Data type
to four, the other need not, can be saved with a time stamp of a string
- Digital: int
- Long numbers: bigint
- String: varchar (len), the most common
- Long strings: longtext
Create a table
create table student(
id int auto_increment primary key, 自增长&&主键
name varchar(20) not null, 非空约束
age int not null, 非空约束
gender bit default 1, 默认约束
address varchar(20) unique, 唯一约束
isDelete bit default 0 默认约束
) CHARSET=utf8;;
Number of current table , do not usecount(*)
select count(1) from t_it
Multi-table query Precautions
# 多表查询如果有一个表是没值的,会导致没有结果,因为笛卡尔积是相乘的,0*100也是0
select
A.*,B.*,C.*
from
t_a A,t_b B,t_c C
# 所以需要要外连接
select
A.*,B.*,C.*
from
t_a A
left join
t_b B
on A.id == B.id
left join
t_c C
on B.id == C.id
Paging query
select * from t_it ORDER BY createTime DESC limit 10,10
Find the one and the next
select A.*,B.id 'nextId',C.id 'lastId'
from
(select * from t_mine where id='001') A
left join
(select id from t_mine where createTime > (select createTime from t_mine where id='001') ORDER BY createTime LIMIT 1) B
on A.id!=B.id
left join
(select id from t_mine where createTime < (select createTime from t_mine where id='001') ORDER BY createTime desc LIMIT 1) C
on A.id!=C.id