（1）关键字and or > < = >= <= between ….and not between…and
select sname
from Student
where studentno='2017151'
（2）属性 in(A,B,C，……)–>属性 = A or 属性=B or 属性= C……
select sname ,studentno
from Student
where point in(777,789,799)
（3）like:只用于文本匹配查询
select sname --查询姓刘的学生，%为任意个数的字
from Student
where sname like '刘%'
select sname --查询刘aa，名字为两个字，_为一个字
from Student
where sname like '刘__'
如在匹配中遇到_或%，则需要在前面加/转义
（4）条件查询中如果某一属性为空，用 is NULL
select studentno,sname,classno
from student
where email is null
（5）聚集函数
COUNT：统计一列中值的个数
SUM：计算一列值的总和
AVG：平均值
MAX：最大值
MIN：最小值
select MIN(point) MIN,MAX(point)
from student
（6）group by
select MIN(point) MIN,MAX(point) MAX
from student
group by classno
(7)HAVING作用对象为组，跟在GROUP BY 后
select studentno
from score
group by studentno
having COUNT(courseno)>=3