MySQL连接限制与两个表

Question

我有两个这样的表->

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->

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

我找到一个密码

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;

由于MySQL在GROUP BY查询上表现不佳，因此在MySQL上往往表现得更好。

Answer 2

如果您对第一个表进行分组，则MySql将始终从第二个表返回第一个记录。

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