Adding Constraints (PRIMARY KEY, FOREIGN KEY, UNIQUE, CHECK)

Constraints help maintain data integrity by enforcing rules on your tables.

Common Constraints

• PRIMARY KEY: Uniquely identifies each row
• FOREIGN KEY: Enforces relationships between tables
• UNIQUE: Ensures all values in a column are distinct
• CHECK: Validates data against a condition

Example: Creating an Enrollments Table

CREATE TABLE enrollments (
id INTEGER PRIMARY KEY,
student_id INTEGER,
course_id INTEGER,
grade INTEGER,
CONSTRAINT fk_student FOREIGN KEY (student_id) 
REFERENCES students(id),
CONSTRAINT chk_grade CHECK (grade >= 0 AND grade <= 100));

This table ensures each enrollment has a valid student reference and a grade between 0 and 100.

Using constraints is essential for building reliable databases.

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