sql - 如何在 SQL Server 的“选择”语句中获取表名

Question

我不得不或多或少地做出这样的UNION表态：

select [table_name], name, address
from Employees
where [my_condition]

UNION

select [table_name], name, address
from Employees_history
where [my_condition]

检索到的数据将在Employees 或Employees_history 中，但不在两个表中。

我需要知道数据来自哪个表。

Answer 1

SELECT 'Employees' AS [table_name],
       name,
       address
FROM   Employees
WHERE  [my_condition]
UNION ALL
SELECT 'Employees_history' AS [table_name],
       name,
       address
FROM   Employees_history
WHERE  [my_condition] 

我使用UNION ALL而不是UNION因为两个分支之间没有重复。因此它可以避免一些不必要的工作，删除整个结果集中的重复项。

如果分支中可能存在重复，请添加DISTINCT到SELECT个人

Answer 2

您可以附加一个新字段，如下所示：

select [table_name], name, address, 'Employees'
from Employees
where [my_condition]

UNION

select [table_name], name, address, 'History'
from Employees_history
where [my_condition]

您也可以使用aliasMartin 在他的回答中显示的那样。

Answer 3

你不能做这样的事情：

select 'Employees' as table_name, name, address
from Employees
where [my_condition]

UNION

select 'Employees_history' as table_name, name, address
from Employees_history
where [my_condition]

Answer 4

可以使用表别名来实现

SELECT 'Employees' AS [table_name],
       name,
       address
FROM   Employees
WHERE  [my_condition]

UNION

SELECT 'History' AS [table_name],
       name,
       address
FROM   Employees_history
WHERE  [my_condition]