Mysql之SQL随笔

1.创建数据库

create database if not exists shop default character set utf8mb4 default collate utf8mb4_unicode_ci;

 2.查询数据库中重复数据

select column,count(*) as count from table group by column HAVING count > 1

3.依据关联表字段数据更新主表字段，并且限制更新条数

UPDATE a
INNER JOIN b ON b.order_id = a.id 
SET a.target_id = b.target_id,
a.item_type = b.type
WHERE
	a.id in (select id from (SELECT id from awhere target_id = 0 order by id limit 100) sub1);

4.删除重复行，仅保留一行

DELETE 
FROM
	table as ta
WHERE
	ta.id <> (
	select t.maxid from (select max(tb.id) as maxid from table as tb where ta.order_id = tb.order_id) t
	);

执行时间会很长，可以对字段加索引

5.select case赋值

select name, (case sex when 0 then '未知' when 1 then '男' when 2 then '女') as '性别' from users;

6.select concat字符串处理

select title, concat('http://host/?u=', type) as url from address;

7.update replace替换字段

update table set url = replace(url, 'qq.com', 'aa.com');

8.where json查询

select * from table where json->'$.oid' = 2;

9.update json字段

update table set json = json_set(json, '$.oid', 3);

或

update table set json = json_object('oid', 3);

 10.You can't specify target table '表名' for update in FROM clause

翻译：不能先select出同一表中的某些值，再update这个表(在同一语句中)

处理方法：将查询结果单独作为结果集

update table set type = 1,type_symbol = 'aa' where order_id in (select id from (select orders.id from orders left join table on table.order_id = orders.id where orders.item_type = 2 and table.type != 1) t);

 11.查询表字段

show full fields from table;

 12.表添加字段

ALTER TABLE `database`.`table` 
ADD COLUMN `column` tinyint(1) UNSIGNED NULL DEFAULT 1 COMMENT '备注' AFTER `column0`;

 13.结果集排序

/*取积分最高的前10条数据*/
SELECT id,nickname,score from users order by score desc limit 10;
等效于
SELECT id,nickname,score from users order by score desc limit 0, 10;
此处可看到limit用法，limit i,n表示返回查询结果集中游标从i开始的n行数据（游标起始为0）。
/*给结果集加上序号*/
SELECT (@i:=@i+1) as rowno,id,nickname,score from users, (select @i:=0) as rowno order by score desc limit 0, 10;
/*取第二行数据，limit 1, 1*/
SELECT (@i:=@i+1) as rowno,id,nickname,total_revenue from users, (select @i:=1) as rowno order by total_revenue desc limit 1,1;
/*取第三行数据，limit 2, 1*/
SELECT (@i:=@i+1) as rowno,id,nickname,total_revenue from users, (select @i:=2) as rowno order by total_revenue desc limit 2,1;

 14.分数名次排名，相同的分数占据位次一样，后续连续不隔断

增加一个变量，用于记录上一条数据的分数，只要当前数据分数跟上一条数据的分数比较，相同分数的排名就不变，不相同分数的排名就加一，并且更新变量的分数值为该条数据的分数，依次比较

select Score, (select count(distinct(Score)) from Scores as t where t.Score >= s.Score) as `Rank` from Scores as s order by Score desc;

以下sql不严谨，有误

SELECT Score, (case when @currentScore = Score then @i when @currentScore := Score then @i := @i +1 end) as `rank` from test, (select @i := 0, @currentScore := null) as t order by Score desc;

 15.各种连接join

https://www.cnblogs.com/fudashi/p/7491039.html

inner join

left join

right join

outer join

full join

cross join

16.第N高的薪水，因为offset不支持运算，必须是定值，故提前得出游标

CREATE FUNCTION getNthHighestSalary(N INT) RETURNS INT
BEGIN 
  set N:=N-1;
  RETURN (
      select (select distinct Salary from Employee order by Salary desc limit 1 offset N) as SecondHightSalary
  );
END

 17.连续出现的数字，查找所有至少连续出现三次的数字，结果去重

select distinct(Num) as ConsecutiveNums from (select Num, case when @current = Num then @t:=@t+1 when @current := Num then @t:=1 end as times from Logs, (select @t:=1, @current:=null) as tmp) as tmp2 where times >= 3;

或

SELECT DISTINCT
    l1.Num AS ConsecutiveNums
FROM
    Logs l1,
    Logs l2,
    Logs l3
WHERE
    l1.Id = l2.Id - 1
    AND l2.Id = l3.Id - 1
    AND l1.Num = l2.Num
    AND l2.Num = l3.Num
;