mysql – GROUP BY和自定义顺序

我已经阅读了MySQL order by before group by的答案,但是将它应用于我的查询最后会在子查询中找到一个子查询,这是一个相当简单的案例,所以我想知道这是否可以简化：

带有样本数据的模式

为简洁起见,我省略了成员表中的其他字段.此外,在实际应用程序中还有更多表加入,但这些表很容易加入.这是给我问题的membership_stack表.

CREATE TABLE members (
  id int unsigned auto_increment,
  first_name varchar(255) not null,
  PRIMARY KEY(id)
);

INSERT INTO members (id, first_name)
     VALUES (1, 'Tyler'),
            (2, 'Marissa'),
            (3, 'Alex'),
            (4, 'Parker');

CREATE TABLE membership_stack (
  id int unsigned auto_increment,
  member_id int unsigned not null,
  sequence int unsigned not null,
  team varchar(255) not null,
  `status` varchar(255) not null,
  PRIMARY KEY(id),
  FOREIGN KEY(member_id) REFERENCES members(id)
);

-- Algorithm to determine correct team:
-- 1. Only consider rows with the highest sequence number
-- 2. Order statuses and pick the first one found:
--    (active, completed, cancelled, abandoned)

INSERT INTO membership_stack (member_id, sequence, team, status)
     VALUES (1, 1, 'instinct', 'active'),
            (1, 1, 'valor', 'abandoned'),
            (2, 1, 'valor', 'active'),
            (2, 2, 'mystic', 'abandoned'),
            (2, 2, 'valor', 'completed'),
            (3, 1, 'instinct', 'completed'),
            (3, 2, 'valor', 'active');

我无法更改数据库架构,因为数据与外部数据源同步.

询问

这是我到目前为止：

    SELECT m.id, m.first_name, ms.sequence, ms.team, ms.status
      FROM membership_stack AS ms
      JOIN (
    SELECT member_id, MAX(sequence) AS sequence
      FROM membership_stack
  GROUP BY member_id
           ) AS t1
        ON ms.member_id = t1.member_id
       AND ms.sequence = t1.sequence
RIGHT JOIN members AS m
        ON ms.member_id = m.id
  ORDER BY m.id, FIELD(ms.status, 'active', 'completed', 'cancelled', 'abandoned');

这可以按预期工作,但如果他们的“最近序列”涉及多个团队,则成员可能会多次出现.我需要做的是再次在id上聚合并选择每个组中的FIRST行.

然而,这会带来一些问题：

>有no FIRST() function in MySQL
>这整个结果集将成为子表(子查询),这在这里不是什么大问题,但查询在应用程序上非常大.
>它需要与ONLY_FULL_GROUP_BY mode兼容,因为它在MySQL 5.7上默认启用.我没有检查但是我怀疑FIELD(ms.status,’active’,’completed’,’cancel’,’abandoned’)被认为是这个结果集的功能依赖字段.该查询还需要与MySQL 5.1兼容,因为这是我们目前正在运行的.

目标

| id | first_name | sequence |     team |    status |
|----|------------|----------|----------|-----------|
|  1 |      Tyler |        1 | instinct |    active |
|  2 |    Marissa |        2 |    valor | completed |
|  3 |       Alex |        2 |    valor |    active |
|  4 |     Parker |     NULL |     NULL |      NULL |

我该怎么办？

编辑：我注意到有些成员不属于任何团队.这些成员应包含在结果集中,并为这些字段使用空值.更新问题以反映新信息.

解决方法:

您可以在WHERE子句中使用LIMIT 1的相关子查询：

SELECT m.id, m.first_name, ms.sequence, ms.team, ms.status
FROM members AS m
JOIN membership_stack AS ms ON ms.member_id = m.id
WHERE ms.id = (
    SELECT ms1.id
    FROM membership_stack AS ms1
    WHERE ms1.member_id = ms.member_id
    ORDER BY ms1.sequence desc, 
             FIELD(ms1.status, 'active', 'completed', 'cancelled', 'abandoned'),
             ms1.id asc
    LIMIT 1
)
ORDER BY m.id;

演示：http://rextester.com/HGU18448

更新

要包含membership_stack表中没有条目的成员,您应该使用LEFT JOIN,并将子查询条件从WHERE子句移动到ON子句：

SELECT m.id, m.first_name, ms.sequence, ms.team, ms.status
FROM members AS m
LEFT JOIN membership_stack AS ms 
    ON  ms.member_id = m.id
    AND ms.id = (
        SELECT ms1.id
        FROM membership_stack AS ms1
        WHERE ms1.member_id = ms.member_id
        ORDER BY ms1.sequence desc, 
                 FIELD(ms1.status, 'active', 'completed', 'cancelled', 'abandoned'),
                 ms1.id asc
        LIMIT 1
    )
ORDER BY m.id;

演示：http://rextester.com/NPI79503

标签：mysql,sql-order-by,group-by,mysql-5-7,mysql-5-1
来源： https://codeday.me/bug/20190710/1428347.html