MS SQL:嵌套选择 - 神秘的"无效列名"错误

Question

当我对MSSQL 2000运行以下查询时

SELECT 
    DISTINCT(Email),
    (SELECT TOP 1 ActivityID
        FROM Activity aa, ActivityType tt 
        WHERE aa.ActivityTypeId = tt.ActivityTypeId 
            AND aa.ConsumerID = c.ConsumerID
            AND tt.ActivityType = 'Something_OptIn') optin,
    (SELECT TOP 1 ActivityID
        FROM Activity aa, ActivityType tt 
        WHERE aa.ActivityTypeId = tt.ActivityTypeId 
            AND aa.ConsumerID = c.ConsumerID
            AND tt.ActivityType = 'Something_OptOut') optout
FROM
    Activity a,
    Consumer c,
    ActivityType t
WHERE
    c.CountryID = '23'
    AND t.ActivityType = 'Something_Create'
    AND a.ActivityTypeId = t.ActivityTypeId
    AND c.ConsumerID = a.ConsumerID
    AND optin > 1

我收到以下错误

Server: Msg 207, Level 16, State 3, Line 1
Invalid column name 'optin'.

为什么会这样？我不明白为什么它会无效.

Answer 1

SQL Server不允许您在同一级别按名称引用别名.要解决此问题,请重复列定义:

WHERE
    c.CountryID = '23'
    AND t.ActivityType = 'Something_Create'
    AND a.ActivityTypeId = t.ActivityTypeId
    AND c.ConsumerID = a.ConsumerID
    AND (SELECT TOP 1 ActivityID
        FROM Activity aa, ActivityType tt 
        WHERE aa.ActivityTypeId = tt.ActivityTypeId 
            AND aa.ConsumerID = c.ConsumerID
            AND tt.ActivityType = 'Something_OptIn'
        ) > 1

或者使用子查询:

SELECT  *
FROM    (
        SELECT 
            DISTINCT(Email),
            (...) optin,
            (...) optout
        FROM
            Activity a,
            Consumer c,
            ActivityType t
        ) as SubqueryAlias
WHERE
    c.CountryID = '23'
    AND t.ActivityType = 'Something_Create'
    AND a.ActivityTypeId = t.ActivityTypeId
    AND c.ConsumerID = a.ConsumerID
    AND optin > 1