A column, or group of columns, that uniquely identifies each row in a table.
A primary key is a column, or group of columns, that uniquely identifies each row in a table.
Every row has one primary-key value. That value lets software refer to one exact record without guessing which row it means.
Simple definition: A primary key is the table's unique label for each row.
Think of a primary key like a passport number or library card number.
Many people can share a name, but each person can have a different official number. A table uses its primary key in the same way: it points to one specific row.
A primary key gives the database and application a stable way to:
Find one exact record in a table.
Stop two rows from using the same key.
Give other tables a value they can reference.
Change or delete the intended row.
Primary keys are used in relational tables whenever software needs to keep records distinct and addressable.
user_id identifies one account.
order_id identifies one purchase.
product_id identifies one catalog item.
payment_id identifies one payment record.
This table uses user_id as its primary key. The value 42 identifies exactly one row.
CREATE TABLE users (
user_id INTEGER PRIMARY KEY,
email TEXT NOT NULL,
display_name TEXT NOT NULL
);
SELECT * FROM users WHERE user_id = 42;| user_id | display_name | |
|---|---|---|
| 41 | alex@example.com | Alex |
| 42 | rebecca@example.com | Rebecca |
| 43 | sam@example.com | Sam |
The query can safely target Rebecca's row because user_id = 42 is unique.
A foreign key in another table can store the primary-key value to connect related records.
Uniquely identifies each row in its own table.
Stores a key from another table to represent a relationship.
Prevents duplicate values in a column, but it is not automatically the table's primary identity.
| ENGLISH TERM | 中文 | SIMPLE ENGLISH | 中文小白理解 |
|---|---|---|---|
| Primary Key | 主键 | A column or group of columns that uniquely identifies each row. | 唯一标识数据表中每一行的一个字段或一组字段。 |
| Table | 表 | A structure that stores related data in rows and columns. | 用行和列保存一类相关数据的结构。 |
| Row | 行 | One complete record in a table. | 数据表中的一条完整记录。 |
| Column | 列 | One named type of information in a table. | 数据表中表示某一类信息的字段。 |
| Unique | 唯一 | Different from every other value in the relevant set. | 和同一范围内的其他值都不相同。 |
| Constraint | 约束 | A rule that limits which data the database accepts. | 限制数据库可以接受什么数据的规则。 |
| Foreign Key | 外键 | A column that refers to a key in another table. | 引用另一张表中键值、用来建立关联的字段。 |
| Composite Key | 复合键 | A key made from two or more columns together. | 由两个或多个字段共同组成的键。 |
| NULL | 空值 | A database value meaning that information is missing or unknown. | 表示信息缺失或未知，而不是普通的空字符串。 |
A primary key uniquely identifies each row in a table.
It helps software find, update, delete, and connect one exact record.
Other tables can refer to it with a foreign key.