How to set foreign keys in database sql statement

Methods for setting foreign keys in database sql statements: 1. Add foreign key constraints [alter table add foreign key (foreign key field) references from table main table (primary key field)]; 2. Delete foreign key constraints [ alter table table name drop foreig].

The operating environment of this article: Windows 7 system, Microsoft SQL Server 2008 version, Dell G3 computer.

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##How to set foreign keys in database sql statement:

-- 为cls_id字段添加外键约束
alter table students add foreign key(cls_id) references classes(id);  【首先会验证的，不符合就会报错】

-- 创建学校表
create table school(
    id int not null primary key auto_increment, 
    name varchar(10)
);

-- 创建老师表
create table teacher(
    id int not null primary key auto_increment, 
    name varchar(10), 
    s_id int not null, 
    foreign key(s_id) references school(id)
);

-- 需要先获取外键约束名称,该名称系统会自动生成,可以通过查看表创建语句来获取名称
show create table teacher;

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