Define primary key and foreign key in MYSQL?

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In MySQL and other relational database management systems (RDBMS), primary keys and foreign keys are essential concepts used to establish relationships between tables in a database. Let me define each term for you:

Primary Key:

A primary key is a unique identifier for a record in a table. It must contain unique values and cannot have NULL values. Every table in a database should have a primary key because it ensures that each record in the table is uniquely identified. In MySQL, you can define a primary key when creating a table, and the primary key column(s) will enforce the uniqueness and non-null constraints for the table.

Here's an example of creating a table with a primary key in MySQL:

sql code
CREATE TABLE employees (
    employee_id INT PRIMARY KEY,
    first_name VARCHAR(50),
    last_name VARCHAR(50),
    email VARCHAR(100) UNIQUE,
    -- other columns...
);

In this example, employee_id is the primary key of the employees table.

Foreign Key:

A foreign key is a field in a table that is used to establish a link between the data in two tables. It creates a relationship between two tables by referencing the primary key of another table. The table containing the foreign key is called the referencing table, and the table with the primary key being referenced is called the referenced table. Foreign keys ensure referential integrity in the database, meaning that relationships between tables are maintained correctly.

Here's an example of creating a table with a foreign key in MySQL:

sql code
CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    customer_id INT,
    order_date DATE,
    -- other columns...
    FOREIGN KEY (customer_id) REFERENCES customers(customer_id)
);
In this example, customer_id in the orders table is a foreign key that references the customer_id column in the customers table. This establishes a relationship between the orders and customers tables, where customer_id in the orders table must correspond to an existing customer_id in the customers table.

In summary, a primary key uniquely identifies records within a table, while a foreign key establishes a relationship between tables by referencing the primary key of another table.

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