Essential Database Constraints to Know for Intro to Database Systems

Database constraints are essential rules that ensure data integrity and relationships within a database. They help maintain accurate and reliable information by enforcing unique identifiers, relationships between tables, and valid data entries, making databases function smoothly.

1. Primary Key Constraint

   • Uniquely identifies each record in a table.
   • Cannot contain NULL values; every entry must have a value.
   • A table can have only one primary key, which may consist of one or multiple columns.

2. Foreign Key Constraint

   • Establishes a relationship between two tables by linking a column in one table to the primary key of another.
   • Ensures referential integrity by restricting actions that would create orphaned records.
   • Can accept NULL values unless specified otherwise, allowing for optional relationships.

3. Unique Constraint

   • Ensures that all values in a column (or a set of columns) are distinct across the table.
   • Allows NULL values unless specified, but only one NULL is permitted per column.
   • Can be applied to multiple columns, creating a composite unique constraint.

4. Not Null Constraint

   • Ensures that a column cannot have NULL values, enforcing data integrity.
   • Essential for columns that require mandatory information, such as names or IDs.
   • Can be applied to any column in a table.

5. Check Constraint

   • Validates that values in a column meet a specific condition or set of conditions.
   • Can enforce rules such as value ranges, specific formats, or logical conditions.
   • Helps maintain data integrity by preventing invalid data entries.

6. Default Constraint

   • Assigns a default value to a column when no value is specified during record insertion.
   • Useful for ensuring that columns have a meaningful value even if not explicitly provided.
   • Can be applied to any data type and can simplify data entry processes.

7. Entity Integrity Constraint

   • Ensures that each table has a primary key and that the primary key is unique and not NULL.
   • Maintains the uniqueness of each entity represented in the database.
   • Fundamental for the relational model, ensuring that each record can be distinctly identified.

8. Referential Integrity Constraint

   • Ensures that foreign keys correctly reference existing primary keys in related tables.
   • Prevents actions that would lead to orphaned records, maintaining the logical relationships between tables.
   • Can enforce cascading actions (e.g., delete or update) to maintain data consistency.

9. Domain Constraint

   • Defines the permissible values for a given column based on its data type and any additional rules.
   • Ensures that data entered into a column adheres to specific formats, ranges, or lists of acceptable values.
   • Helps maintain data quality by restricting invalid entries.

10. Assertion

    • A condition that must always be true for the database to maintain integrity.
    • Can encompass complex rules that involve multiple tables or columns.
    • Used to enforce business rules and constraints that go beyond standard constraints.