以下是我的SELECT声明，可以很好地调整我的数据。
我的数据看起来像这样：
col_a | col_b | col_c | col_d | Score
-------------------------------------
stuff | stuff | stuff | null | 5
stuff | stuff | stuff | title_a | 3
stuff | stuff | stuff | title_x | 4
我目前的Pivot声明如下：
SELECT `col_a`, `col_b`, `col_c`,
MAX(CASE `col_d` WHEN 'title_a' THEN `col_d` end) AS 'Title',
MAX(CASE `col_d` WHEN 'title_a' THEN `score` end) AS 'Score'
MAX(CASE `col_d` WHEN 'title_x' THEN `col_d` end) AS 'Title',
MAX(CASE `col_d` WHEN 'title_x' THEN `score` end) AS 'Score'
.....
这给了我以下结果：
col_a | col_b | col_c | Title | Score | Title | Score
---------------------------------------------------------
stuff | stuff | stuff | title_a | 3 | title_x | 4
我想做的是检查更多标题，但我只想在数据透视表中有四列。最多只有2行需要转动到上面的记录。但col_d可以包含任何标题。
例如，我尝试了以下内容：
我的数据现在看起来像这样：
col_a | col_b | col_c | col_d | Score
-------------------------------------
stuff | stuff | stuff | null | 5
stuff | stuff | stuff | title_a | 3
stuff | stuff | stuff | title_x | 4
stuff | stuff | stuff | null | 5
stuff | stuff | stuff | title_a | 3
stuff | stuff | stuff | title_bx | 4
我的Pivot声明现在看起来像这样：
SELECT `col_a`, `col_b`, `col_c`,
MAX(CASE `col_d` WHEN 'title_a' THEN `col_d` end) AS 'Title',
MAX(CASE `col_d` WHEN 'title_a' THEN `score` end) AS 'Score'
MAX(CASE `col_d` WHEN 'title_x' THEN `col_d` end) AS 'Title',
MAX(CASE `col_d` WHEN 'title_x' THEN `score` end) AS 'Score'
MAX(CASE `col_d` WHEN 'title_bx' THEN `col_d` end) AS 'Second Title',
MAX(CASE `col_d` WHEN 'title_bx' THEN `score` end) AS 'Score'
.....
因为你可以看到我试图检查另一个标题，但是这只给了我六列，其中2列为null，因为在这种情况下这两行包含title_a和title_bx，所以中间两列填充null。
我想从上面的数据输出：
col_a | col_b | col_c | Title | Score | Title | Score
---------------------------------------------------------
stuff | stuff | stuff | title_a | 3 | title_x | 4
stuff | stuff | stuff | title_a | 3 | title_bx | 4
所以我的问题是如何在col_d中检查多个可能的标题，并且只有4列。
答案 0 :(得分：8)
这有点混乱，因为MySQL没有窗口函数，并且您希望在第一组Title / Score列中包含非常具体的值。您可以使用一些user variables为col_d不等于title_a的行创建行号来获得最终结果，然后将其连接回您的表格。
语法类似于以下内容：
select a.col_a, a.col_b, a.col_c,
max(case when a.col_d = 'title_a' then a.col_d end) title1,
max(case when a.col_d = 'title_a' then a.score end) score1,
max(case when na.col_d <> 'title_a' then na.col_d end) title2,
max(case when na.col_d <> 'title_a' then na.score end) score2
from yourtable a
left join
(
-- need to generate a row number value for the col_d rows
-- that aren't equal to title_a
select n.col_a, n.col_b, n.col_c, n.col_d,
n.score,
@num:=@num+1 rownum
from yourtable n
cross join
(
select @num:=0
) d
where n.col_d <> 'title_a'
order by n.col_a, n.col_b, n.col_c, n.col_d
) na
on a.col_a = na.col_a
and a.col_b = na.col_b
and a.col_c = na.col_c
-- in the event you have more than 2 row only return 2
and na.rownum <= 2
where a.col_d = 'title_a'
group by a.col_a, a.col_b, a.col_c, na.rownum;
见SQL Fiddle with Demo。这得到一个结果：
| COL_A | COL_B | COL_C | TITLE1 | SCORE1 | TITLE2 | SCORE2 |
|-------|-------|-------|---------|--------|----------|--------|
| stuff | stuff | stuff | title_a | 3 | title_bx | 4 |
| stuff | stuff | stuff | title_a | 3 | title_x | 4 |
有人向我指出，如果你只有2个其他值，那么你可以简单地加入数据而不使用用户变量：
select distinct a.col_a, a.col_b, a.col_c,
a.col_d title1,
a.score score1,
na.col_d title2,
na.score score2
from yourtable a
left join
(
select n.col_a, n.col_b, n.col_c, n.col_d,
n.score
from yourtable n
where n.col_d <> 'title_a'
) na
on a.col_a = na.col_a
and a.col_b = na.col_b
and a.col_c = na.col_c
where a.col_d = 'title_a';
见SQL Fiddle with Demo。这给出了相同的结果：
| COL_A | COL_B | COL_C | TITLE1 | SCORE1 | TITLE2 | SCORE2 |
|-------|-------|-------|---------|--------|----------|--------|
| stuff | stuff | stuff | title_a | 3 | title_x | 4 |
| stuff | stuff | stuff | title_a | 3 | title_bx | 4 |
根据您对col_a，col_b和col_c中的数据实际拥有的内容，您可能需要更改此内容，但它可以为您提供所需的结果。
更新：根据您的评论，您不会知道col_d列中的值，但只需要将数据拆分为两个透视列，这个过程变得复杂，因为MySQL没有窗口功能。如果有NTILE函数，这将非常容易。 NTILE函数将行分配到特定数量的组中。在这种情况下，您的数据将分为两组。
我已修改SO用户this blog中的代码，Quassnoi以使用用户变量复制NTILE函数。变量用于创建两个东西，一个行号（在旋转期间使用）和ntile值。
代码将被修改为：
select
x.col_a,
x.col_b,
x.col_c,
max(case when x.splitgroup = 1 then x.col_d end) as Title1,
max(case when x.splitgroup = 1 then x.Score end) as Score1,
max(case when x.splitgroup = 2 then x.col_d end) as Title2,
max(case when x.splitgroup = 2 then x.Score end) as Score2
from
(
select src.col_a, src.col_b, src.col_c, src.col_d, src.score,
src.splitGroup,
@row:=case when @prev=src.splitGroup then @row else 0 end +1 rownum,
@prev:=src.splitGroup
from
(
-- mimic NTILE function by splitting the total count of rows
-- over the number of columns we want (2)
select d.col_a, d.col_b, d.col_c, d.col_d, d.score,
FLOOR((@r * @n) / cnt) + 1 AS splitGroup
from
(
select a.col_a, a.col_b, a.col_c, a.col_d, a.score, grp.cnt
from yourtable a
inner join
(
select col_a, col_b, col_c, count(*) as cnt
from yourtable
where col_d is not null
group by col_a, col_b, col_c
) grp
on a.col_a = grp.col_a
and a.col_b = grp.col_b
and a.col_c = grp.col_c
where a.col_d is not null
order by a.col_a, a.col_b, a.col_c
) d
cross join
(
-- @n is equal to the number of new pivoted columns we want
select @n:=2, @group1:='N', @group2:='N', @group3:='N'
) v
WHERE
CASE
WHEN @group1 <> col_a AND @group2<> col_b AND @group3 <> col_c
THEN @r := -1
ELSE 0 END IS NOT NULL
AND (@r := @r + 1) IS NOT NULL
) src
cross join
(
-- these vars are used to get the row number once the data is split
-- this will be needed for the aggregate/group by on the final select
select @row:=0, @prev:=1
) v2
order by src.splitGroup
) x
group by x.col_a, x.col_b, x.col_c, x.rowNum;
见SQL Fiddle with Demo。这给出了结果：
| COL_A | COL_B | COL_C | TITLE1 | SCORE1 | TITLE2 | SCORE2 |
|-------|-------|-------|----------|--------|----------|--------|
| stuff | stuff | stuff | title_a | 3 | title_tt | 1 |
| stuff | stuff | stuff | title_bx | 0 | title_qq | 1 |
| stuff | stuff | stuff | title_x | 4 | title_a | 8 |
| stuff | stuff | stuff | title_yy | 3 | title_h | 4 |
| stuff | stuff | stuff | title_a | 2 | title_o | 6 |
答案 1 :(得分：0)
如果我理解你的话。你可以这样做：
SELECT `col_a`, `col_b`, `col_c`,
MAX(CASE WHEN `col_d` IN('title_a','title_x','title_bx') THEN `col_d` end) AS 'Title',
MAX(CASE WHEN `col_d` IN('title_a','title_x','title_bx') THEN `score` end) AS 'Score'
...