增:
insert into 表名 (列名1, 列名2,) values(值1, 值2);
insert into 表名 (列名1, 列名2,) values(值1, 值2),(值1,值2),(值n,值n);
insert into 表名 (列名1, 列名2,) select 列名1, 列名2 from 表名;
删除:
delete from 表名;
delete from 表名 where id > 10
delete from 表名 where id < 10
delete from 表名 where id <= 10
delete from 表名 where id >= 10
delete from 表名 where id != 10
delete from 表名 where id = 10 and name='xxx'; and : 并且 两个条件都必须要成立
delete from 表名 where id = 10 or name='xxx'; or : 或者 只要满足一个条件成立
修改:
update 表名 set name='zekai', age=23 where id > 10;
查询:
基本:
select * from 表名;
select name , age from 表名;
高级:
a. where 条件查询:
select * from 表名 where id=10;
select * from 表名 where id >10 and id<15;
select * from 表名 where id > 10;
!= : 不等于
between and: between是一个闭区间
select * from t4 where id between 9 and 12;
in: 在某一个集合中
select * from t4 where id in (9,10,11....);
select * from t4 where id in (select id from t3 where id between 2 and 4)
b. 通配符:
alex
select * from 表 where name like 'ale%' - ale开头的所有（多个字符串）
select * from 表 where name like 'ale_' - ale开头的所有（一个字符）
c. 限制取几条:
select * from 表名 limit 索引偏移量, 取出多少条数据;
select * from t3 limit 0, 10; 第一页
select * from t3 limit 10, 10; 第二页
page = input('page:')
page 索引偏移量 数据量(offset)
1 0 10
2 10 10
3 20 10
4 30 10
page (page-1)*offset offset
分页核心SQL:
select * from t3 limit (page-1)*offset, offset;
d. 排序:
order by
降序:
select * from t4 order by 列名 desc; descending
升序:
select * from t4 order by 列名 asc; ascending
多列:
create table t7(
id int auto_increment primary key,
num int not null default 0,
age int not null default 0
)charset=utf8;
insert into t7 (num, age) values (2, 12),(3,13),(4, 12);
select * from t4 order by num desc, name asc;
如果前一列的值相等的话, 会按照后一列的值进行进一步的排序.
e. 分组
select age, 聚合函数(count(num)/sum(num)/max(num)/min(num)/avg(num)) from 表名 group by 列名;
select age, avg(num) from t7 group by age;
select age, count(num) from t7 group by age;
select age, count(num) as cnt from t7 group by age; 显示别名 as
having的二次删选:
select age, count(num) as cnt from t7 group by age having cnt>1;
where 和 having的区别:
1). having与where类似,可筛选数据
2). where针对表中的列发挥作用,查询数据
3). having针对查询结果中的列发挥作用,二次筛选数据, 和group by配合使用
f. 连表操作
select * from userinfo, department; (笛卡尔积)
select * from userinfo, department where userinfo.depart_id=department.id;
左连接:
select * from userinfo left join department on userinfo.depart_id=department.id;
左边的表全部显示, 右边没有用到不显示
右连接:
select * from userinfo right join department on userinfo.depart_id=department.id;
右边的表全部显示, 左边没关联的用null表示
内连接:
左右两边的数据都会显示
ps:
a.只需要记住左连接 left join
b.可以连接多张表 通过某一个特定的条件
注意查询的顺序:
select name,sum(score) from 表 where id > 10 group by score having age> 12 order by age desc limit 2, 10