巧用子查询和CTE引用HAVING子句中的另一列

2024-03-05 05:06:15

HAVING 子句中引用另一列的巧妙方法

简介

在查询数据时，HAVING 子句允许我们根据组聚数据进一步过滤结果。然而，如果需要引用另一列来进行过滤，就会遇到 "未知列..." 的错误。本文将探究使用子查询和公用表表达式 (CTE) 来解决此问题的巧妙方法。

问题

想象我们有一个表，其中包含从当前日期开始的过去 12 个月的开始和结束日期。我们想要查询一个包含每个患者最小就诊日期在指定时间段内的患者数量。

方法 1：子查询

SELECT COUNT(b.PatNum)
FROM
    (SELECT pl.PatNum
    FROM procedurelog pl 
    WHERE pl.ProcStatus=2
    GROUP BY pl.PatNum
    HAVING MIN(pl.ProcDate) BETWEEN (
        SELECT Start
        FROM (
            SELECT t.order,
                LAST_DAY(CURDATE() - INTERVAL (t.order+1) MONTH) + INTERVAL 1 DAY  AS 'Start',
                LAST_DAY(CURDATE() - INTERVAL t.order MONTH) AS 'End'
            FROM 
            (SELECT 0 as 'order' 
            UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6
            UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9 UNION ALL SELECT 10 UNION ALL SELECT 11 UNION ALL SELECT 12) t
        ) AS n
        WHERE n.order = 1
    )
) b

子查询 的方法利用子查询来创建临时表或视图，其中包含开始和结束日期。HAVING 子句随后引用该子查询的 Start 列。

方法 2：公用表表达式 (CTE)

WITH MonthRange AS (
    SELECT t.order,
        LAST_DAY(CURDATE() - INTERVAL (t.order+1) MONTH) + INTERVAL 1 DAY  AS 'Start',
        LAST_DAY(CURDATE() - INTERVAL t.order MONTH) AS 'End'
    FROM 
    (SELECT 0 as 'order' 
    UNION ALL SELECT 1 UNION ALL SELECT 2 UNION ALL SELECT 3 UNION ALL SELECT 4 UNION ALL SELECT 5 UNION ALL SELECT 6
    UNION ALL SELECT 7 UNION ALL SELECT 8 UNION ALL SELECT 9 UNION ALL SELECT 10 UNION ALL SELECT 11 UNION ALL SELECT 12) t
)

SELECT COUNT(b.PatNum)
FROM
    (SELECT pl.PatNum
    FROM procedurelog pl 
    WHERE pl.ProcStatus=2
    GROUP BY pl.PatNum
    HAVING MIN(pl.ProcDate) BETWEEN Start AND End
) b
JOIN MonthRange ON MonthRange.order = 1