T-SQL - 在 SELECT 语句期间设置变量答案

【问题标题】：T-SQL - Set variable during SELECT statementT-SQL - 在 SELECT 语句期间设置变量
【发布时间】：2020-11-28 06:09:36
【问题描述】：

我想在 SQL 查询中间设置一个变量。

我有多个“标志”，我希望查询在结果列中报告有关客户的所有真实标志。因此，我不能使用 CASE 语句，因为一旦找到第一个“真”语句，它就会停止评估。为了解决这个问题，我使用一组 if IF 语句将相关标志附加到变量，然后在 Result 列中报告该变量。但是，我对此无能为力。

下面的代码当然不会运行。如何重新组织此查询以使其符合犹太教规？

想要的结果是这样的：

代码如下：

DECLARE 
@CountOfOrders int = 0,
@resultstring varchar(max) = ''

SELECT 
Customers.CustomerName,
CountOfOrders.CountOfOrders,
@resultstring as Result

From 
--Get some basic data from the query's base table:
(Select 'James Bond' as CustomerName) Customers


--Get some extra data about James Bond's orders from a different table,
--Let's just say he's got 35 orders:
OUTER APPLY (
                SELECT CAST(35 AS INT) as CountOfOrders
            ) CountOfOrders


--Now do some number crunching:
Set @CountOfOrders = CountOfOrders.CountOfOrders
IF @CountOfOrders > 10 set @resultstring = @resultstring + 'James Bond has greater than 10 orders. '
IF @CountOfOrders > 20 set @resultstring = @resultstring + 'James Bond has greater than 20 orders. '
IF @CountOfOrders > 30 set @resultstring = @resultstring + 'James Bond has greater than 30 orders. '
IF @CountOfOrders > 40 set @resultstring = @resultstring + 'James Bond has greater than 40 orders. '
IF @CountOfOrders > 50 set @resultstring = @resultstring + 'James Bond has greater than 50 orders. '

【问题讨论】：

标签： sql sql-server string tsql count

【解决方案1】：

在一系列case 表达式之上使用concat_ws() 怎么样？

concat_ws(
    ' ',
    case when CountOfOrders > 10 
        then concat(CustomerName, ' has more than 10 orders.')
    end,
    case when CountOfOrders > 20 
        then concat(CustomerName, ' has more than 20 orders.')
    end,
    case when CountOfOrders > 30 
        then concat(CustomerName, ' has more than 30 orders.')
    end
) as result

您可以根据需要向 concat_ws() 添加更多 epwresssions。这个函数应该直接放在你的select 子句中（然后你就不需要这个变量了）。

【讨论】：

【解决方案2】：

您可以只使用CONCAT_WS()（或旧版本 SQL Server 中的类似构造：

select @resultstring = CONCAT_WS(' ',
                                 (CASE WHEN CountOfOrders > 10 THEN 'James Bond has greater than 10 orders.' END),
                                 (CASE WHEN CountOfOrders > 20 THEN 'James Bond has greater than 20 orders.' END),
                                 . . .
                                )
from . . .;

【讨论】：

【解决方案3】：

重新整理代码

DECLARE 
@CountOfOrders int = 0,
@resultstring varchar(max) = ''

--Get some extra data about James Bond's orders from a different table,
--Let's just say he's got 35 orders:

--Now do some number crunching:
Set @CountOfOrders = (
                SELECT CAST(35 AS INT) as CountOfOrders
            ) 
IF @CountOfOrders > 10 set @resultstring = @resultstring + 'James Bond has greater than 10 orders. '
IF @CountOfOrders > 20 set @resultstring = @resultstring + 'James Bond has greater than 20 orders. '
IF @CountOfOrders > 30 set @resultstring = @resultstring + 'James Bond has greater than 30 orders. '
IF @CountOfOrders > 40 set @resultstring = @resultstring + 'James Bond has greater than 40 orders. '
IF @CountOfOrders > 50 set @resultstring = @resultstring + 'James Bond has greater than 50 orders. ' 

SELECT 
Customers.CustomerName,
@CountOfOrders,
@resultstring as Result

From 
--Get some basic data from the query's base table:
(Select 'James Bond' as CustomerName) Customers

【讨论】：