MySQL数据库-数据的查询操作大全
查询
基本查询
查询所有字段
select * from 表名;
- select * from students;
select * from classes;
- select id, name from classes;
查询指定字段
select 列1,列2,… from 表名;
- select name, age from classes;
select 表名.字段 … from 表名;
- select students.name, students.age from students;
使用 as 给字段起别名
select 字段 as 名字… from 表名;
- select name as 姓名, age as 年龄 from classes;
可以通过 as 给表起别名
select 别名.字段 … from 表名 as 别名;
- select students.name, students.age from students;
- select s.name, s.age from students as s;
- 失败的select students.name, students.age from students as s;
消除重复行
distinct 字段
- select distinct gender from students;
条件查询
elect … from 表名 where …
比较运算符
>
- 查询大于18岁的信息
select * from students where age>18;
select id,name,gender from students where age>18;
<
- 查询小于18岁的信息
select * from students where age<18;
>= <=
- 查询小于或等于18岁的信息
=
- 查询年龄为18岁的所有学生的名字
select * from students where age=18;
!= 或者 <>
逻辑运算符
and
18到28岁之间所有学生的信息
select * from students where age>18 and age<28;
失败select * from students where age>18 and <28;18岁以上的女性
select * from students where age>18 and gender=“女”;
select * from students where age>18 and gender=2;
or
- 18岁以上或者身高查过180(包含)以上
select * from students where age>18 and height>=180;
not
不在 18岁以上的女性 这个范围内的信息
select * from students where not age>18 and gender=2;
select * from students where not (age>18 and gender=2);年龄不是小于或者等于18 并且是女性
select * from students where (not age<=18) and gender=2;
模糊查询
like
% 替换1个或者多个
_ 替换1个
查询姓名中 以 “小” 开始的名字
select name from students where name=“小”;
select name from students where name like “小%”;查询姓名中 有"小" 所有的名字
select name from students where name like “%小%”;查询有2个字的名字
select name from students where name like “__”;查询有3个字的名字
select name from students where name like “___”;查询至少有2个字的名字
select name from students where name like “__%”;
rlike 正则
查询以 周开始的姓名
select name from students where name rlike “^周.*”;查询已 周开始、伦结尾的姓名
select name from students where name rlike “^周.*伦$”;
范围查询 in(1, 3,8 )表示在一个非连续的范围内
查询 年龄为18、34的名字
select name,age from students where age=18 or age=34;
select name,age from students where age=18 or age=34 or age=12;
select name,age from students where age in (12, 18, 34);not in 不非连续的范围之内
年龄不是 18、34岁之间的信息
select name,age from students where age not in (12, 18, 34);
between … and …表示在一个连续的范围内
- 查询 年龄在18到34之间的信息
select name,age from students where age between 18 and 34;
not between … and …表示不在一个连续的范围内
- 查询 年龄不在在18到34之间的的信息
select * from students where age not between 18 and 34;
select * from students where not age between 18 and 34;
失败的select * from students where age not (between 18 and 34);
空判断
判空is null
- 查询身高为空的信息
select * from students where height is null;
select * from students where height is NULL;
select * from students where height is Null;
判非空is not null
- select * from students where height is not null;
排序
order by 字段
** asc从小到大排列,即升序**
desc从大到小排序,即降序
查询年龄在18到34岁之间的男性,按照年龄从小到到排序
select * from students where (age between 18 and 34) and gender=1;
select * from students where (age between 18 and 34) and gender=1 order by age;
select * from students where (age between 18 and 34) and gender=1 order by age asc;查询年龄在18到34岁之间的女性,身高从高到矮排序
select * from students where (age between 18 and 34) and gender=2 order by height desc;
order by 多个字段
查询年龄在18到34岁之间的女性,身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序
select * from students where (age between 18 and 34) and gender=2 order by height desc, age asc;查询年龄在18到34岁之间的女性,身高从高到矮排序, 如果身高相同的情况下按照年龄从小到大排序,如果年龄也相同那么按照id动大到小排序
select * from students where (age between 18 and 34) and gender=2 order by height desc, age asc, id desc;按照年龄从小到大、身高从高到矮的排序
select * from students order by age asc, height desc;
聚合函数
总数 count
- 查询男性有多少人,女性有多少人
select * from students where gender=1;
select count(*) from students where gender=1;
select count(*) as 男性人数 from students where gender=1;
select count(*) as 女性人数 from students where gender=2;
最大值max
查询最大的年龄
select age from students;
select max(age) from students;查询女性的最高 身高
select max(height) from students where gender=2;
最小值min
求和sum
- 计算所有人的年龄总和
select sum(age) from students;
平均值avg
计算平均年龄
select avg(age) from students;计算平均年龄 sum(age)/count(* )
select sum(age)/count(*) from students;
四舍五入 round(123.23 , 1) 保留1位小数
计算所有人的平均年龄,保留2位小数
select round(sum(age)/count(*), 2) from students;
select round(sum(age)/count(*), 3) from students;计算男性的平均身高 保留2位小数
select round(avg(height), 2) from students where gender=1;
select round(avg(height), 2) from students where gender=1;
分组 group by
group by
# 对原始数据筛选 分组 在聚合函数的基础上分组,否则没有意义
按照性别分组, 查询所有的性别
select gender from students group by gender;
失败select * from students group by gender;
失败 select name from students group by gender;计算每种性别中的人数
select gender,count(*) from students group by gender;计算男性的人数
select gender,count(*) from students where gender=1 group by gender;
group_concat(…)
分组后显示group_concat()内的内容 ()内写什么,就连接显示什么
- select gender, group_concat(name) from students where gender=1 group by gender;
select gender, group_concat(name, age, id) from students where gender=1 group by gender;
select gender, group_concat(name, “_”, age, " ", id) from students where gender=1 group by gender;
having
# 对结果进行筛选 对分组进行条件判断
注:where对原始表进行条件判断,having 对分组进行条件判断;where在分组前,having在分组后
查询平均年龄超过30岁的性别,以及姓名 having avg(age) > 30
select gender, group_concat(name), avg(age) from students group by gender having avg(age)>30;查询每种性别中的人数多于2个的信息
select gender, group_concat(name) from students group by gender having count(*)>2;
分页
limit start,count
限制查询出来的数据格式
select * from students where gender=1 limit 2;查询前5个数据
select * from students limit 0,5;查询id6-10(包含)的书序
select * from students limit 5,5;每页显示2个,第1个页面
select * from students limit 0,2;每页显示2个,第2个页面
select * from students limit 2,2;每页显示2个,第3个页面
select * from students limit 4,2;每页显示2个,第4个页面
select * from students limit 6,2; – -----> limit (第N页-1)*每页的个数, 每页的个数;每页显示2个,显示第6页的信息, 按照年龄从小到大排序
select * from students order by age asc limit 10,2;
select * from students where gender=2 order by age desc limit 0,2;
失败select * from students limit 2*(6-1),2;
失败select * from students limit 10,2 order by age asc;
注:limit 需放在最后
链接查询(多表查询)
inner join … on
select * from 表A inner join 表B on 条件;
注: inner join 两个表同时存在才显示;inner join on 条件 显示相同条件的信息
查询 有能够对应班级的学生以及班级信息
select * from students inner join classes on students.cls_id=classes.id;按照要求显示姓名、班级
select students.* classes.name from students inner join classes on students.cls_id=classes.id;
select students.name, classes.name from students inner join classes on students.cls_id=classes.id;给数据表起名字
select s.name, c.name from students as s inner join classes as c on s.cls_id=c.id;查询 能够对应班级的学生以及班级信息-显示学生的所有信息, 只显示班级名称
select s.*,c.name from students as s inner join classes as c on s.cls_id=c.id;在以上的查询中,将班级姓名显示在第1列
select c.name,s.* from students as s inner join classes as c on s.cls_id=c.id;查询 有能够对应班级的学生以及班级信息, 按照班级进行排序
select c.xxx s.xxx from student as s inner join clssses as c on … order by …;
select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name;当时同一个班级的时候, 按照学生的id进行从小到大排序
select c.name, s.* from students as s inner join classes as c on s.cls_id=c.id order by c.name,s.id;
left join
# 左表中特有的数据,右表中没有与之对应的数据,则结果中用null填充
查询每位学生对应的班级信息
select * from students as s left join classes as c on s.cls_id=c.id;查询没有对应班级信息的学生
select … from xxx as s left join xxx as c on… where …
select … from xxx as s left join xxx as c on… having …select s.*, c.name from students as s left join classes as c on s.cls_id=c.id where c.name is null;
select s.*, c.name from students as s left join classes as c on s.cls_id=c.id having c.name is null;
right join
# 将数据表名字互换位置,用left join完成
自关联
注:chcp 65008 Windows改变为utf8编码
省级联动 url:http://demo.lanrenzhijia.com/2014/city0605/
查询所有省份
select * from areas where pid is null;查询aid
select aid from areas where “山东省”;查询出山东省有哪些市
select * from areas as province inner join areas as city on city.pid=province.aid having province.atitle=“山东省”;
select province.atitle, city.atitle from areas as province inner join areas as city on city.pid=province.aid having province.atitle=“山东省”;查询出青岛市有哪些县城
select * from areas as city inner join areas as province on city.pid=province.aid where province.atitle=“青岛市”;
子查询
标量子查询
查询出高于平均身高的信息
select * from students where height > (select avg(height) from students);查询最高的男生信息
select * from students where height = 188;select * from students where height = (select max(height) from students);
列级子查询
- 查询学生的班级号能够对应的学生信息
select * from students where cls_id in (1, 2);
select * from students where cls_id in (select id from classes);
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