Question

我有两张桌子

Customer (idCustomer, ecc.. ecc..)
Comment (idCustomer, idComment, ecc.. ecc..)

显然这两个表连接在一起，例如

SELECT * FROM Comment AS co
  JOIN Customer AS cu ON cu.idCustomer = co.idCustomer

有了这个，我选择与客户关联的该表中的所有评论，但现在我想限制每个客户最多2条评论的评论数量。

我看到的第一件事就是使用GROUP BY cu.idCustomer，但每位客户只限制1条评论，但我希望每位客户2条评论。

我怎样才能做到这一点？

Answer 1

MySQL中的一个选项是服务器端变量。例如：

set @num := 0, @customer := -1;

select  *
from    (
        select  idCustomer
        ,       commentText
        ,       @num := if(@customer = idCustomer, @num + 1, 1) 
                    as row_number
        ,       @customer := idCustomer
        from    Comments
        order by 
                idCustomer, PostDate desc
        ) as co
join    Customer cu
on      co.idCustomer = cu.idCustomer
where   co.row_number <= 2

Answer 2

此版本不需要SET操作：

select  *
from    (select  idCustomer
         ,       commentText
         ,       @num := if(@customer = idCustomer, @num + 1, 1) as row_number
         ,       @customer = idCustomer
         from    Comments
         JOIN(SELECT @num := 0, @customer := 1) r
         order by idCustomer, PostDate desc) as co
 join    Customer cu on co.idCustomer = cu.idCustomer
 where   co.row_number <= 2

Answer 3

SELECT * FROM Comments AS cm1 
         LEFT JOIN Comments AS cm2 ON cm1.idCustomer = cm2.idCustomer 
         LEFT JOIN Customer AS cu ON cm1.idCustomer = cu.idCustomer
WHERE cm1.idComment != cm2.idComment
GROUP BY cm1.idCustomer

但是，如果您要更改评论数量，最好使用Andomar的解决方案。

Answer 4

没有必要使用光标，这非常慢。请参阅我对Complicated SQL Query About Joining And Limitting的回答。 DENSE_RANK将在没有所有游标错综复杂的情况下完成这项工作。

Answer 5

如果您使用PHP等脚本语言来处理结果，则可以在运行查询后限制每个客户显示的结果数。设置一个数组来保存所有结果，设置另一个数组来保存每个客户的结果数，并在计数超过限制后停止将查询结果添加到结果集中，如下所示：

$RESULTS = array();
$COUNTS = array();
$limit = 2;
$query = "SELECT customer_id, customer_name, customer_comment FROM customers ORDER BY RAND()";
$request = mysql_query($query); 
while ($ROW = mysql_fetch_assoc($request))
{
    $c = $ROW['customer_id'];
    $n = $COUNTS[$c];
    if ($n<$limit)
    {
         $RESULTS[] = $ROW;
         $COUNTS[$c]++;
    }
}

这样可以保证每个客户只会随机显示两条评论或者您想要的，其余的评论会被抛弃。当然，你要提取所有结果，但这比（进行复杂连接）要快得多。

Mysql限制列值重复N次

5 个答案: