Data Constraints and Classification of Constraints in Database Tables

Column Constraints

• A column-level constraint is applied specifically to a single column in a database table.

NOT NULL Constraint

• The NOT NULL constraint ensures that a specific value within a column cannot be NULL, enforcing data integrity.

UNIQUE Constraint

• The UNIQUE constraint prevents duplicate values in a column, ensuring that each entry remains distinct across the dataset.

Table Level Constraints

• A table-level constraint applies to the entire table instead of individual columns, affecting multiple columns or the table as a whole.

Primary Keys

• A table can have only one primary key, which uniquely identifies each record within the table.

Components of a Primary Key

• A PRIMARY KEY consists of one or more columns that must have unique values for each row, ensuring no two records are the same.

NULL Value Prevention

• The UNIQUE constraint does not prevent NULL values and allows duplicates as long as the non-null entries are unique.

DEFAULT Constraint Behavior

• If a column includes a DEFAULT constraint, it automatically assigns a specified value to that column when no value is provided during an INSERT operation.

CHECK Constraint

• The CHECK constraint allows for the definition of specific conditions for a column's values, ensuring that only valid data is entered into the database.

Significance of Composite Keys

• In a composite key, having multiple columns as part of the key ensures a unique combination of values, enhancing record identification beyond single columns.