二.查询语句

一.分组查询

SELECT MAX(age) FROM student;         # 查询年龄最大的数字为32
SELECT * FROM student WHERE age=32;   # 查询32的学生信息
SELECT * FROM student WHERE age=(SELECT MAX(age) FROM student);   # 利用子查询合并

# like关键字模糊查询：用%代替任意字符，用_代替一个字符

# 查询学生中姓“张”的学生
SELECT * FROM student WHERE sname LIKE '张%'; 
 
# 查询学生中姓“李”的学生
SELECT * FROM student WHERE sname LIKE '李__';

# 查询学生中包含“雨”的学生
SELECT * FROM student WHERE sname LIKE '%雨%';

# 查询一班的学生平均年龄
SELECT AVG(age) FROM student WHERE classid=1;

# 查询某个条件下的行数
SELECT COUNT(studentid) FROM student 

# 计算C09课程5月的考试平均成绩
SELECT COUNT(studentid) FROM grade WHERE courseid='C09' 
AND BETWEEN createtime '2022-05-01' AND '2022-05-31';