What are Database Constraints?

Database constraints are rules that are applied to the data in a database to ensure its accuracy, integrity, and reliability. These constraints define limitations or conditions on the data that can be stored in the database. They help maintain the quality of the data and prevent inconsistencies or errors. Here are some common types of database constraints:

Primary Key Constraint:

• Definition: Ensures that a column or set of columns uniquely identifies each record in a table.
• Usage: Applied to the primary key column(s) of a table.
• Example: Each employee in an Employee table is uniquely identified by their EmployeeID.

Unique Constraint:

• Definition: Ensures that values in a column or set of columns are unique across all records in a table.
• Usage: Applied to columns where uniqueness is required but not necessarily for primary key purposes.
• Example: Ensuring that all email addresses in a Users table are unique.

Foreign Key Constraint:

• Definition: Establishes a relationship between two tables by linking the foreign key in one table to the primary key in another table.
• Usage: Applied to the foreign key column(s) of a table.
• Example: Connecting the EmployeeID column in an Orders table to the EmployeeID column in an Employees table.

Check Constraint:

• Definition: Enforces a condition that values in a column must satisfy for a record to be valid.
• Usage: Applied to a column to specify a condition that must be true for each record.
• Example: Ensuring that a "Quantity" column in an Order table is always greater than zero.

Default Constraint:

• Definition: Specifies a default value for a column in case no value is explicitly provided during an insert operation.
• Usage: Applied to a column to set a default value.
• Example: Setting the default value of a "Status" column to "Active."

Not Null Constraint:

• Definition: Ensures that a column cannot have a null (empty) value.
• Usage: Applied to a column where the presence of a value is mandatory.
• Example: Ensuring that the "Username" column in a Users table cannot be null.

These constraints collectively play a crucial role in maintaining the integrity and consistency of the data stored in a database, providing a set of rules that govern the allowable values and relationships within the database tables.

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