MySql（二）

Mysql

行列转换

Select username,kills from user1 a join user_kills b on a.id = b.id;

Select username,SUM(kills) from user1 join user1 a join user_kills b on a.id=b.id;
第一种方法用
cross join：
select * from (
select sum(kills) as '孙悟空' from user1 a
join user_kills b
on a.id = b.use_id and a.user_name='孙悟空'
) a cross join (
select sum(kills) as '猪八戒' from user1 a
join user_kills b
on a.id = b.use_id and a.user_name='猪八戒'
) 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

第二种方法采用CASE方式

select sum(case when user_name='孙悟空' then kills end) as '孙悟空',
sum(case when user_name='猪八戒' then kills end) as '猪八戒',
sum(case when user_name='沙僧' then kills end) as '沙僧'
from user1 a join user_kills b on a.id=b.user_id;

列转行：

1. 创建序列表

2.

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;

第二种列转行

3.

select user_name,'arms' as equipment, arms from user1 a join user1_equipment b on a.id=user_id
union all
select 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;

a156

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

生成唯一序列号使用自增序列设置序列号

1. 使用sql编写特殊序列号

2.

DECLARE v_cnt INT;
DECLARE v_timestr INT;
DECLARE rowcount BIGINT;
SET v_timestr = DATE_FORMAT(NOW(),'%Y%m%d');
SELECT ROUND(RAND()*100,0)+1 INTO v_cnt;
START TRANSACTION;
UPDATE order_seq SET order_sn = order_sn + v_cnt WHERE timestr = v_timestr;
IF ROW_COUNT() = 0 THEN
INSERT INTO order_seq(timestr,order_sn) VALUES(v_timestr,v_cnt);
END IF;
SELECT CONCAT(v_timestr,LPAD(order_sn,7,0))AS order_sn
FROM order_seq WHERE timestr = v_timestr;
COMMIT;

删除重复数据

1. 判断是否重复

select  user_name ，count(*)  from  test  group  by  user_name  having count(*)>1

删除重复数据，同时保留id最大的那条记录

delete a
from user1_test a join(
select user1_name count(*),max(id) as id
from user1_test
group by user_name having count(*) >1
)b on a.user_name = b.user_name
where a.id<b.id

较为复杂的重复数据的处理：

sql:
select username,GROUP_CONCAT(phone) mobile from
(
select
DISTINCT a.username,
REPLACE(SUBSTRING(SUBSTRING_INDEX(a.mobile,',',s.id),LENGTH(SUBSTRING_INDEX(a.mobile,',',s.id-1))+1),',','') phone
from t_sequence s join
(select
username,
CONCAT(mobile,',') mobile,
(LENGTH(mobile)-LENGTH(REPLACE(mobile,',',''))+1) count
from t_user where mobile is not null) a
on s.id <= a.count
)as b
GROUP BY username

相关视频参考慕课网http://www.imooc.com/learn/427