DML：data manipulation language
General CRUD
1. Data increase (insert)
insert into t_user(name,address) values('jwe','china');
2. Data deletion (delete)
delete from t_user where id=5;
3. Data modification (update)
update t_user set name='xixi' where id=1;
4. Data query (select)
- General query
select name,address from t_user;
- Deduplication query
select distinct age from t_user;
- Fuzzy query
select * from t_user where name like '%to%';
- Aggregate query
select max(age) from t_user;
- Group query
select count(*),age from t_user group by age having count(*)>1;
- Sort query
select * from t_user order by age asc/desc;
- Paging query (physical paging)
# limit 2、4指索引(第3到第5个)，左闭右闭
select * from t_user limit 2,4;
Two: multi-table query
The essence of query: query XX target from XX range according to XX conditions
1. Subquery
- where type [query condition]
select sno,cno from score where degree=(select max(degree) from score);
- from type [query range]
- exist type
2. Connect query
- Foundation: Cartesian product t1*t2 【Scope of inquiry】
- Equivalent connection
from t1, t2 where t1.deptid = t2.id
- Internal connection
from t1 inner join t2 on t1.x = t2.x
- Left join [Table 1 is the main table]
from t1 left join t2 on t1.x = t2.x
- Right connection [Table 2 is the main table]
from t1 right join t2 on t1.x=t2.x
3. Joint query
-
Application scenario:
The results to be queried come from multiple tables, and multiple tables have no direct connection relationship, but the query information is consistent. -
Condition: The results of multiple query statements can be combined into one table (the number of fields and types are the same).
-
Union query: Combine the results of multiple query statements into one result (not including duplicate data)
SELECT * FROM employees WHERE email LIKE '%a%'
UNION
SELECT * FROM employees WHERE department_id>90;
- Union all query: Combine the results of multiple query statements into one result (including duplicate data)
SELECT id,cname FROM t_ca WHERE csex='男'
UNION ALL
SELECT t_id,tname FROM t_ua WHERE tGender='male';
4. Single table self-connection
select * from t_user a,t_user b where a.deptid = b.id