1.环境部署:
syntax语法错误
查询基本使用(条件,排序,聚合函数,分组,分页)
--创建学生表
create table students (
id int unsigned not null auto_increment primary key,
name varchar(20) default '',
age tinyint unsigned default 0,
high decimal(5,2),
gender enum('男', '女', '中性', '保密') default '保密',
cls_id int unsigned default 0,
is_delete bit default 0
);
--创建班级表
create table classes(
id int unsigned auto_increment primary key not null,
name varchar(20) not null
);
--往students表里插入数据
insert into students values
(0,'小明',18,180.00,1,1,0),
(0,'小月月',19,180.00,1,2,0),
(0,'彭于晏',28,185.00,1,1,0),
(0,'刘德华',58,175.00,1,2,0),
(0,'黄蓉',108,160.00,2,1,0),
(0,'凤姐',44,150.00,4,2,1),
(0,'王祖贤',52,170.00,2,1,1),
(0,'周杰伦儿',34,null,1,1,0),
(0,'程坤',44,181.00,1,2,0),
(0,'和珅',55,166.00,1,2,0),
(0,'刘亦菲',29,162.00,2,3,0),
(0,'金星',45,180.00,3,4,0),
(0,'静香',18,170.00,2,4,0),
(0,'郭靖',22,167.00,1,5,0),
(0,'周杰',33,178.00,1,1,0),
(0,'钱小豪',56,178.00,1,1,0),
(0,'谢霆锋',38,175.00,1,1,0),
(0,'陈冠希',38,175.00,1,1,0);
2.查询所有列
select * from 表名
一定条件查询(where)
select * from where id=5;
查询制定列
select id,name from students;
使用as给字段起别名
select id,name as '姓名', age, high, gender from students;
通过表名字段查询
select students.name from students; #多个表中有相同字段,通过表名字查询
给表起别名查询
select s.id,s.name,s.age from students as s;
消除重复行
distinct
select distinct age from students;
条件查询
比较运算符
查询年纪大于18岁的信息
select * from students where age > 18;
18岁到28岁之间(and,&&)
select * from students where age >= 18 and age =< 28;
select * from students where age between 18 and 28;
在18岁以上或者身高180以上的人(or,||)
select * from students where age > 18 or high > 180;
找出18岁和28岁的人
select * from students where age=18 or age=28;
模糊查询
like
% 替代1个或者多个甚至是没有
查询姓名中有‘小’的所有名字
select * from students where name like '%小%';
查询两个字人的名字 _表示一个字符
select * from students where name like '__';
查询至少有3个字的名字
select * from students where name like '%___';
范围查询
in (1,3,8)表示在一个非连续的范围内,在1,3,8范围中
查询 年纪为18和34的人
select * from students where age in (18, 34);
查询 年龄在17岁到34岁之间的信息
select * from students where age between 17 and 34;
查询 年纪不在18到34岁的信息
select * from students where age not between 17 and 34;
空判断
判断is null
查询身高为空的信息
select * from students where high is null;
判断非空is not null
select * from students where high is not null;
排序
order by 字段默认为从小到大
asc从小到大排列,即升序
desc从大到小排序,即降序
查询年纪在18到34岁之间的男性,按照年纪从小到大
select * from students where gender=1 and age between 18 and 34 order by age;
查询年纪在18到34岁之间的女性,身高从高到矮
select * from students where gender=2 and age between 18 and 34 order by high desc;
order by 多字段
将students表中数据按照年龄从大到小排列,年龄相同的情况下按照身高从大到小排列
select * from students order by age desc,high desc;
查询年纪在18到34岁的女性,身高从高到矮排序,如果身高相同的情况下按照年纪从小到大排序
select * from students where (age between 18 and 34) and gender=2 order by high desc,age asc; #如果条件多可以用()来区分
查询年纪在18到34岁的男性,身高从高到矮排序,如果身高相同的情况下按照年纪从小到大排序,如果年龄也相等那么按照id从小到大排序;
select * from students where gender=1 and age between 18 and 34 order by high desc,age asc,id asc;
聚合函数
总数
count(*) #()里可以添加字段,但是如果该字段为空,统计总数就会不准,因此用(*)
查询男性有多少人
select count(*) from students where gender=1;
最大值
max
查询最大的年纪
select max(age) from students;
查询女性的最高身高
select max(high) from students where gender=2;
最小值
min
select min(high) from students;
求和
sum
计算所有人的年龄总和
select sum(age) from students;
计算所有男人的年龄总和
select sum(age) from students where gender=1;
平均值
avg
计算平均年纪
计算平均年纪 sum(age)/count(*)
select sum(age)/count(*) from students;
select avg(age) from students;
保留2位小数round(,2)
select round(avg(age),2) from students;
分组
group by
按照性别分组,查询所有的性别
select gender from students group by gender;
计算每组性别的人数
select gender, count(*) from students group by gender;
查询男性组中的姓名 group_concat
select gender,group_concat(name) from students where gender=1 group by gender;
having
查询每个性别平均年纪超过30岁的性别,以及姓名
select gender, group_concat(name) from students group by gender having avg(age) > 30;
查询每种性别中的人数多于4个的组的信息
select gender,group_concat(name) from students group by gender having count(*)>4;
查询每种性别中的名字,身高,年龄 #查询多个内容之间要加分隔符(自定义),否则输出的字段是连在一起的,容易混淆。
select gender,group_concat(name,'|',high,'|',age) from students group by gender ;
分页
limit m,n m:从m+1行开始展示 n:显示的行数
显示5页
select * from students limit 5;
分页显示,每页显示2条数据
select * from students limit 0, 2;
按照身高从高到矮排序,查找出所有女性,并且分页显示,每页显示2条数据
select * from students where gender=2 order by high desc limit 0,2;