从我之前提到的question开始，我在下面进行了查询，但不能限制每个组的特定行数，
SELECT *
FROM (SELECT p.page_id AS page_id,
p.page_url AS page_url,
SUBSTRING(p.page_title,LOCATE('{',p.page_title),LOCATE('}',p.page_title)) AS group_title,
p.page_created AS page_created,
@row_number := @row_number + 1 AS row_number
FROM root_pages AS p
WHERE p.parent_id = '8'
AND p.page_id != '8'
AND p.page_hide != '1'
AND p.category_id = '1'
ORDER BY p.page_backdate DESC) a
WHERE row_number <= 2
结果，
page_id page_url group_title page_created row_number
----------------------------------------------------------
44 abc {a} 2011-10-21... NULL
43 def {a} 2011-10-21... NULL
42 qwe {b} 2011-10-21... NULL
41 rty {b} 2011-10-21... NULL
40 tyu {c} 2011-10-21... NULL
39 ghj {c} 2011-10-21... NULL
59 sss {c} 2011-10-21... NULL
但我希望得到这个结果，
结果，
page_id page_url group_title page_created row_number
----------------------------------------------------------
44 abc {a} 2011-10-21... 1
43 def {a} 2011-10-21... 2
42 qwe {b} 2011-10-21... 1
41 rty {b} 2011-10-21... 2
40 tyu {c} 2011-10-21... 1
39 ghj {c} 2011-10-21... 2
有什么想法我错过了吗？
编辑1：
查询，
SELECT a.*
FROM (SELECT p.page_id,
p.page_url,
SUBSTRING(p.page_title, LOCATE('{',p.page_title),LOCATE('}', p.page_title)) AS group_title,
p.page_created,
@row_number := @row_number + 1 AS row_number
FROM root_pages AS p
JOIN (SELECT @row_number := 0, @url) r -- Initialize the variable
WHERE p.parent_id = '8'
AND p.page_id != '8'
AND p.page_hide != '1'
AND p.category_id = '1'
ORDER BY p.page_backdate DESC) a
结果，
page_id page_url group_title page_created row_number
----------------------------------------------------------
44 abc {a} 2011-10-21... 1
43 def {a} 2011-10-21... 2
42 qwe {b} 2011-10-21... 3
41 rty {b} 2011-10-21... 4
40 tyu {c} 2011-10-21... 5
39 ghj {c} 2011-10-21... 6
59 sss {c} 2011-10-21... 7
查询，
SELECT a.*
FROM (SELECT p.page_id,
p.page_url,
SUBSTRING(p.page_title, LOCATE('{',p.page_title),LOCATE('}', p.page_title)) AS group_title,
p.page_created,
@row_number := @row_number + 1 AS row_number
FROM root_pages AS p
JOIN (SELECT @row_number := 0, @url) r -- Initialize the variable
WHERE p.parent_id = '8'
AND p.page_id != '8'
AND p.page_hide != '1'
AND p.category_id = '1'
ORDER BY p.page_backdate DESC) a
WHERE a.row_number <= 2
结果，
page_id page_url group_title page_created row_number
----------------------------------------------------------
44 abc {a} 2011-10-21... 1
43 def {a} 2011-10-21... 2
查询，
SELECT *
FROM(
SELECT
p.page_id AS page_id,
p.page_url AS page_url,
SUBSTRING(p.page_title,LOCATE('{',p.page_title),LOCATE('}',p.page_title)) AS group_title,
p.page_created AS page_created,
@url = p.page_url,
@row_number := CASE
WHEN p.page_url = @url THEN @row_number + 1
ELSE 1
END AS row_number
FROM root_pages AS p
JOIN (SELECT @row_number := 0, @url) r -- Initialize the variable
WHERE p.parent_id = '8'
AND p.page_id != '8'
AND p.page_hide != '1'
AND p.category_id = '1'
ORDER BY p.page_backdate DESC
) a
where row_number <= 2
结果，
page_id page_url group_title page_created row_number
----------------------------------------------------------
44 abc {a} 2011-10-21... 1
43 def {a} 2011-10-21... 1
42 qwe {b} 2011-10-21... 1
41 rty {b} 2011-10-21... 1
40 tyu {c} 2011-10-21... 1
39 ghj {c} 2011-10-21... 1
59 sss {c} 2011-10-21... 1
编辑2：
查询，
SELECT
a.*,
@r := CASE WHEN @g = a.group_title THEN @r+1 ELSE 1 END AS r,
@g := a.group_title AS dummy
FROM(
SELECT
p.page_id AS page_id,
p.page_url AS page_url,
SUBSTRING(p.page_title,LOCATE('{',p.page_title),LOCATE('}',p.page_title)) AS group_title,
p.page_created AS page_created
FROM root_pages AS p
WHERE p.parent_id = '8'
AND p.page_id != '8'
AND p.page_hide != '1'
AND p.category_id = '1'
ORDER BY p.page_backdate DESC
) a
JOIN (SELECT @r := 0, @g) r -- Initialize the variable
结果，
page_id page_url group_title page_created dummy row_number
44 abc {a} 2011-10-21... {a} 1
43 def {a} 2011-10-21... {a} 1
42 qwe {b} 2011-10-21... {b} 1
41 rty {b} 2011-10-21... {b} 1
40 tyu {c} 2011-10-21... {c} 1
39 ghj {c} 2011-10-21... {c} 1
59 sss {c} 2011-10-21... {c} 1
解答：
SELECT *
FROM
(
SELECT
a.*,
@r := CASE WHEN @g = a.group_title THEN @r+1 ELSE 1 END AS r,
@g := a.group_title AS dummy
FROM(
SELECT
p.page_id AS page_id,
p.page_url AS page_url,
SUBSTRING(p.page_title,LOCATE('{',p.page_title),LOCATE('}',p.page_title)) AS group_title,
p.page_created AS page_created
FROM root_pages AS p
WHERE p.parent_id = '8'
AND p.page_id != '8'
AND p.page_hide != '1'
AND p.category_id = '1'
ORDER BY p.page_backdate DESC
) a
JOIN (SELECT @g:=null, @r:= 0) n -- Initialize the variable
) b
WHERE r <= 2
答案 0 :(得分：1)
您的查询未初始化@row_number变量：
SELECT a.*
FROM (SELECT p.page_id,
p.page_url,
SUBSTRING(p.page_title, LOCATE('{',p.page_title),LOCATE('}', p.page_title)) AS group_title,
p.page_created,
@row_number := @row_number + 1 AS row_number
FROM root_pages AS p
JOIN (SELECT @row_number := 0, @url) r -- Initialize the variable
WHERE p.parent_id = '8'
AND p.page_id != '8'
AND p.page_hide != '1'
AND p.category_id = '1'
ORDER BY p.page_backdate DESC) a
WHERE a.row_number <= 2
否则，您需要在发布的查询之前使用SET声明。
其次，您发布的内容不包括重置变量所需的CASE语句，因此每个page_url获得1+
SELECT a.*
FROM (SELECT p.page_id,
p.page_url,
SUBSTRING(p.page_title, LOCATE('{',p.page_title),LOCATE('}', p.page_title)) AS group_title,
p.page_created,
@url = p.page_url,
@row_number := CASE
WHEN p.page_url = @url THEN @row_number + 1
ELSE 1
END AS row_number
FROM root_pages AS p
JOIN (SELECT @row_number := 0, @url) r -- Initialize the variable
WHERE p.parent_id = '8'
AND p.page_id != '8'
AND p.page_hide != '1'
AND p.category_id = '1'
ORDER BY p.page_url, p.page_backdate DESC) a
WHERE a.row_number <= 2
IME，该变量适用于InnoDB表，但与MyISAM不一致。