sender_id,receiver_id,date_added,message, subject,book_id, from trading_message where (receiver_id=1132 or receiver_id=66) and (sender_id=1132 or sender_id=66)
union
select id,sender_id,receiver_id,date_added,message,'','' from personal_message where (receiver_id=1132 or receiver_id=66) and (sender_id=1132 or sender_id=66) order by date_added asc

注意：您在第二个子查询中不需要别名，引擎从您的第一个选择结果中获取字段名

Answer 2

我为MS Sql Server运行了此程序。 我认为问题之一是'B'作为book_id返回，我猜这是联合查询的第一部分中的int。 正如“ TheOneWhoPrograms”中提到的那样，列也必须顺序排列。

Create Table dbo.trading_message
(
 id int
 ,sender_id int
 ,receiver_id int
 ,date_added datetime
 ,subject varchar(100)
 ,book_id int
 ,message varchar(1000)
)

Create Table dbo.personal_message
(
 id int
 ,sender_id int
 ,receiver_id int
 ,date_added datetime
 ,message varchar(1000)
)

select 
 id
 ,sender_id
 ,receiver_id
 ,date_added
 ,subject
 ,book_id -- <- I *guessing* this is an int
 ,message 
from trading_message 
where (receiver_id=1132 or receiver_id=66) 
 and (sender_id=1132 or sender_id=66)

union

select 
 id
 ,sender_id
 ,receiver_id
 ,date_added
 ,'A' as subject
 ,0 as book_id -- <- Changed this from 'B' to zero
 ,message
from personal_message 
where (receiver_id=1132 or receiver_id=66) 
 and (sender_id=1132 or sender_id=66) 

order by date_added asc

上面的两个列（或更具体地说）的列数据类型具有相同的顺序

Answer 3

select id,sender_id,receiver_id,date_added,subject,book_id,message from trading_message where (receiver_id=1132 or receiver_id=66) and (sender_id=1132 or sender_id=66)
union
select id,sender_id,receiver_id,date_added,'A', 'B', null from personal_message where (receiver_id=1132 or receiver_id=66) and (sender_id=1132 or sender_id=66) order by date_added asc

试试看..我想我以正确的顺序匹配了您的列，每个列的数量都正确...

Answer 4

尝试请设置mysql变量而不是char'A'

@ var1 AS主题

代替

以“ A”为主题