DML:data manipulation language
一般CRUD
1.数据增加(insert)
insert into t_user(name,address) values('jwe','china');
2.数据删除(delete)
delete from t_user where id=5;
3.数据修改(update)
update t_user set name='xixi' where id=1;
4.数据查询(select)
- 普通查询
select name,address from t_user;
- 去重查询
select distinct age from t_user;
- 模糊查询
select * from t_user where name like '%to%';
- 聚合查询
select max(age) from t_user;
- 分组查询
select count(*),age from t_user group by age having count(*)>1;
- 排序查询
select * from t_user order by age asc/desc;
- 分页查询(物理分页)
# limit 2、4指索引(第3到第5个),左闭右闭
select * from t_user limit 2,4;
二:多表查询
查询的本质:从XX范围根据XX条件来查询XX目标
1.子查询
- where型【查询条件】
select sno,cno from score where degree=(select max(degree) from score);
- from型【查询范围】
- exist型
2.连接查询
- 根基:笛卡尔积 t1*t2 【查询范围】
- 等值连接
from t1, t2 where t1.deptid = t2.id
- 内连接
from t1 inner join t2 on t1.x = t2.x
- 左连接【表1为主表】
from t1 left join t2 on t1.x = t2.x
- 右连接【表2为主表】
from t1 right join t2 on t1.x=t2.x
3.联合查询
-
应用场景:
要查询的结果来自于多个表,且多个表没有直接的连接关系,但查询的信息一致。 -
条件:多条查询语句的结果可以合并成一张表(字段个数、类型一致)。
-
union查询:将多条查询语句的结果合并成一个结果(不包括重复数据)
SELECT * FROM employees WHERE email LIKE '%a%'
UNION
SELECT * FROM employees WHERE department_id>90;
- union all查询:将多条查询语句的结果合并成一个结果(包括重复数据)
SELECT id,cname FROM t_ca WHERE csex='男'
UNION ALL
SELECT t_id,tname FROM t_ua WHERE tGender='male';
4.单表自连接
select * from t_user a,t_user b where a.deptid = b.id