对mysql数据库进行增删改查

#1.添加字段
ALTER TABLE student ADD stuAge INT NOT NULL;
#2.修改字段
ALTER TABLE student CHANGE stuAge sAge VARCHAR (100)NOT NULL;
#3.修改表名
ALTER TABLE students RENAME student;
#4.删除字段
ALTER TABLE student DROP sAge;



#1.添加唯一约束
ALTER TABLE grade 
ADD CONSTRAINT stuName UNIQUE KEY(stuName);
#2.删除唯一约束
ALTER TABLE grade
DROP INDEX stuName;


#1.添加默认值约束 (地址默认北京)
ALTER TABLE student
ALTER COLUMN stuAddress SET DEFAULT '北京'
#2.删除默认值约束
ALTER TABLE student
ALTER COLUMN stuAddress DROP DEFAULT


#1.给student表中添加一个stuAge的字段
ALTER TABLE student ADD stuAge INT 
#2.给stuAge添加一个检查约束，验证是否可以成功
ALTER TABLE student ADD CONSTRAINT ck_stuAge CHECK (stuAge>=18)
#3.验证stuAge字段的检查约束是否成功
INSERT INTO student(stuName,stuGradeid,stuAge)
VALUES("陈七",1,17)
#4.删除检查约束是不成功的
ALTER TABLE student
DROP CONSTRAINT ck_stuAge


#1.删除外键约束

ALTER TABLE student2
DROP FOREIGN KEY FK_Grade_Student_GradeId

#
ALTER TABLE student2 ADD CONSTRAINT FK_Grade_Student_GradeId FOREIGN KEY (stuGradeid) REFERENCES `grade`(gid)