Primary keys uniquely identify rows in a table, while foreign keys establish relationships between tables. A primary key must have unique, non-null values and each table can have only one; it can be a single column or a composite of multiple columns. Foreign keys reference the primary key in another table to ensure referential integrity, preventing orphaned records. They support actions like ON DELETE CASCADE or ON UPDATE CASCADE to automate data consistency. Primary keys are essential for uniqueness and fast access, often using auto-incrementing integers or UUIDs, while foreign keys are used whenever tables are related, such as linking users to orders or blog posts to authors. Common pitfalls include missing indexes on foreign keys, improper handling of dependent records, using strings for IDs, and choosing storage engines that do not support foreign keys.
Primary keys and foreign keys are the backbone of relational databases like MySQL. They help maintain data integrity and establish relationships between tables. If you're just starting with MySQL, understanding how these keys work is crucial for building solid database structures.
What’s a Primary Key?
A primary key is a column (or combination of columns) that uniquely identifies each row in a table. Think of it as an ID card for your records — no two rows can have the same value in the primary key column(s).
Here’s what makes a primary key special:
- It must contain unique values.
- It cannot contain
NULLvalues. - Each table can have only one primary key, though it may consist of multiple columns (called a composite key).
For example, in a users table, you might use user_id as the primary key:
CREATE TABLE users (
user_id INT PRIMARY KEY,
name VARCHAR(100)
);Or if you want to define it separately (especially useful when creating composite keys):
CREATE TABLE orders (
order_id INT,
product_code CHAR(10),
amount DECIMAL(10,2),
PRIMARY KEY (order_id, product_code)
);This ensures that the combination of order_id and product_code is unique across the table.
What’s a Foreign Key?
A foreign key is a field (or collection of fields) in one table that refers to the primary key in another table. The main purpose is to ensure referential integrity — meaning you can't have a record pointing to something that doesn’t exist.
Let's say we have a users table and an orders table. To link each order to a specific user, we’d add a foreign key:
CREATE TABLE orders (
order_id INT PRIMARY KEY,
user_id INT,
total DECIMAL(10,2),
FOREIGN KEY (user_id) REFERENCES users(user_id)
);Now, if someone tries to insert an order with a user_id that doesn’t exist in the users table, MySQL will stop them from doing it.
You can also set what happens when the referenced row changes:
ON DELETE CASCADE: If the user gets deleted, all their orders get deleted too.ON UPDATE CASCADE: If theuser_idchanges (unlikely but possible), the change propagates to theorderstable.ON DELETE SET NULL: If the user is deleted, the foreign key becomesNULL. Note: the column must allowNULLvalues for this to work.
Example:
FOREIGN KEY (user_id) REFERENCES users(user_id) ON DELETE CASCADE ON UPDATE CASCADE
This helps automate cleanup and avoid orphaned records.
When to Use Primary and Foreign Keys
Use a primary key whenever you need to guarantee uniqueness and fast access to rows. Most tables should have one — typically an auto-incrementing integer (AUTO_INCREMENT) or a UUID.
Use a foreign key whenever there's a clear relationship between tables. For example:
- A blog post belongs to an author.
- An invoice has multiple line items.
- A customer can place many orders.
Some common pitfalls to watch out for:
- Forgetting to create indexes on foreign key columns (MySQL usually does this automatically).
- Trying to delete or update a referenced row without handling dependent records first.
- Using strings instead of numeric IDs for foreign keys — slower and more error-prone.
Also, keep in mind that not all storage engines support foreign keys. InnoDB does; MyISAM does not. So always make sure you're using InnoDB unless you have a good reason not to.
That’s basically how primary and foreign keys work in MySQL. They’re pretty straightforward once you get used to thinking in terms of relationships and constraints.
The above is the detailed content of mysql tutorial about foreign keys and primary keys. For more information, please follow other related articles on the PHP Chinese website!
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