1.在已有字段建立外键和主键
ALTER TABLE score
ADD CONSTRAINT fk_score_course
FOREIGN KEY(cid)
REFERENCES course(cid);
ALTER TABLE stu
ADD CONSTRAINT pk_stu
PRIMARY KEY(stuid);
2.左右表链接
SELECT a.stuid 学号,b.name 姓名,a.s 成绩
FROM score a RIGHT JOIN stu b
ON a.stuid=b.`stuid`;
3.查询年龄最大的学生编号，姓名，生日
SELECT stuid,name,birthday FROM emp WHERE YEAR(CURDATE())-YEAR(birthday)= (SELECT MAX(YEAR(CURDATE())-YEAR(birthday)) FROM emp);
原文：https://www.cnblogs.com/lt1845265171/p/10437435.html