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Understanding SQL Constraints: A Guide to Data Integrity and Security

Constraints are essential tools for ensuring data integrity and security in a database. In our brief overview, we will explain the five common SQL constraints and their significance in shaping your databases.

When designing a database, you often want to impose specific restrictions on the data in certain columns. For example, when creating a table for skyscrapers, you want to ensure that the height column does not contain negative values.

Relational Database Management Systems (RDBMS) allow you to achieve this using constraints. A constraint is a rule applied to a column or table, specifying what changes can be made to the data in a table, whether through an INSERT, UPDATE, or DELETE statement.

What are Constraints?

In SQL, a constraint is a rule applied to a column or table, governing what data can be entered into it. When you perform an operation that alters the data in a table, such as an INSERT, the RDBMS checks if the data violates existing constraints and returns an error if it does.

Constraints are useful for enforcing business rules and ensuring data integrity. They also aid in maintaining relationships between tables.

The Five Common SQL Constraints

PRIMARY KEY Constraint

The PRIMARY KEY constraint ensures that values in a column are unique and not NULL, allowing you to identify rows in a table. In relational databases, a primary key is a special type of key used to uniquely identify rows. An SQL primary key combines UNIQUE and NOT NULL constraints and automatically generates an index for efficient data queries. A primary key consists of a minimal set of attributes to uniquely identify rows. In some cases, a natural key derived from observable data is suitable, while in others, a surrogate or synthetic key is recommended for consistency.

FOREIGN KEY Constraint

The FOREIGN KEY constraint enforces that values in a table (child table) already exist in another table (parent table), creating relationships between tables. For example, the primary key of the EMPLOYEES table is referenced by the foreign key in the SALES table. When a record is added to the child table, the value in the foreign key must exist in the parent table to maintain relationship integrity. The foreign key can reference UNIQUE or PRIMARY KEY columns in the parent table.

UNIQUE Constraint

The UNIQUE constraint prevents duplicates in a column, ensuring unique values. It is useful for enforcing one-to-one relationships between tables. UNIQUE constraints can be defined at the column or table level. At the table level, they can be applied to multiple columns, requiring each row to have a unique combination of values in the constrained columns.

CHECK Constraints

CHECK constraints are requirements for columns defined as predicates. These predicates are expressions that return either TRUE, FALSE, or UNKNOWN. When a value is inserted into such a column and the predicate evaluates to TRUE or UNKNOWN (including NULL values), the operation is successful. If it evaluates to FALSE, the operation fails. CHECK predicates often use mathematical comparison operators (e.g., <, >, <=, >=) to restrict data in the column. They ensure compliance with specific rules, such as ensuring no negative prices appear in a product table. The CREATE TABLE statement provides an example of how to create a CHECK constraint for positive prices. CHECK predicates can use various SQL operators, including LIKE, BETWEEN, and IS NOT NULL. Some implementations even allow subqueries but typically do not permit references to other tables in these predicates.

NOT NULL Constraint

The NOT NULL constraint prevents NULL values in a column. In SQL, NULL is used to represent unknown values. The constraint enforces that a value must always be provided for this column; otherwise, the INSERT operation will fail.


Constraints are indispensable tools for anyone looking to design a database with a high level of data integrity and security. By limiting what can be entered into a column, you can ensure that relationships between tables are maintained correctly and that the database adheres to the business rules defining its purpose.

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