Question

我有两张这样的表 - ＆gt;

Categories
-----------
id      name
---------
1 -->      a
2    -->   b
3    -->   c



    Messages
    ------------
    id        catid    message
    ---------------------------
    1   -->      1   -->     aa
    2      -->   1   -->     bb
    3        --> 1   -->     cc
    4        --> 2   -->     dd
    5        --> 3    -->    ee
    6        --> 3    -->    ff

    i want to join the tables for get FIRST message from messages table,

我想要这样的查询结果 - ＆gt;

-------------------------------
id         name       message
1          a          aa
2          b          dd
3          c          ee

我找到了一个代码

select * from categories c, items i
    -> where i.categoryid = c.id
    -> group by c.id;

但没有任何ORDER BY程序

Answer 1

SELECT c.*, i.*
FROM categories c
JOIN items i ON (i.categoryid = c.id)
LEFT OUTER JOIN items i2 ON (i2.categoryid = c.id 
  AND (i.message > i2.message OR i.message = i2.message AND i.id > i2.id))
WHERE i2.categoryid IS NULL
ORDER BY c.id;

这往往在MySQL上表现更好，因为MySQL在GROUP BY查询时表现得非常糟糕。

Answer 2

如果按第一个分组，MySql将始终返回第二个表中的第一个记录。

SELECT a.id, a.name, b.message FROM Categories a, Messages b WHERE b.catid = a.id GROUP BY a.id ORDER BY a.id

mysql连接限制有两个表

2 个答案: