內容介紹：
如何進行行列轉換
如何生成唯一序列號
如何刪除重複資料
一：如何進行行列轉換
需要用到的場景：主要兩個場景（報表統計+彙總顯示）
使用自連線實現行列轉換
行轉列比如成績
1：分別查詢出不同同學的成績，並將欄位名改為同學的名字。
2：通過交叉連線，將不同的語句連線起來
select *
from (
select
sum(kills) as
'a'from a inner
join b on a.name=b.user_name
where a.name='a') as a cross
join(
select
sum(kills) as
'b'from a inner
join b on a.name=b.user_name
where a.name='b') as b cross
join(
select
sum(kills) as
'c'from a inner
join b on a.name=b.user_name
where a.name='c')as c;
select * from (
select
sum(kills) as
'孫悟空'
from user1 a
join user_kills b
on a.id = b.use_id and a.user_name='孫悟空'
) a cross
join (
select
sum(kills) as
'豬八戒'
from user1 a
join user_kills b
on a.id = b.use_id and a.user_name='豬八戒'
) b cross
join(
select
sum(kills) as
'沙僧'
from user1 a
join user_kills b
on a.id = b.use_id and a.user_name='沙僧'
) c
select
sum(kills) as
'孫悟空'
from user1 a
join user_kills b
on a.id = b.use_id and a.user_name='孫悟空'
缺點：是將原來查詢的結果每一行單獨查詢出來，再進行拼接，因此每增加乙個同學就增加乙個select語句，並且是通過交叉連線，要保證每個查詢的結果只能是乙個，不然沒辦法通過交叉連線實現轉換。
使用case實現行列轉換
select
sum(case
when user_name='孫悟空'
then kills end) as
'孫悟空',
sum(case
when user_name='豬八戒',thwn kills end) as
'豬八戒'）
sum(case
when user_name='沙僧'
then kills end) as
'沙僧'
from user_name a join user_kills b on a.id = b.user_id；
select
sum(case user_name='a'
then kills end) as
'a',
sum(case username='b'
then kill end) as
'b',
sum(case username='c'
then kill end) as
'c'from a inner
join b on a.name=b.username;
一般形式
select name as
'姓名',
sum(case subject when
'數學'
then mark else
null
end) as
'數學',
sum(case subject when
'英語'
then mark else
null
end) as
'英語',
sum(case subject when
'語文'
then mark else
null
end) as
'語文'
from subject
group
by name;
注意：要用聚合函式，最後要group by。
列轉行的實現
單列轉多行：屬性拆分，etl資料處理
利用序列表處理列轉行的資料
select user_name,replace(substring(substring_index(mobile,',',a_id),char_length(
substring_index(mobile,',',a.id-1),',',")as mobile from tb_sequernce a
cross join select user_name,
concat(mobile,',') as mobile,length(mobile)-length(replace(mobile,',',"))+1
size
from user1 b ) b on a.id<=b.size;
直接替換user1
使用union all將結果集合並
select user_name,'arms'
as equipment, arms from user1 a join user1_equipment b on a.id=user_id
union
allselect user_name,'clothing'
as equipment, clothing from user1 a join user1_equipment b on a.id=user_id;
union all
select user_name,'shoe'
as equipment, shoe from user1 a join user1_equipment b on a.id=user_id;
select username,
(case
when s.id=1
then
'arms'
when s.id=2
then
'clothing'
when s.id=3
then
'shoe'
end) equipment,
(case
when s.id=1
then arms
when s.id=2
then clothing
when s.id=3
then shoe end) eq_name
from t_equipment e join t_user u
on e.userid = u.userid
cross
join t_sequence s where s.id<=3
order
by username
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