【MySQL】经典查询例题笔记

1.查询生日在“1988-1-1"后的学生姓名、专业名称；

设计日期

SELECT studentname, majorname
FROM student s
JOIN major m
ON s .majorid = m. majorid
WHERE DATEDIFF (borndate, '1988-1-1')>0;

2.查询每个专业的男生人数和女生人数分别是多少；

#方式一-:
SELECT COUNT(*) 个数，sex, majorid
FROM student
GROUP BY sex, majorid;
#方式二:
SELECT majorid,
(SELECT COUNT (*)FROM student WHERE sex='男' AND majorid = s.majorid) 男，
(SELECT COUNT (*)FROM student WHERE sex='女' AND majorid = s.majorid) 女
FROM student s
GROUP BY majorid;

3.查询学生名、专业名、分数

三表关联

SELECT studentname, score , majorname

FROM student s

JOIN major m ON s. majorid=m.majorid

LEFT JOIN result r ON s.studentno=r.studentno