Question

我有3张桌子＆＃34;维护＆＃34;，＆＃34;汽车＆＃34;，＆＃34;用户＆＃34; 。我想从表维护中选择所有数据，使用不同的car_id和每个不同的最后一条记录（基于max maintenance_date）

SELECT
     m. * , u.username, c.Model, c.Make, c.License, c.Milage, COUNT( m.process_id ) AS count_nr
    FROM
      maintenances AS m

    LEFT JOIN users AS u ON u.id = m.user_id
    LEFT JOIN cars AS c ON c.id = m.car_id

    WHERE
    maintenance_date = (SELECT MAX(maintenance_date) FROM maintenances WHERE car_id = m.car_id)

问题是此查询只返回一条记录，该记录具有所有记录的最大日期。我想要所有记录（不同的car_id和具有相同car_id的记录仅显示max（maintenance_date）的值）

Answer 1

添加GROUP BY u.username。

 WHERE
maintenance_date = (SELECT MAX(maintenance_date) FROM maintenances WHERE car_id = m.car_id)
 GROUP BY u.username

Answer 2

这是您的查询：

SELECT m. * , u.username, c.Model, c.Make, c.License, c.Milage, COUNT( m.process_id ) AS count_nr
----------------------------------------------------------------^
FROM maintenances AS m LEFT JOIN
     users AS u
     ON u.id = m.user_id LEFT JOIN
     cars AS c
     ON c.id = m.car_id
WHERE maintenance_date = (SELECT MAX(maintenance_date) FROM maintenances WHERE car_id = m.car_id);

这是一个聚合查询。如果没有group by，则只返回一行（所有行都在一个组中）。所以，添加group by：

SELECT m. * , u.username, c.Model, c.Make, c.License, c.Milage, COUNT( m.process_id ) AS count_nr
FROM maintenances AS m LEFT JOIN
     users AS u
     ON u.id = m.user_id LEFT JOIN
     cars AS c
     ON c.id = m.car_id
WHERE maintenance_date = (SELECT MAX(m2.maintenance_date) FROM maintenances m2 WHERE m2.car_id = m.car_id);
GROUP BY c.id

我还修复了关联语句，清楚它与外部查询相关。

sql选择distinct最大日期

2 个答案: