SQL Server與CASE一起使用

Question

我們如何在WHERE子句中從CASE指定值的范圍？
我的此查詢失敗

declare @ProductType int

select * from Products 
where ProductLine in 
(
    case @ProductType
        when 1 then ('TVs')
        when 2 then ('Handsets', 'Mobiles') --fails here
        else ('Books')
    end
)

這也不起作用：

declare @ProductType int

select * from Products 
where (case @ProductType 
             when 1 then (ProductLine = 'TVs')
             when 2 then (ProductLine in  ('Handsets', 'Mobiles'))
             else (ProductLine = 'Books')
       end)

Answer 1

您無法做到這一點-您需要將其拆分為幾項檢查：

WHERE (ProductLine = 'TVs' AND @ProductType = 1)
   OR (ProductLine IN ('Handsets', 'Mobiles') AND @ProductType = 2)
   OR (ProductLine = 'Books' AND @ProductType NOT IN (1, 2))

Answer 2

您可以采用兩種方法。 我的第一個選擇是使用子查詢或通用表表達式來反轉邏輯並返回產品類型，然后匹配產品類型。 第二個是使用“ sp_executesql”。

第一種選擇：

declare @ProductType int

WITH cte (Product_key, ProductType) AS (
    SELECT Product_key, CASE WHEN ProductLine IN ('TVs') THEN 1
        WHEN ProductLine IN ('Handsets', 'Mobiles') THEN 2  
        ELSE 3 END FROM Products
)
select p.* from Products p, cte 
where p.product_key = cte.product_key AND cte.ProductType = @ProductType

第二種選擇：

declare @ProductType int, @sql NVARCHAR(MAX)

SET @sql = 'select * from Products 
    where ProductLine in (' +
        case @ProductType
            when 1 then '''TVs'''
            when 2 then '''Handsets'', ''Mobiles'''
            else '''Books'''
        end + ')'
EXEC sp_executesql @sql

Answer 3

declare @ProductType int

select * from Products 
where (case @ProductType 
             when 1 then ProductLine in ('TVs') 
             when 2 then ProductLine in  ('Handsets', 'Mobiles') 
             else ProductLine in ('Books') end)

Answer 4

CASE是一個返回值的表達式。 IN是可能是查詢一部分的子句。 而且SQL Server只勉強支持布爾數據類型。

您可以這樣組合它們：

declare @ProductType int = 1
declare @Products as Table ( ProductLine VarChar(16) )
insert into @Products ( ProductLine ) values ( 'TVs' ), ( 'Books' )
select *
  from @Products  
  where
    case @ProductType 
      when 1 then ( select 1 where ProductLine in ('TVs') )
      when 2 then ( select 1 where ProductLine in ('Handsets', 'Mobiles') )
      else ( select 1 where ProductLine in ('Books') )
      end is not NULL