原文: http://blog.csdn.net/jx_870915876/article/details/52403472sql
add by zhj: 本文是以MySQL为例说明的,但其实它适用于全部关系型数据库。数据库
行转列的需求以下,能够发现,行转列时实际上是先group by,而后将每一个组中的行转为列,使用case-when + 聚合函数,函数
每一个由行转为列的列,都要使用case-when或when-case语句。case-when是怎么做用的呢?组中的每行都通过case-when,spa
若是组中有N行,那经过case-when就产生N个值,再使用聚合函数,变为一个单一值,即新列的值。.net
例如:把图1转换成图2结果展现3d
图1 code
图2blog
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CREATE
TABLE
`TEST_TB_GRADE` (
`ID`
int
(10)
NOT
NULL
AUTO_INCREMENT,
`USER_NAME`
varchar
(20)
DEFAULT
NULL
,
`COURSE`
varchar
(20)
DEFAULT
NULL
,
`SCORE`
float
DEFAULT
'0'
,
PRIMARY
KEY
(`ID`)
) ENGINE=InnoDB AUTO_INCREMENT=1
DEFAULT
CHARSET=utf8;
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insert
into
TEST_TB_GRADE(USER_NAME, COURSE, SCORE)
values
(
"张三"
,
"数学"
, 34),
(
"张三"
,
"语文"
, 58),
(
"张三"
,
"英语"
, 58),
(
"李四"
,
"数学"
, 45),
(
"李四"
,
"语文"
, 87),
(
"李四"
,
"英语"
, 45),
(
"王五"
,
"数学"
, 76),
(
"王五"
,
"语文"
, 34),
(
"王五"
,
"英语"
, 89);
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行转列SQL:ci
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SELECT
user_name ,
MAX
(
CASE
course
WHEN
'数学'
THEN
score
ELSE
0
END
) 数学,
MAX
(
CASE
course
WHEN
'语文'
THEN
score
ELSE
0
END
) 语文,
MAX
(
CASE
course
WHEN
'英语'
THEN
score
ELSE
0
END
) 英语
FROM
test_tb_grade
GROUP
BY
USER_NAME;
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例如:把图1转换成图3结果展现get
图3
列转行SQL:
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select
user_name,
'语文'
COURSE , CN_SCORE
as
SCORE
from
test_tb_grade2
union
select
user_name,
'数学'
COURSE, MATH_SCORE
as
SCORE
from
test_tb_grade2
union
select
user_name,
'英语'
COURSE, EN_SCORE
as
SCORE
from
test_tb_grade2
order
by
user_name,COURSE;
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