[英]SQL - Limiting results from one table, based on criteria specified in another
[英]Limiting based on value selected from another table
我有以下表格结构:
类别: ID,名称,限制
帖子: id,类别,标题,正文,日期,作者
我要做的是显示类别表中“限制”字段所限制的每个类别的最新帖子,并按帖子日期排序。
如果我了解您:
SELECT * FROM posts LEFT JOIN Category ON posts.category = Category.id WHERE limit = 'some limit' ORDER BY category, date DESC
(假设:posts.category是对Category表的主键的引用)
尝试这样的事情:(不测试是否有错误)
$result = mysql_fetch_array(mysql_query("<query to select your category>"));
$limit = $result['limit'];
$queryResult = mysql_query("select * from posts order by date desc limit $limit");
//do whatever you want with the $queryResult
每组N个最大组的有趣变化
SELECT id, name, limit, postid, title, body, date, author
FROM
(
SELECT
c.id, c.name, c.limit, p.id postid, p.title, p.body, p.date, p.author
@r:=case when @g=C.id then @r+1 else 1 end r,
@g:=C.id
FROM
(select @g:=null,@r:=0) n
CROSS JOIN CATEGORY C
INNER JOIN POSTS p on p.category = c.id
ORDER BY
C.ID, postid
) X
WHERE r<=c.limit
ORDER BY ID, postid
侧面影响变量基本上产生2列
这可能很昂贵,因为没有捷径,它会在类别/后联接中的WHERE r<=c.limit
行编号,并且只有在此之后,它才会在WHERE r<=c.limit
位中被截断。
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