在以下情况下使用最近在sql中定义的别名

Question

我正在尝试在查询时清理sql表中的产品列。我的（假示例）查询如下所示：

select
Id
, Name
, ProductName
, CASE
   WHEN ProductName IN ('macbook', 'dell', 'air', 'hp') THEN 'Laptop'
   WHEN ProductName IN ('ipod', 'walkman', 'headset') THEN 'Music_Device'
   WHEN ProductName IN ('mop', 'broom', 'sponge') THEN 'Household Utilities'
   WHEN ProductName IN ('bike', 'bike_2', 'bike_3') THEN 'Bicycle'
   WHEN ProductName IN ('tesla', 'ford', 'prius') THEN 'Car'
   ELSE null
   END AS Prod_Group
, CASE
   WHEN Prod_Group IN ('Laptop', 'Music_Device') THEN 'Electronics'
   WHEN Prod_Group IN ('Bicycle', 'Car') THEN 'Transportation'
   WHEN Prod_Group IN ('Household Utilities') THEN 'Utilities'
   ELSE null
   END AS Line_of_Business
from prod_database

有没有办法引用别名列？在声明并将Laptop改为Electronics等时，我是否必须重复我的整个案例？我来自R并学习SQL，所以这对我来说有点新鲜。

Answer 1

在Sql Server中，我们可以使用Cross Apply

SELECT Id,
       NAME,
       ProductName,
       cs.Prod_Group,
       CASE
         WHEN cs.Prod_Group IN ( 'Laptop', 'Music_Device' ) THEN 'Electronics'
         WHEN cs.Prod_Group IN ( 'Bicycle', 'Car' ) THEN 'Transportation'
         WHEN cs.Prod_Group IN ( 'Household Utilities' ) THEN 'Utilities'
         ELSE NULL
       END AS Line_of_Business
FROM   prod_database
       CROSS apply (SELECT CASE
                             WHEN ProductName IN ( 'macbook', 'dell', 'air', 'hp' ) THEN 'Laptop'
                             WHEN ProductName IN ( 'ipod', 'walkman', 'headset' ) THEN 'Music_Device'
                             WHEN ProductName IN ( 'mop', 'broom', 'sponge' ) THEN 'Household Utilities'
                             WHEN ProductName IN ( 'bike', 'bike_2', 'bike_3' ) THEN 'Bicycle'
                             WHEN ProductName IN ( 'tesla', 'ford', 'prius' ) THEN 'Car'
                             ELSE NULL
                           END) cs (Prod_Group) 

Answer 2

您可以参考我想的CTE ......

    WITH CTE
    AS
        (
          SELECT CASE WHEN 1>0 THEN 'A' END AS derivedCol
        )
    SELECT CASE WHEN derivedCol < 'B' THEN 'Pos1' ELSE 'Other' END
    FROM CTE

Answer 3

--could use a real table here
declare @prodTypes as table
  (ProductName varchar(50),
  ProductGroup varchar(50),
  Line_of_Business varchar(50));

insert into @prodTypes
values ('macbook', 'Laptop', 'Electronics'),
('dell', 'Laptop', 'Electronics'),
('hp', 'Laptop', 'Electronics'),
('ipod', 'Music_Device', 'Electronics')
-- etc.


-- the results you want:
select
  p.Id
, p.Name
, p.ProductName
, pt.ProductGroup
, pt.Line_of_Business
from prod_database p
left join @prodTypes pt
on p.ProductName = pt.ProductName


-- to double check your settings you can do:
select Line_of_business, ProductGroup, ProductName
from @prodTypes 
order by Line_of_business, ProductGroup, ProductName