MySQL基础讲义——约束与表管理

-- 主键约束   
create table s_user(   
u_id int auto_increment primary key,   
u_name varchar(20),   
u_pwd varchar(20)   
);    
# 添加自增列
create table s_user(
u_id int auto_increment primary key,
u_name varchar(20),
u_pwd varchar(20)
);
-- 外键约束   
-- 方法1   
create table s_order(   
o_id int auto_increment primary key,   
o_buyer_id int,   
o_totalprices float,   
foreign key(o_buyer_id) references s_user(u_id)   
);   
-- 方法2   
create table s_order2(   
o_id int auto_increment primary key,   
o_buyer_id int,   
o_totalprices float    
);  
alter table s_order2 add foreign key(o_buyer_id)   references s_user(u_id);   

-- 添加数据   
insert into s_user(u_name,u_pwd) values("carry","123");   
insert into s_user(u_name,u_pwd) values("carry","123"),("harry","456"),("marry","789");   
-- 修改数据   
set sql_safe_updates=0; #设置数据库安全等级  
update s_user set u_pwd="963" where u_name="harry";   
-- 删除数据   
delete from s_user where u_name="marry";