在两个表mysql上左连接时获取最后修改日期

Posted 2023-02-16

【中文标题】在两个表mysql上左连接时获取最后修改日期

【英文标题】：Get last modified date when left join on two tables mysql

【发布时间】：2021-09-08 15:26:15

【问题描述】：

我有主表 advertiser 并希望在 ads_id 上左连接两个表 LayoutVariables 和 LayoutMapping 并在这两个表中获取最后修改日期最终结果中的表格列。

下面的查询只给出一条记录，我想要 advertiser 表中的所有值，即使这两个表 LayoutVariables 和 LayoutMapping

SELECT  
    ADV.av_id,
    ADV.av_advertiser_name,
    ADV.av_is_active, 
    IF(AVLV.alv_advertiser_id IS NOT NULL, TRUE, FALSE) AS isBrandConfigDone,
    IF(AVLM.alm_advertiser_id IS NOT NULL, TRUE, FALSE) AS isLayoutMappingDone,
   ---here i want the max date between two tables
    IF(AVLM.alm_last_modified_on IS NOT NULL AND AVLV.alv_last_modified_on IS NOT NULL,IF(MAX(AVLM.alm_last_modified_on)>MAX(AVLV.alv_last_modified_on), "AVLM", "AVLV"),NULL) AS mm
    FROM DB.Advertiser AS ADV 
    LEFT JOIN DB.LayoutVariables AS AVLV ON ADV.av_id = AVLV.alv_advertiser_id
    LEFT JOIN DB.LayoutMapping AS AVLM ON ADV.av_id = AVLM.alm_advertiser_id 
    WHERE ADV.av_is_active=1;

表格布局变量

alv_id	alv_advertiser_id	alv_layout_variable_key	alv_layout_variable_value	alv_last_modified_on	alv_last_modified_by
11	8167	var4	var val 40	1624550014	2025
12	7690	amazon_usa	amazon_usa_vairaible	1624589928	2021

表格布局映射

alm_id	alm_advertiser_id	alm_layout_id	alm_last_modified_on	alm_last_modified_by
5	8167	334455	1624436972	2025
12	7690	123	1624320000	2021

餐桌广告商

av_id	av_advertiser_name	av_is_active
8167	TestAdvertiser	1
7690	amazon_usa	1

CREATE TABLE LayoutVariable (
 alv_id INT,alv_advertiser_id INT,alv_layout_variable_key VARCHAR(200),
 alv_layout_variable_value VARCHAR(200),alv_last_modified_on INT,alv_last_modified_by INT);

INSERT INTO LayoutVariable VALUES
 (11,8167,'var4','var val 40',1624550014,2025),
 (12,7690,'amazon_usa','amazon_usa_vairaible',1624589928,2021);

CREATE TABLE LayOutmapping (    
alm_id INT,alm_advertiser_id INT,alm_layout_id INT,
alm_last_modified_on INT,alm_last_modified_by INT);

INSERT INTO LayOutmapping VALUES
 (5  ,8167 ,334455 ,1624436972 ,2025)
 (12  ,7690 ,123 ,1624320000 ,2021);

CREATE TABLE Advertiser (
av_id INT,av_advertiser_name VARCHAR(200),av_is_active INT);

INSERT INTO Advertiser VALUES
(8167,'TestAdvertiser',1),
(7690,'amazon_usa',1);

有人可以在这里指导我吗

【问题讨论】：

在输出列表中使用相关子查询。

你能举一些例子来满足上述要求吗

为相关表格提供样本数据并显示所需结果。

@FaNo_FN 提供，请看

【参考方案1】：

你可以这样做：

SELECT
    ADV.av_id,
    ADV.av_advertiser_name,
    ADV.av_is_active, 
    AVLV.alv_advertiser_id ,
    AVLM.alm_advertiser_id ,
    IF(AVLV.alv_advertiser_id IS NOT NULL, TRUE, FALSE) AS isBrandConfigDone,
    IF(AVLM.alm_advertiser_id IS NOT NULL, TRUE, FALSE) AS isLayoutMappingDone,
    (SELECT MAX(lmd) FROM 
            (SELECT alm_last_modified_on lmd FROM LayoutMapping UNION
             SELECT alv_last_modified_on FROM LayoutVariables) v) AS mm
    FROM Advertiser AS ADV 
    LEFT JOIN LayoutVariables AS AVLV ON ADV.av_id = AVLV.alv_advertiser_id
    LEFT JOIN LayoutMapping AS AVLM ON ADV.av_id = AVLM.alm_advertiser_id 
;

或在SELECT 列表中使用相同的子查询，将其设为CROSS JOIN，例如：

SELECT
    ADV.av_id,
    ADV.av_advertiser_name,
    ADV.av_is_active, 
    AVLV.alv_advertiser_id ,
    AVLM.alm_advertiser_id ,
    IF(AVLV.alv_advertiser_id IS NOT NULL, TRUE, FALSE) AS isBrandConfigDone,
    IF(AVLM.alm_advertiser_id IS NOT NULL, TRUE, FALSE) AS isLayoutMappingDone,
    mm
    FROM Advertiser AS ADV 
    LEFT JOIN LayoutVariables AS AVLV ON ADV.av_id = AVLV.alv_advertiser_id
    LEFT JOIN LayoutMapping AS AVLM ON ADV.av_id = AVLM.alm_advertiser_id 
    CROSS JOIN (SELECT MAX(lmd) AS mm FROM 
            (SELECT alm_last_modified_on lmd FROM LayoutMapping UNION
             SELECT alv_last_modified_on FROM LayoutVariables) v) mx

您最初尝试在整个查询中使用MAX()，而上面的这些方法是与查询的其余部分分开的子查询。

Demo fiddle

【讨论】：

非常感谢您的回答，我已经运行了上面的查询，它为所有行提供 mm，这个值 1624589928 也适用于其他行

不，只匹配行 mm shd 来，不匹配它 shd 来 null

你的意思是这样？ dbfiddle.uk/…

是的，但是对于 8167，LayoutVariables 表中的这个值 1624550014 大于 LayoutMapping 中的值，并且在您的查询中，它将为 null，如果 av_id 没有值，则 null shd 出现

好的，那这个怎么样？ dbfiddle.uk/…