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feikeq
V2EX  ›  MySQL

求一个 SQL 语句不知去重还是分组来实现

  •  
  •   feikeq ·
    feikeq · 2016-06-27 11:13:51 +08:00 · 5269 次点击
    这是一个创建于 3050 天前的主题,其中的信息可能已经有所发展或是发生改变。
    现在学过的东西想再用时却忘干净了,就像解一元二次方程,小时候很会解,现在怎么不知怎么开始解。。。。


    原表内容
    +--------------------+
    | id | name | fid |
    +--------------------+
    | 101 | aaaa | |
    +--------------------+
    | 102 | bbbb | 101 |
    +--------------------+
    | 103 | cccc | 102 |
    +--------------------+
    | 104 | dddd | 101 |
    +--------------------+

    查询结果
    +--------------------+
    | id | name | sum |
    +--------------------+
    | 101 | aaaa | 2 |
    +--------------------+
    | 102 | bbbb | 1 |
    +--------------------+
    24 条回复    2016-06-27 16:44:25 +08:00
    feikeq
        1
    feikeq  
    OP
       2016-06-27 11:14:51 +08:00
    还是一定要子查询?
    Ouyangan
        2
    Ouyangan  
       2016-06-27 11:18:50 +08:00
    没看懂提问意思
    birdccc
        3
    birdccc  
       2016-06-27 11:19:21 +08:00
    看不懂你这结果集是怎么来的啊 。
    techme
        4
    techme  
       2016-06-27 11:24:32 +08:00
    是不是在本表中查询每个 id 与 fid 的大于零的关联数 要用 join 吗?
    delavior
        5
    delavior  
       2016-06-27 11:24:47 +08:00
    去重怎么实现?一般都是分组吧
    dxfree
        6
    dxfree  
       2016-06-27 11:27:45 +08:00
    select distinct(id),distinct(name),sum(*)
    from table_name
    --where optional
    group by distinct(id),distinct(name)

    大概是这样吧
    sunchen
        7
    sunchen  
       2016-06-27 11:36:36 +08:00
    select id, name, count(*) as sum
    from
    (
    select a.id, a.name
    from x as a, x as b
    where a.id = b.fid
    )
    group by 1, 2
    ;
    feikeq
        8
    feikeq  
    OP
       2016-06-27 11:38:02 +08:00
    正确的查询结果
    +----------------------+
    | id | name | count |
    +----------------------+
    | 101 | aaaa | 2 |
    +----------------------+
    | 102 | bbbb | 1 |
    +----------------------+


    select id, name, count(fid) as sum from table group by 'fid'
    直接 GROUP BY 结果
    +----------------------+
    | id | name | count |
    +----------------------+
    | 104 | dddd | 2 |
    +----------------------+
    | 103 | cccc | 1 |
    +----------------------+
    这样是不对的,我要的是 101 用户 aaaa 带来 2 个人,而不是 104 这用户。
    feikeq
        9
    feikeq  
    OP
       2016-06-27 11:40:27 +08:00
    能不用子查询做到吗?我想提高查询性能不想用子查询,数据库表设计就是这样的也不能去再修改。
    Martin9
        10
    Martin9  
       2016-06-27 11:41:05 +08:00
    @feikeq 之前做过这个,是用子查询的。
    feikeq
        11
    feikeq  
    OP
       2016-06-27 11:43:50 +08:00
    select id, name, count(fid) as count from table group by 'fid'


    查出来虽然 count 对了,但 id 和 name 不匹配.
    fireapp
        12
    fireapp  
       2016-06-27 11:44:13 +08:00
    ```sql

    select
    id, name, (select count(*) from table t1 where t1.fid = t.id) as sum
    from
    table t
    where
    exists(select 1 from table t2 where t.id = t2.fid)
    -- order by sum desc

    ```
    feikeq
        13
    feikeq  
    OP
       2016-06-27 11:44:23 +08:00
    @Martin9 没别的办法了吗?
    Martin9
        14
    Martin9  
       2016-06-27 11:46:06 +08:00
    @feikeq 额暂时不知道别的。
    lxy
        15
    lxy  
       2016-06-27 12:00:00 +08:00
    子查询统计一下 fid 数量,然后跟原表链接起来。我设原表为 t1 。
    select id, name, t2.fid_count from t1
    left join (
    select fid, count(fid) as fid_count from t1 where fid is not null group by fid
    ) as t2 on t1.id=t2.fid
    where t2.fid is not null order by id
    lxy
        16
    lxy  
       2016-06-27 12:02:54 +08:00
    @lxy 漏了个, t2.fid_count 就是 sum 。
    txoooy
        17
    txoooy  
       2016-06-27 13:07:59 +08:00
    mysql> select * from ref1;
    +----+------+-----+
    | id | name | fid |
    +----+------+-----+
    | 1 | aaa | 0 |
    | 2 | bbb | 1 |
    | 3 | ccc | 2 |
    | 4 | ddd | 1 |
    +----+------+-----+
    4 rows in set

    mysql> SELECT
    r1.id AS user_id,
    r1. NAME AS user_name,
    count(DISTINCT r2. NAME) AS ref_count
    FROM
    ref1 r1
    INNER JOIN ref1 r2 ON r1.id = r2.fid
    GROUP BY
    r1. NAME;

    +---------+-----------+-----------+
    | user_id | user_name | ref_count |
    +---------+-----------+-----------+
    | 1 | aaa | 2 |
    | 2 | bbb | 1 |
    +---------+-----------+-----------+
    2 rows in set
    feikeq
        18
    feikeq  
    OP
       2016-06-27 15:02:02 +08:00
    @txoooy 我用你这 SQL 语句查出来不是上面这表的结果呀,我查出来是:
    1 aaaa 1
    2 bbbb 1
    3 cccc 1
    4 dddd 1
    txoooy
        19
    txoooy  
       2016-06-27 15:07:23 +08:00
    你的表结构, 数据 和我一样吗? 数据库是 mysql 吗?
    feikeq
        20
    feikeq  
    OP
       2016-06-27 15:12:13 +08:00
    @txoooy 抱歉,是我 SQL 语句写错了你的方法是可行的,谢谢。
    NNER JOIN 对性能影响大吗?
    feikeq
        21
    feikeq  
    OP
       2016-06-27 15:20:03 +08:00
    @txoooy 的方法查询要 0.07 秒,采用下面方法只需 0.02 秒

    SELECT T1.`uid`,T1.`headimg`, T1.`nickname` ,T1.`referer` ,T2.`num`
    FROM `user_center` AS T1 LEFT JOIN (
    SELECT C.`referer` AS uid, count(`referer`) AS num
    FROM `user_center` C, `user_open` O
    WHERE C.`uid`= O.`uid`
    AND O.`subscribe`=1
    GROUP BY C.`referer`
    ) AS T2
    ON T1.`uid`= T2.`uid`
    WHERE T2.`num` IS NOT NULL
    ORDER BY T2.`num` DESC
    txoooy
        22
    txoooy  
       2016-06-27 15:22:20 +08:00
    这个要分情况, 有些情况中利用子查询构建中间表可以提高性能, 不过你现阶段并不需要考虑这些, sql 优化东西太多了, 多写多看, 自然知道什么时候用什么方法
    wavingclear
        23
    wavingclear  
       2016-06-27 16:41:37 +08:00
    地球思维的写法
    SELECT id, name, num FROM test INNER JOIN ( SELECT fid, COUNT(fid) as num FROM `test` GROUP BY fid) AS temp ON ( test.id = temp.fid )

    修改之后的写法,不用子查询
    SELECT a.id, a.name, count(b.fid) as num FROM test as a INNER JOIN test as b ON (a.id = b.fid) GROUP BY b.fid
    wavingclear
        24
    wavingclear  
       2016-06-27 16:44:25 +08:00
    啊 17 楼已经写过了……
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