SQL Constraints

In SQL, a constraint is a rule that is enforced on a table to ensure that data is entered and stored correctly. Constraints can be applied to one or more columns in a table, and can be used to enforce rules such as uniqueness, data type, and referential integrity.

Here are some common types of constraints in SQL:

1. Primary key constraint: Ensures that each row in a table has a unique identifier. The primary key can be composed of one or more columns, and is used to link tables together in relationships.

2. Foreign key constraint: Ensures that a value in one table corresponds to a value in another table. A foreign key is a column in one table that refers to the primary key of another table.

3. Unique constraint: Ensures that each value in a column is unique. A table can have multiple unique constraints.

4. Not null constraint: Ensures that a column cannot contain null values. This is useful for columns that require data to be entered.

5. Check constraint: Ensures that a value in a column meets a specific condition. For example, a check constraint can be used to ensure that a date is entered in a specific format.

Here is an example of how to create a primary key constraint on a table:

CREATE TABLE customers (
    customer_id INT PRIMARY KEY,
    customer_name VARCHAR(50),
    customer_email VARCHAR(50)
);

In this example, the customer_id column is designated as the primary key for the customers table. This ensures that each row in the table has a unique identifier.

Constraints are an important aspect of SQL table design, as they help ensure data integrity and prevent errors or inconsistencies in the data.