如何在mysql中加入3个表

Posted 2023-02-24

【中文标题】如何在mysql中加入3个表

【英文标题】：how to left join 3 tables in mysql

【发布时间】：2013-12-01 08:43:00

【问题描述】：

我有 3 张桌子：

成员（网名，姓名）

奖项(nic,aw_name)

冠军（nic,ch_name）

一个人可以拥有多个奖项或冠军。比如 Richard 有 2 个奖项和 3 个冠军，而 Steve 没有任何奖项和冠军。

现在我想列出获得奖项和冠军的人。例如：

|    nic   |    name    |   aw_name   |      ch_name      |
|----------|------------|-------------|-------------------|
| 1        | Richard    | award 1     | championship 1    |
| 1        | Richard    | award 2     | championship 2    |
| 1        | Richard    |             | championship 3    |
| 2        | Steve      |             |                   |

谁能帮帮我？谢谢！

【问题讨论】：

如何你认为你可以做到..？

***.com/questions/9853586/sql-join-multiple-tables

【参考方案1】：

试试这个：

SELECT
    members.nic,
    members.name,
    awards.aw_name,
    championships.ch_name
FROM
    (
        SELECT
            CASE WHEN @curNic = members.nic THEN @curRow := @curRow + 1 ELSE @curRow := 1 END num,
            @curNic := members.nic nic,
            name
        FROM
            members LEFT JOIN
            (
                SELECT nic FROM awards UNION ALL
                SELECT nic FROM championships
            ) tmp ON tmp.nic = members.nic JOIN
            (SELECT @curNic := null, @curRow := 0) r
    ) members LEFT JOIN
    (
        SELECT
            CASE WHEN @curNic1 = nic THEN @curRow1 := @curRow1 + 1 ELSE @curRow1 := 1 END num,
            @curNic1 := nic nic,
            aw_name
        FROM
            awards JOIN
            (SELECT @curNic1 := null, @curRow1 := 0) r
    ) awards ON awards.nic = members.nic AND awards.num = members.num LEFT JOIN
    (
        SELECT
            CASE WHEN @curNic2 = nic THEN @curRow2 := @curRow2 + 1 ELSE @curRow2 := 1 END num,
            @curNic2 := nic nic,
            ch_name
        FROM
            championships JOIN
            (SELECT @curNic2 := null, @curRow2 := 0) r
    ) championships ON championships.nic = members.nic AND championships.num = members.num
WHERE
    awards.nic IS NOT NULL OR championships.nic IS NOT NULL OR members.num = 1

如果mysql中有FULL OUTER JOIN会更容易，但是没有。

小提琴：http://www.sqlfiddle.com/#!2/31d47/43

【讨论】：

但这会为理查德返回 6 行并重复获奖和冠军

@MohamadMehdiHabibi 使用这种模式并不容易，但请参阅我的更新。

【参考方案2】：

试试这个。

SELECT m.nic,
       m.name,
       a.aw_name,
       c.ch_name
FROM members m
LEFT JOIN awards AS a ON m.nic = a.nic
LEFT JOIN championships AS c ON m.nic = c.nic
GROUP BY m.nic, m.name

【参考方案3】：

您可以根据自己的要求使用连接来合并表。

SELECT mem.nic,mem.name, awd.aw_name, chm.ch_name
FROM members mem
LEFT JOIN awards AS awd ON mem.nic = awd.nic
LEFT JOIN championship AS chm ON mem.nic = chm.nic
GROUP BY mem.nic, mem.name;

更多请点击：http://www.w3schools.com/sql/sql_join.asp