Mysql (二) - 增删改查

1. 插入语句

insert into Persons values (1,'Adams','John','Oxford Street','London');

insert into Persons (LastName,FirstName)values ('Bush','George');

2. 修改语句

update Persons set Address = 'Fifth Avenue',City = 'New York' where LastName = 'Bush';

3. 删除语句

delete from Persons where id =2;

4. 基本查询

· 比较运算符：
1. <
2. <=
3. !=或 <>
4. >=
5. =
6. >
7. in
8. between 值1 and 值 2

· 逻辑运算符：
1. not !
2. or ||
3. and &&

· 模糊查询：like

select * from Persons where FirstName like 'J%';

#通配单个字符
select * from Persons where FirstName like 'Joh_';

· 算术函数

select max(age) from Persons;

select min(age) from Persons;

select sum(age) from Persons;

select avg(age) from Persons;

select count(*) from Persons;

· 分组查询：group by

select count(*),City from Persons group by City;

· 取别名： as

select count(*),City as c from Persons group by c; #可省略 as select count(*),City c from Persons group by c;

· 对结果集再次查询： having

select count(*),City as c from Persons group by c having c is not null;

· 排序： order by

可以根据字段排序，可以升序或降序，默认升序排列
字段名 desc 按降序排序
asc 按升序排序

select * from Persons order by age desc;
# 先按字段1 ，字段1相同 按 字段2
select * from Persons order by age desc, FirstName;
# limit [offset][N]    offset : 偏移量
# 在语句最后，起到限制条目的作用
select * from Persons order by age desc limit 2;
select * from Persons order by age desc limit 1,2;

5.子查询

1. where 型
   把内层查询结果作为外层查询比较条件

select * from Persons where age in (select max(age) from Persons);

1. From 型
   把内层查询结果作为临时表

select * from (select * from Persons where City is not null) as temp where Id > 1;

1. exist 型
   把外层查询结果放在内层，看内层条件是否符合
   select * from Persons p where exists (select * from Car c where p.Id =c.PersonId);

6.多表查询

1.union
作用：把2次或多次查询结果联合起来

select Id , FirstName from Persons union select Id , CarName from Car;

要求：两次查询的列数一致
建议：查询每一列，相应的列类型也一样
注意：
· 可以来自多张表；
· 多次sql语句取出的列名可以不一致，以第一次为准
· 如果不同语句，取出的行结果完全一致，相同的行会被合并，可以使用 union all ，不合并相同结果
· 如果子句中有 order by，limit 需加小括号包起来，推荐放在所有子句后，即对合并结果来排序
· 在子句中，order by 配合 limit 使用才有意义，不配合使用会被语法分析去除

select Id , FirstName from Persons order by age limit 2) union select Id , CarName from Car;

2.左连接
左连接以左表为准，去右表匹配数据，匹配不到用 null 补充

select p.Id , FirstName , CarName from Persons p left join Car c on p.Id = c.PersonId; 

3.右连接
右连接以右表为准，去左表匹配数据，匹配不到用 null 补充

select p.Id , FirstName , CarName from Persons p right join Car c on p.Id = c.PersonId; 

4.内连接
查询左右表都有的数据，不要左右中 null 部分，即左右表的交集

select p.Id , FirstName , CarName from Persons p inner join Car c on p.Id = c.PersonId; 

5.外连接
mysql 不支持 全外连接