数据库学习笔记——DDL

数据库学习笔记——DDL

建立EMPLOYEE数据库：

CREATE TABLE employee(
	employee_ID int not null,
	employee_name varchar(20) not null,
	street varchar(20) not null,
	city varchar(20) not null,
	PRIMARY KEY(employee_ID)
);

CREATE TABLE company(
	company_name varchar(30) not null,
	city varchar(20) not null,
	PRIMARY KEY(company_name)
);

create table manages(
	employee_ID int not null,
	manager_ID int,
	primary key(employee_ID),
    foreign key(employee_ID) references employee(employee_ID) on delete cascade,
	foreign key(manager_ID) references employee(employee_ID) on delete set null			
);

create table works(
	employee_ID int not null,
	company_name varchar(30),
	salary numeric(8,2) check (salary>3000),  
	primary key(employee_ID),
	foreign key(employee_ID) references employee(employee_ID) on delete cascade,
	foreign key(company_name) references company(company_name) on delete set null		
);

向基本表employee 中增加“性别”属性列，其属性名为sex，数据类型为字符型：

ALTER TABLE employee add sex char(1) comment '性别';

向基本表employee中增加“年龄”属性列，其属性名为age，数据类型为SMALLINT, 并且年龄在15至30之间：

ALTER TABLE employee add age SMALLINT comment '年龄';
ALTER TABLE employee add CONSTRAINT check1 check (age>=15 and age<=30);

将employee中的age年龄的数据类型改为INT型：

ALTER TABLE employee MODIFY age int;

将employee中的age字段改名为emp_age：

ALTER TABLE employee drop check check1;
ALTER TABLE employee change age emp age int;
ALTER TABLE employee add CONSTRAINT check1 check (emp_age>=15 and emp_age<=30);

将employee中的sex列允许为空值的属性更改为不允许为空值：

ALTER TABLE employee change sex sex char(1) not null;

向company表中增加id字段，并添加自增约束：

create table works(
	employee_ID int not null,
	company_name varchar(30),
	salary numeric(8,2) check (salary>3000),  
	primary key(employee_ID),
	foreign key(employee_ID) references employee(employee_ID) on delete cascade,
	CONSTRAINT works_fk foreign key(company_name) references company(company_name) on delete set null		
);

# 删除works的外键约束
ALTER TABLE works drop foreign key works_fk;
# 删除company原始主键
ALTER TABLE company drop PRIMARY KEY;
# 主键才能添加自增约束
ALTER TABLE company add company_ID int PRIMARY KEY auto_increment;
# 恢复从表的外键约束
ALTER TABLE works add company_ID int;
ALTER TABLE works add CONSTRAINT works_fk1 FOREIGN KEY (company_ID) references company (company_ID) on delete set null;

desc company;
desc works;
# 显示works建表语句
show create TABLE works;

删除employee表中的sex列：

ALTER TABLE employee drop sex;

删除employee表：

drop TABLE works;
drop TABLE manages;
drop TABLE employee;

manages表中的manager_ID字段上无法添加not null约束：

create table manages(
	employee_ID int not null,
	manager_ID int,
	primary key(employee_ID),
    foreign key(employee_ID) references employee(employee_ID) on delete cascade,
	foreign key(manager_ID) references employee(employee_ID) on delete set null			
);

当主表中一条记录被删除时，如何处理子表中的外键字段：

• on delete cascade : 删除子表中所有的相关记录

• on delete set null : 将所有相关记录的外键字段值设置为NULL

• on delete no action: 不做任何操作（默认）

manager_ID作为外键字段，被设置为on delete set null，即当主表中一条记录被删除时，manager_ID设置为NULL，与“添加not null约束”冲突，即无法成功添加约束。