从联合查询中获取表名？

Question

这是我的查询

SELECT Id, productName, Largeimagepath, Discount, Price, Image FROM tablename1 where Active =1 
union 
SELECT Id, productName, Largeimagepath, Discount, Price, Image FROM tablename2 where Active =1
union  
SELECT Id, productName, Largeimagepath, Discount, Price, Image FROM tablename3 where Active =1

它工作正常。

现在我想productName在将来获取各自的表名。

1. 那么我怎样才能取到它呢？

我试过AS..像这样：

SELECT Id, productName, Largeimagepath, Discount, Price, Image
FROM tablename3 AS tablename
where Active = 1;

但没有得到输出。

2. 我应该如何更正查询并提高查询性能？

Answer 1

通常，在使用UNION并且您需要知道特定行来自哪个表时，您会使用存储在类似于以下列的硬编码值：

SELECT Id, productName, Largeimagepath, Discount, Price, Image, 'Tablename1' as Source
FROM tablename1 
where Active =1 
union 
SELECT Id, productName, Largeimagepath, Discount, Price, Image, 'Tablename2' as Source
FROM tablename2 
where Active =1
union  
SELECT Id, productName, Largeimagepath, Discount, Price, Image, 'Tablename3' as Source
FROM tablename3 
where Active =1;

这将返回一个Source带有标识符的新列，该列显示该行来自哪个表。

正如@ypercube 在评论中所建议的那样，您可能还想考虑将其更改为 a UNION ALL- 这将包括重复项，但您还将拥有它来自哪个表的标识符。使用 aUNION ALL将消除删除重复项对性能的影响。

SELECT Id, productName, Largeimagepath, Discount, Price, Image, 'Tablename1' as Source
FROM tablename1 
where Active =1 
union all
SELECT Id, productName, Largeimagepath, Discount, Price, Image, 'Tablename2' as Source
FROM tablename2 
where Active =1
union all
SELECT Id, productName, Largeimagepath, Discount, Price, Image, 'Tablename3' as Source
FROM tablename3 
where Active =1;