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 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 students;
-- select 表名.字段 .... from 表名;
select students.name, students.age from students;
-- 可以通过 as 给表起别名
-- select 表名.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
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;
select id,name,gender from students where age>18;
-- 查询小于18岁的信息
select * from students where age<18;
-- 查询年龄为18岁的所有学生的名字
select * from students where age=18;
逻辑运算符
-- and
-- 18到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;
-- 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以上或者身高查过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);
-- 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.*";
-- 查询以 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的姓名
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;