Enforcing Primary key and foreign key.

What is the Key?

In the realm of database management systems, a key refers to a particular attribute or a group of attributes employed to distinguish each record, often called a tuple, within a table uniquely. Keys hold significant importance in the structure of a relational database, as they serve to organize data and create connections between tables. Within a database, various types of keys exist, each serving a distinct function. Keys are pivotal components in relational databases, as they ensure record uniqueness, facilitate table relationships, and enhance overall database performance.

1. Enforcing a Primary Key

A primary key is a unique identifier for each record in a table. To enforce a primary key constraint, follow these steps:

Define a primary key when creating a table using the 'PRIMARY KEY' constraint.

CREATE TABLE employees ( employee_id INT PRIMARY KEY, first_name VARCHAR(50), last_name VARCHAR(50) );

If you're altering an existing table to add a primary key, use the ALTER TABLE statement.

ALTER TABLE employees ADD PRIMARY KEY (employee_id);

The primary key constraint enforces uniqueness, meaning that no two rows in the table can have the same value for the primary key column(s).

2. Enforcing a Foreign Key

A foreign key is a field in one table that refers to the primary key in another table, establishing a relationship between the tables. To enforce a foreign key constraint, follow these steps:

Define a foreign key when creating a table using the 'FOREIGN KEY' constraint.

CREATE TABLE orders (

order_id INT PRIMARY KEY,

customer_id INT, order_date DATE,

FOREIGN KEY (customer_id) REFERENCES customers(customer_id)

);

The 'REFERENCES' clause specifies the table and column that the foreign key references. In this example, it references the 'customer_id' column in the 'customers' table.
Ensure that the referenced column in the referenced table (in this case, 'customers' in the 'customers' table) has a primary key constraint.

Foreign key constraints enforce referential integrity, ensuring that data in the referencing table (in this case, the 'orders' table) is consistent with data in the referenced table (in this case, the 'customers' table).

Enforcing these constraints helps maintain data quality and consistency in your database, preventing the insertion of invalid or inconsistent data and facilitating data relationships between tables.

Difference between Primary Key and Foreign Key

Primary keys and foreign keys are both fundamental components of relational database design, but they serve different purposes and have distinct characteristics. Here are the key differences between primary keys and foreign keys:

Primary Key

1. **Uniqueness:** A primary key is a column or a set of columns in a table that uniquely identifies each row. Every value in the primary key column(s) must be unique within the table.

2. **Required:** A primary key is required for every table. It ensures that each row in the table can be uniquely identified.

3. **Constraints:** Primary keys enforce data integrity by preventing duplicate and null values in the primary key column(s).

4. **Indexed:** By default, primary key columns are automatically indexed, which can improve query performance.

5. **One per Table:** Each table can have only one primary key.

6. **Used for Joins:** Primary keys are often used as reference points for creating relationships with other tables through foreign keys.

**Foreign Key:**

1. **Relationships:** A foreign key is a column or a set of columns in one table that establishes a link or relationship between data in two tables. It references the primary key of another table.

2. **Referential Integrity:** Foreign keys enforce referential integrity by ensuring that data in the referencing table (child table) corresponds to data in the referenced table (parent table).

3. **Optional:** While foreign keys are commonly used to create relationships, they are not required in every table. Tables can exist without foreign keys.

4. **Values Must Match:** Values in the foreign key column(s) must match values in the primary key column(s) of the referenced table.

5. **Can Have Multiple:** A table can have multiple foreign keys, each referencing a different table and primary key.

6. **Used for Joins:** Foreign keys are used to join related tables, allowing you to retrieve data from multiple tables based on their relationships.

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