SQL Server在子查询中重用别名

这听起来像是一个愚蠢的问题-抱歉，我是SQL Server新手，我只想确认我的理解。 我有一个查询，它以不同的方式将表中的值聚合为不同列的子查询，例如，对于给定日期的事务、前一个月的事务、前6个月的事务、之前的事务、之后的事务

我将主表别名为

tx

，然后子查询别名为

tx1

，因此可以使用以下示例：

tx1.TransactionDate < tx.TransactionDate

---

我认为下面的脚本可以解释一切

SELECT 1,1,GETDATE()
INSERT INTO @t ( Id, UserId, TranDate )
SELECT 2,1,GETDATE()
INSERT INTO @t ( Id, UserId, TranDate )
SELECT 3,1,GETDATE()



SELECT tx.Id/*main alias*/,
       tx1.Id /*First subquery alias*/,
       tx2.Id /*Second subquery alias*/,
       (SELECT Id FROM @t txs /*alias only in this one subquery/must be different from main if you want use main alias in it...*/ 
        WHERE txs.Id = tx.Id+2 /*here is used main value = subquery value+2*/) AS Id
FROM @t tx /*main*/
JOIN (SELECT * 
      FROM @t tx 
      WHERE tx.Id =  1 /*this one using subquery values + you are not able to use here main value*/ 
      ) tx1 --alias of subquery
      ON tx.Id = tx1.Id /*here is used main value = subquery value*/
CROSS APPLY (SELECT TOP 1 * 
             FROM @t txc /*This one must be different from main if you want use it to comparison with main*/
             WHERE txc.Id > tx.Id /*this one using subquery value > main value*/ 
             ) tx2 --alias of subquery
WHERE tx.Id = 1 AND /*Subquery alias canot reference on First subquery value*/
      tx1.Id = 1 AND/*Subquery alias*/
      tx2.Id = 2 /*Subquery alias*/

---

这意味着是的，它可以被重用，但只有当您不想比较main/sub时，因为如果您重用它，例如，您尝试在subquery

tx.Id>tx.Id

中执行以下语句，则只会比较子查询中的值。在我们的示例中，这会导致您得不到任何结果，因为您将值放在同一行中…

问题中的Post代码。我已经编辑了你的问题。以下是一篇相关文章，可以帮助您：

SELECT 1,1,GETDATE()
INSERT INTO @t ( Id, UserId, TranDate )
SELECT 2,1,GETDATE()
INSERT INTO @t ( Id, UserId, TranDate )
SELECT 3,1,GETDATE()



SELECT tx.Id/*main alias*/,
       tx1.Id /*First subquery alias*/,
       tx2.Id /*Second subquery alias*/,
       (SELECT Id FROM @t txs /*alias only in this one subquery/must be different from main if you want use main alias in it...*/ 
        WHERE txs.Id = tx.Id+2 /*here is used main value = subquery value+2*/) AS Id
FROM @t tx /*main*/
JOIN (SELECT * 
      FROM @t tx 
      WHERE tx.Id =  1 /*this one using subquery values + you are not able to use here main value*/ 
      ) tx1 --alias of subquery
      ON tx.Id = tx1.Id /*here is used main value = subquery value*/
CROSS APPLY (SELECT TOP 1 * 
             FROM @t txc /*This one must be different from main if you want use it to comparison with main*/
             WHERE txc.Id > tx.Id /*this one using subquery value > main value*/ 
             ) tx2 --alias of subquery
WHERE tx.Id = 1 AND /*Subquery alias canot reference on First subquery value*/
      tx1.Id = 1 AND/*Subquery alias*/
      tx2.Id = 2 /*Subquery alias*/