Question

如何让它工作，它没有Where子句，否则使用Where子句，我得到明显的错误，但基本上需要做什么，谁知道如何处理这个？

select ID, 
       Name,
       case T.N 
         when 1 then City1
         when 2 then City2
         when 3 then City3
       end as City,
       case T.N 
         when 1 then State1
         when 2 then State2
         when 3 then State3
       end as State
from YourTable
  cross join (values(1),(2),(3)) as T(N)

    Where City is NOT Null

Answer 1

您不能在WHERE子句中使用别名。要么重复表达式（杂乱），要么将SELECT放在子查询中，然后将WHERE子句放在外部查询中：

SELECT Id, Name, City, State
FROM
(
     SELECT
         ID, 
         Name,
         CASE T.N 
             WHEN 1 THEN City1
             WHEN 2 THEN City2
             WHEN 3 THEN City3
         END AS City,
         CASE T.N 
             WHEN 1 THEN State1
             WHEN 2 THEN State2
             WHEN 3 THEN State3
         END AS State
     FROM YourTable
     CROSS JOIN (VALUES(1),(2),(3)) AS T(N)
) T1
WHERE City IS NOT NULL

Answer 2

您不能在SELECT子句中使用别名（来自WHERE子句），因为logical processing order（部分：Logical Processing Order of the SELECT statement）是WHERE然后SELECT：

FROM    
ON
JOIN
WHERE <--
GROUP BY
WITH CUBE or WITH ROLLUP
HAVING
SELECT <--
DISTINCT
ORDER BY <--
TOP

但是，您可以在ORDER BY中使用别名：

SELECT  h.SalesOrderID, YEAR(h.OrderDate) OrderYear
FROM    Sales.SalesOrderHeader h
ORDER BY OrderYear;

解决方案：请参阅Mark Byers提供的解决方案。

Tibor Karaszi: Why can't we have column alias in ORDER BY?

在Where子句或替代选项中使用别名？

2 个答案: