条件
- 使用where子句对表中的数据筛选，结果为true的行会出现在结果集中
- 语法如下：
select * from 表名 where 条件;
比较运算符
- 等于=
- 大于>
- 大于等于>=
- 小于<
- 小于等于<=
- 不等于!=或<>
- 查询编号大于3的学生
select * from students where id>3;
select * from subjects where id<=4;
select * from students where sname!='黄蓉';
select * from students where isdelete=0;
逻辑运算符
select * from students where id>3 and gender=0;
select * from students where id<4 or isdelete=0;
模糊查询
- like
- %表示任意多个任意字符
- _表示一个任意字符
- 查询姓黄的学生
select * from students where sname like '黄%';
select * from students where sname like '黄_';
select * from students where sname like '黄%' or sname like '%靖%';
范围查询
- in表示在一个非连续的范围内
- 查询编号是1或3或8的学生
select * from students where id in(1,3,8);
- between ... and ...表示在一个连续的范围内
- 查询学生是3至8的学生
select * from students where id between 3 and 8;
select * from students where id between 3 and 8 and gender=1;
空判断
- 注意：null与''是不同的
- 判空is null
- 查询没有填写地址的学生
select * from students where hometown is null;
- 判非空is not null
- 查询填写了地址的学生
select * from students where hometown is not null;
select * from students where hometown is not null and gender=0;
优先级
- 小括号，not，比较运算符，逻辑运算符
- and比or先运算，如果同时出现并希望先算or，需要结合()使用
