When learning SQL, understanding keys and constraints is an important step toward designing reliable and well‑structured databases. These concepts help maintain data accuracy, avoid duplication, and create meaningful relationships between tables. The handwritten notes on “Keys and Constraints in SQL” present these ideas in a simple, visual, and exam‑friendly way, making them easier for beginners to grasp.
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Why Keys and Constraints Matter in SQL
In a database, data is stored in tables, and each table can grow large very quickly. Without proper rules, data can become messy, duplicated, or incorrect. Keys and constraints act like rules and identifiers, ensuring the data remains organized and trustworthy.
Primary Key: Identifying Each Record Uniquely
A Primary Key is a column (or set of columns) used to uniquely identify each row in a table.
Key points about a Primary Key:
- It must be unique
- It cannot be NULL
- Each table can have only one primary key
Simple example:
Here, the id column ensures that every record in the table can be identified without confusion. In handwritten notes, this concept is often shown with a key symbol next to a table, helping students visually remember that the primary key is special.
Foreign Key: Connecting Tables Together
A Foreign Key is used to create a link between two tables. It refers to the primary key of another table.
Why it is useful:
- Connects related data across tables
- Maintains data consistency
- Avoids duplicate information
Simple example:
- A
Studenttable has a primary keyid - A
Coursetable containsstudent_idas a foreign key
This relationship shows which student is enrolled in which course.
Constraints: Rules That Protect Your Data
Constraints are rules applied to table columns to control the type of data that can be stored. T
NOT NULL Constrain
The NOT NULL constraint ensures that a column cannot have empty values.
Meaning:
Every record must contain data in that column.
Example:
name VARCHAR(50) NOT NULL
This means the name field cannot be left blank.
UNIQUE Constraint
The UNIQUE constraint ensures that all values in a column are different.
Purpose:
- Prevents duplicate entries
- Helps maintain data uniqueness
Example:
No two records can have the same email address.
DEFAULT Constraint
The DEFAULT constraint assigns a value automatically when no value is provided.
Helpful when:
- A common value is needed frequently
- Data should never remain empty
Example:
If no salary is entered, the value 30000 is stored automatically.
CHECK Constraint
The CHECK constraint ensures that data follows a specific condition.
Used to:
- Validate data before storing it
Example:
CHECK (age >= 18
This ensures that only valid ages (18 or above) are allowed.
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