-- 查询练习-- 查询所有字段-- select * from 表名;select * from students;-- 查询指定字段-- select 列1,列2,... from 表名;select name,gender 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",gender as "性别" from students;-- select 表名.字段 .... from 表名;select students.name,students.gender from students;-- 可以通过 as 给表起别名-- select 别名.字段 .... from 表名 as 别名;select s.name,s.gender from students as s;失败的select students.name, students.age from students as s;-- 消除重复行(查性别)-- distinct 字段 select distinct gender from students;-- 条件查询-- 比较运算符-- select .... from 表名 where .....-- >-- 查询大于18岁的信息select * from students where age > 18;-- <-- 查询小于18岁的信息select * from students where age < 18;-- >=-- <=-- 查询小于或者等于18岁的信息select * from students where age <= 18;-- =-- 查询年龄为18岁的所有学生的名字select name,age from students where age = 18;-- != 或者 <>select * from students where age != 18;select * from students where age <> 18;-- 逻辑运算符-- and-- 18和28之间的所以学生信息select * from students where age > 18 and age < 28;失败select * from students where age>18 and <28;select * from students where 18<age<28;-- 18岁以上的女性select * from students where age > 18 and gender=2;-- or-- 18以上或者身高高过180(包含)以上select * from students where age > 18 or 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);(注意)-- 模糊查询(where name like 要查询的数据)-- like -- % 替换任意个-- _ 替换1个-- 查询姓名中 以 "小" 开始的名字select * from students where name like "小%";-- 查询姓名中 有 "小" 所有的名字select * from students where name like "%小%";-- 查询有2个字的名字select * from students where name like "__";-- 查询有3个字的名字select * from students where name like "___";-- 查询至少有2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-- 查询 年龄为18、34的姓名select name,age from students where age in (18,34);-- not in 不非连续的范围之内-- 年龄不是 18、34岁的信息select name,age from students where age not in (18,34);(注意)select name from students where not age in (18,34);-- between ... and ...表示在一个连续的范围内-- 查询 年龄在18到34之间的的信息select * 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;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;-- 判非空is not nullselect * from students where height is not null;失败select * from students where height not is  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，按照年龄从小到大到排序(默认是asc升序)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;-- 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从大到小排序select * from students where (age between 18 and 34) and gender = 2 order by height desc,age,id desc;-- 聚合函数-- 总数-- count-- 查询男性有多少人，女性有多少人select count(*) from students where gender=1;select count(*) from students where gender=2;-- 最大值-- max-- 查询最大的年龄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-- minselect min(height) from students;-- 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(avg(age),2) 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;-- 分组(重点)-- group by-- 按照性别分组,查询所有的性别select gender from students group by gender;select name,gender from students group by gender;错误-- select name,gender from students group by gender;-- 失败select * from students group by gender;-- 计算每种性别中的人数select count(*),gender from students group by gender;-- group_concat(...)-- 查询同种性别中的姓名select gender,group_concat(name) from students group by gender;-- 查询每组性别的平均年龄select avg(age),gender from students group by gender;-- 查询平均年龄超过30岁的性别，以及姓名 having avg(age) > 30(重点)select gender,group_concat(name) from students  group by gender having avg(age) > 30;-- 查询每种性别的平均年龄和名字select avg(age),group_concat(name) from students group by gender;-- 查询每种性别中的人数多于2个的性别和姓名（重点）select gender,group_concat(name) from students group by gender having count(*) > 2;-- with rollup 汇总的作用(了解)select gender,count(*) from students group by gender with rollup;-- 分页-- limit start, count-- 限制查询出来的数据个数-- 查询前5个数据select * from students limit 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;-- 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;错误1 select * from students limit 10,2 order by age asc;-- 错误的写法错误2 select * from students limit 2*(6-1),2;-- limit 放在最后面(注意)-- 连接查询(重点)-- inner join ... on-- select ... from 表A inner join 表B;select * from students inner join classes;-- 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.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 有能够对应班级的学生以及班级信息，显示学生的所有信息 students.*，只显示班级名称 classes.name.select students.*,classes.name from students inner join classes on students.cls_id=classes.id;-- 在以上的查询中，将班级姓名显示在第1列select classes.name,students.* from students inner join classes on students.cls_id=classes.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 students as s inner join clssses as c on .... order by ....;select classes.name,students.* from students inner join classes on students.cls_id=classes.id order by classes.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 classes.name,students.* from students inner join classes on students.cls_id=classes.id order by classes.name,students.id;-- left join-- 查询每位学生对应的班级信息select * from students inner join classes on students.cls_id=classes.id;select * from students left join classes on students.cls_id=classes.id;-- select * from students right join classes on students.cls_id = classes.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 * from students left join classes on students.cls_id=classes.id where classes.id is null;(注意)不建议使用 select * from students left join classes on students.cls_id=classes.id having classes.id is null;-- right join   on-- 将数据表名字互换位置，用left join完成-- 子查询-- 标量子查询: 子查询返回的结果是一个数据(一行一列)-- 列子查询: 返回的结果是一列(一列多行)-- 行子查询: 返回的结果是一行(一行多列)-- 查询出高于平均身高的信息(height)-- 1 查出平均身高select avg(height) from students;-- 2 查出高于平均身高的信息select * from students where height > (select avg(height) from students);-- 查询学生的班级号能够对应的 学生名字-- select name from students where cls_id in (select id from classes);-- 1 查出所有的班级idselect id from classes;(1,2)-- 2 查出能够对应上班级号的学生信息select * from students where cls_id in (select id from classes);