Mysql之sql开发技巧

如何进行行转列

create table t_amount(
create_time date not null,
amount float(8, 2) not null default 0
);

insert into t_amount values('2012-10-01', 111422.12), ('2012-10-10', 111424.12), ('2012-10-21', 112321.13);
insert into t_amount values('2012-11-01', 131422.12), ('2012-11-10', 111524.12), ('2012-11-21', 112678.34);
insert into t_amount values('2012-12-02', 1122721.78), ('2012-12-10', 112761.78), ('2012-12-21', 122231.23);

----行转列
select '销售额' as '月份',
sum(case month(create_time) when 10 then amount end) as '10月',
sum(case month(create_time) when 11 then amount end) as '11月',
sum(case month(create_time) when 12 then amount end) as '12月'
from t_amount;

create table stu_01(
name varchar(10) not null,
course varchar(20) not null,
score float(5,2) not null default 0
);

insert into stu_01 values('张三', '语文', 98);
insert into stu_01 values('张三', '数学', 100);
insert into stu_01 values('张三', '英语', 95);
insert into stu_01 values('李四', '语文', 76);
insert into stu_01 values('李四', '数学', 80);
insert into stu_01 values('李四', '英语', 79);
insert into stu_01 values('王五', '语文', 90);
insert into stu_01 values('王五', '数学', 67);
insert into stu_01 values('王五', '英语', 78);
select * from stu_01;

--行转列
select name, max(case course when '语文' then score end) as '语文',
max(case course  when '数学' then score end) as '数学',
max(case course  when '英语' then score end) as '英语'
from stu_01 group by name;

--行转列(带总分和平均分)
select name, max(case course when '语文' then score end) as '语文',
max(case course  when '数学' then score end) as '数学',
max(case course  when '英语' then score end) as '英语',
sum(score) as '总分',
avg(score) as '平均分'
from stu_01 group by name;

如何进进行列转行

drop table user_01;
create table user_01(
username varchar(20) not null,
mobile varchar(100) not null
);
insert into user_01 values('张三', '13889278278,17628278987,19087826678');
insert into user_01 values('李四', '13628798689,13783937637');
insert into user_01 values('王五', '13039889876');

--第一步：生成一个序列表(最大的序列值需要大于上面待拆分mobile字段里面含有元素的最大个数)
create table seq_01(
seq_num int primary key auto_increment
);
insert into seq_01 values(),(),(),(),(),(),(),(),();
select * from seq_01;

--第二步，将序列表和稍微处理过的带拆分的表进行内连接
select * from seq_01 s inner join(
select username, concat(mobile, ',') as mobile,
length(mobile)-length(replace(mobile, ',', '')) +1 as size
from  user_01) u on s.seq_num<=u.size;

--第三步，将内连接后的表进行字符串截取操作得到最终的表
SELECT
username,
REPLACE(SUBSTRING(SUBSTRING_INDEX('13889278278,17628278987,19087826678,',
',',
s.seq_num),
CHAR_LENGTH(SUBSTRING_INDEX('13889278278,17628278987,19087826678,',
',',
s.seq_num - 1)) + 1),
',',
'') AS mobile
FROM
seq_01 s
INNER JOIN
(SELECT
username,
CONCAT(mobile, ',') AS mobile,
LENGTH(mobile) - LENGTH(REPLACE(mobile, ',', '')) + 1 AS size
FROM
user_01) u ON s.seq_num <= u.size;

create table book_01(
book_name varchar(255) not null,
author varchar(255) not null,
amount int unsigned
);

insert into book_01 values('java in action', 'zhangsan', 20), ('c++ programming', 'lisi', 30), ('python thinking', 'wangwu', 25);

SELECT
book_name,
CASE s.seq_num
WHEN 1 THEN 'author'
WHEN 2 THEN 'amount'
END AS intem,
COALESCE(CASE s.seq_num
WHEN 1 THEN author
END,
CASE s.seq_num
WHEN 2 THEN amount
END) AS item_value
FROM
seq_01 s
INNER JOIN
book_01 b ON s.seq_num <= 2;

如何生成唯一序列号

--存储当天当前订单号表的表
create table order_seq(
date_str varchar(8) not null,
order_sn int unsigned
);

--订单号生成的存储过程
delimiter //
create procedure gener_order_seq()
begin
declare v_cnt int unsigned;
declare v_timestr varchar(8);
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 date_str=v_timestr;
if row_count()=0 then
insert into order_seq(date_str, 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 date_str=v_timestr;
commit;
end //
delimiter ;

--调用存储过程生成订单号(大约1000条/s)
call gener_order_seq;

如何删除重复数据

create table user_03(
id int unsigned not null auto_increment,
name varchar(10) not null,
primary key(id)
) engine=InnoDB charset=utf8;
insert into user_03 value(default, '张三'), (default,'李四'),(default,'小娟'),(default,'李四'), (default,'李四'), (default,'王五'),(default,'王五');

--删除重复数据，只保留id最大的那条记录
delete u1 from user_03 u1 left join(
select name, max(id)  as maxid from user_03 group by name having count(name)>1
) u2 on u1.name=u2.name where u1.id<u2.maxid;