SQL Server和MySQL主外键写法对比

SQL Server主键的写法：

--列级
create table dept
(
dept_no int primary key,
dept_name nvarchar(20) not null
)
--表级
create table dept
(
dept_no int not null,
dept_name nvarchar(20) not null,
primary key (dept_no)
)
--修改
create table dept
(
dept_no int not null,
dept_name nvarchar(20) not null
);
alter table dept add primary key (dept_no)
--添加个主键名字

alter table dept add constraint dept_no_pk primary key (dept_no)
--测试
insert into dept values
(10,'IT'),(20,'Finance'),(30,'Engineer');
insert into dept values(10,'IT2')

MySQL主键的写法：

--列级，同SQL Server一样
create table dept
(
dept_no int primary key,
dept_name varchar(20) not null
)
--表级,同SQL Server也一样
create table dept
(
dept_no int ,
dept_name varchar(20) not null,
primary key(dept_no)
)
--修改，同SQL Server也一样
create table dept
(
dept_no int not null,
dept_name varchar(20) not null
);
alter table add  primary key (dept_no);
--另一种修改的方法，就是加个名字
alter table add constraint dept_no_pk primary key(dept_no)

--测试

insert into dept values (10,'IT'),(20,'Finance'),(30,'Engineer'); 

insert into dept values(10,'IT2')

为毛都是一样啊，难道是标准SQL？

SQL Server外键的写法：

--列级
create table employee
(
employee_id int primary key,
employee_name nvarchar(20) not null,
dept_id int foreign key references dept(dept_no)
)
--表级
create table employee
(
employee_id int primary key,
employee_name nvarchar(20) not null,
dept_id int,
constraint dept_no_fk foreign key(dept_id) references dept(dept_no)
)
--修改
create table employee
(
employee_id int primary key,
employee_name nvarchar(20) not null,
dept_id int
);
alter table employee add foreign key (dept_id) references dept(dept_no);
alter table employee add constraint dept_no_fk foreign key (dept_id) references dept(dept_no)
--测试
insert into employee(1001,'zhangsan',10);
insert into employee(1002,'lisi',50)

MySQL外键的写法：

--这下终于和SQL Server 不一样了,好像没有列级的了哦
create table employee
(
employee_id int primary key,
employee_name varchar(20) not null,
dept_id int,
foreign key (dept_id) references dept(dept_no)
)；
create table employee
(
employee_id int primary key,
employee_name varchar(20) not null,
dept_id int,
constraint dept_no_fk foreign key (dept_id) references dept(dept_no)
)

--测试
insert into employee values(1001,'zhangsan',10);
insert into employee values(1002,'lisi',50)

其实主外键写法就是“constraint 主键名/外键名”这个的区别，我理解就是加了个别名。

删除主键和外键

SQL Server（好像必须有主外键别名的才行）：

create table employee
(
employee_id int,
employee_name nvarchar(20) not null,
dept_id int ,
constraint employee_id_pk primary key(employee_id),
constraint dept_no_fk foreign key(dept_id) references dept(dept_no)
);
alter table employee drop constraint employee_id_pk;
alter table employee drop constraint dept_no_fk;
--测试
insert into employee values(1001,'zhangsan',10);
insert into employee values(1003,'wangwu',20);
insert into employee values(1001,'niuliu',30);
insert into employee values(1002,'lisi',50)；

MySQL:

create table employee
(
employee_id int primary key,
employee_name varchar(20) not null,
dept_id int ,
constraint dept_no_fk foreign key (dept_id) references dept(dept_no)
);
alter table employee drop primary key;
alter table employee drop foreign key dept_no_fk;
--测试语句
insert into employee values(1001,'zhangsan',10);
insert into employee values(1003,'wangwu',20);
insert into employee values(1001,'niuliu',30);
insert into employee values(1002,'lisi',50)；

单独测试下MySQL的check约束

create table employee
(
employee_id int primary key,
employee_name varchar(20) not null,
age int check (18<=age and age<=55),
dept_id int,
constraint dept_no_fk foreign key(dept_id) references dept(dept_no)
);
--测试
insert into employee values(1001,'zhangsan'10,10);
insert into employee values(1002,'lisi',29,10);

真的可以insert into 进去，使用show create table employee查看，果真没有check约束

在网上搜的：

“所有的存储引擎均对CHECK子句进行分析，但是忽略CHECK子句。”
The CHECK clause is parsed but ignored by all storage engines.