过滤别名列

Question

我在 MS-Access 中有以下查询，运行良好：

SELECT A1.LRN, A1.StartDate, A1.Destination, 
(
    SELECT TOP 1 A2.StartDate
    FROM testEnrolment As A2
    WHERE A2.LRN = A1.LRN AND A2.StartDate > A1.StartDate
    ORDER BY A2.StartDate
) As NextStartDate, 
(
    SELECT TOP 1 B2.Destination
    FROM testEnrolment As B2
    WHERE B2.LRN = A1.LRN AND B2.StartDate > A1.StartDate
    ORDER BY B2.StartDate
) As NextDestination
FROM testEnrolment As A1

NextStartDate、NextDestination 这两个别名列从当前LRN 的下一条记录的StartDate 和Destination 字段中获取数据。

所以如果表 testEnrolment 有这个数据：

LRN    StartDate    Destination
--------------------------------
L0001  01/08/2014   Unemployed
L0001  02/08/2014   Education
L0001  03/08/2014   Unemployed
L0002  20/09/2014   Education
L0002  21/09/2014   

查询将导致：

LRN    StartDate   Destination  NextStartDate  NextDestination
--------------------------------------------------------------
L0001  01/08/2014  Unemployed   02/08/2014     Education
L0001  02/08/2014  Education    03/08/2014     Unemployed
L0001  03/08/2014  Unemployed
L0002  20/09/2014  Education    21/09/2014
L0002  21/09/2014

接下来我要做的是通过不等于“教育”的记录过滤列别名NextDestination。

WHERE 子句不适用于列别名，我似乎也无法让 HAVING 工作。

Answer 1

将您的 sql 包装到子查询中，以便您可以过滤别名

SELECT * FROM (
SELECT A1.LRN, A1.StartDate, A1.Destination, 
(
    SELECT TOP 1 A2.StartDate
    FROM testEnrolment As A2
    WHERE A2.LRN = A1.LRN AND A2.StartDate > A1.StartDate
    ORDER BY A2.StartDate
) As NextStartDate, 
(
    SELECT TOP 1 B2.Destination
    FROM testEnrolment As B2
    WHERE B2.LRN = A1.LRN AND B2.StartDate > A1.StartDate
    ORDER BY B2.StartDate
) As NextDestination
FROM testEnrolment As A1
) AS s
WHERE NextDestination <> 'Education'