9.2.1查询所有的列
*表示所有列
##查询所有的员工信息
Select * from emp;
9.2.2查询指定列
输入特指表里的单独列名进行查询
Select empno,ename,depton from emp;
9.3条件查询where
9.3.1常见的运算符
关系运算符：=、!=、<>、>=、<=
区间：between A and B -》[A,B]
And:并且，和
Or：或者
Not：否，非
Is Not null：非空
In：在什么里面
9.3.2查询年龄为65的女学生（where，and）
Select * from stu where age="65" and gender="female";
+--------+-------+------+--------+
| sid | sname | age | gender |
+--------+-------+------+--------+
| S_1004 | liSi | 65 | female |
+--------+-------+------+--------+
9.3.3查询学号为S_1001或名字为lisi的记录（where，or）
Select * from stu where sid="S_1001"or sname="lisi";
+--------+-------+------+--------+
| sid | sname | age | gender |
+--------+-------+------+--------+
| S_1001 | liuYi | 35 | male |
| S_1004 | liSi | 65 | female |
+--------+-------+------+--------+
9.3.4查询学号是S_1001，S_1002，S_1003的记录（where，in）
写法一：select * from stu where sid = "S_1001" or sid = "S_1002" or sid = "S_1003";
写法二：select * from stu where sid in("S_1001","S_1002","S_1003");
+--------+----------+------+--------+
| sid | sname | age | gender |
+--------+----------+------+--------+
| S_1001 | liuYi | 35 | male |
| S_1002 | chenEr | 15 | female |
| S_1003 | zhangSan | 95 | male |
+--------+----------+------+--------+
9.3.5查询学号不是S_1001，S_1002，S_1003的记录（where，not，in）
select * from stu where sid not in("S_1001","S_1002","S_1003");
9.3.6查询年龄为null的记录（where）
Select * from stu weher age is null;
9.3.7查询年龄在20-40之间的记录（between）
方式一：Select * from stu where age >=20 and <=40;
方式二：Select * from stu where age between 20 and 40;
9.3.8查询性别为非男的记录（where，not）
方式一：Select * from stu where gender !=’male’
方式二：Select * from stu where not gender =’male’
方式三：Select * from stu where gender <> ’male’
9.3.9查询名字不为空的学生信息（is not null）
Select * from stu where sname is not null;
