A key that references a key in another table and helps enforce a relationship between tables.
A foreign key references a key in another table and helps enforce a relationship between tables.
It usually stores a value that points to a primary key in a related table.
Simple Definition: A foreign key is a link from one table to another.
Think of a foreign key as a label on an order that says which customer owns it.
The customer keeps the identity. The order keeps the reference.
A foreign key helps a database:
Point a row in one table to a row in another.
Prevent links to records that do not exist.
Make related records agree about their connections.
Give SQL a reliable path for combining related data.
Foreign keys are used wherever one kind of record belongs to another.
Each order points to the customer who placed it.
Each review points to the product being reviewed.
Each comment points to the post it belongs to.
Each employee can point to a department.
An orders table can store the customer relationship directly in the row.
CREATE TABLE orders (
id INTEGER PRIMARY KEY,
customer_id INTEGER,
FOREIGN KEY (customer_id)
REFERENCES customers(id)
);orders.customer_id is the foreign key.
customers.id is the referenced key.
The database can reject an order that points to a customer that does not exist.
The foreign key gives a query a safe, meaningful connection between child and parent rows.
Points from a row in one table to a key in another table.
Uniquely identifies a row inside its own table.
| ENGLISH TERM | 中文 | SIMPLE ENGLISH | 中文小白理解 |
|---|---|---|---|
| Foreign Key | 外键 | A key that references a key in another table. | 指向另一个数据表中某个键的字段。 |
| Primary Key | 主键 | A value that uniquely identifies a row in a table. | 在表中唯一标识一行数据的值。 |
| Parent Table | 父表 | The table whose key is referenced by another table. | 被另一个表引用的那张表。 |
| Child Table | 子表 | The table that stores the foreign key. | 保存外键、用来指向另一张表的表。 |
| Reference | 引用 | A link that points to a related key or record. | 指向相关键或记录的连接。 |
| Constraint | 约束 | A rule the database checks for stored data. | 数据库保存数据时必须遵守的规则。 |
| Relationship | 关系 | A connection between records in different tables. | 不同数据表中的记录之间建立的联系。 |
| One-to-Many | 一对多 | One parent record is related to many child records. | 一条父记录可以对应多条子记录。 |
| NULL | 空值 | A value that means no value is present. | 表示当前没有填写或没有对应值。 |
| Referential Integrity | 引用完整性 | The rule that table references remain valid. | 保证表之间的引用不会指向不存在记录的规则。 |
A foreign key connects a row to a key in another table.
It helps the database protect relationships and keep related data consistent.
In a common one-to-many relationship, the child table stores the foreign key.