SQL Constraints

SQL Constraints are rules that define the relationships and dependencies between tables in a relational database. They act as a safeguard, preventing the entry of inconsistent or incorrect data into the database. By enforcing these constraints, SQL ensures that the data adheres to a predefined structure, maintaining the overall integrity of the database.

In the below PDF we discuss about SQL Constraints in detail in simple language, Hope this will help in better understanding.

SQL Tutorial

Types of SQL Constraints:

1. Primary Key Constraint:

  • A primary key is a unique identifier for each record in a table.
  • It ensures that each record can be uniquely identified, preventing the entry of duplicate or null values.
  • The primary key constraint enforces the integrity of relationships between tables.

2. Foreign Key Constraint:

  • A foreign key establishes a link between two tables, creating a relationship between them.
  • It ensures referential integrity by enforcing that values in one table correspond to existing values in another table.
  • Foreign keys are crucial in maintaining consistency across interconnected tables.

3. Unique Constraint:

  • The unique constraint ensures that values in a specified column or a combination of columns are unique across all records in the table.
  • It prevents the insertion of duplicate values, promoting data accuracy.

4. Check Constraint:

  • Check constraints are used to limit the range of values that can be inserted into a column.
  • They enforce specific conditions, ensuring that only valid data is entered into the database.

5. Not Null Constraint:

  • The not null constraint ensures that a column does not contain any null values.
  • It is employed to enforce the presence of data in a particular field, avoiding inconsistencies.

Benefits of SQL Constraints:

  • Data Integrity: SQL constraints maintain the accuracy and consistency of data by preventing the entry of invalid or conflicting information.
  • Relationships and Dependencies: Foreign key constraints establish relationships between tables, fostering a structured and interconnected database design.
  • Query Optimization: Well-defined constraints enable the database management system to optimize query execution, resulting in improved performance.
  • Enforcing Business Rules: Check constraints play a crucial role in enforcing business rules, ensuring that data adheres to specific conditions.

Related Question

SQL constraints are rules defined on a table column or a set of columns that enforce data integrity within the database. They specify the conditions that data in a table must meet, ensuring accuracy, consistency, and reliability.

Constraints play a crucial role in maintaining the quality of data stored in a database. They prevent the insertion of incorrect or inconsistent data, enforce relationships between tables, and contribute to overall data integrity.

To remove a constraint from a table in SQL, you can use the ALTER TABLE statement with the DROP CONSTRAINT clause.

To add a Foreign Key constraint to an existing table, you can use the ALTER TABLE statement.

A primary key is a unique identifier for a record in a table. It ensures that each row in a table can be uniquely identified and helps establish relationships between tables. A primary key column cannot contain null values, and there can be only one primary key in a table.


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