In a database management system (DBMS), a key constraint is a rule that is used to enforce data integrity and maintain the relationships between data in different tables. The following are some common key constraints in DBMS:

1. Primary key: A primary key is a column or set of columns that uniquely identifies each row in a table. No two rows can have the same primary key value. A primary key can be used to enforce relationships between tables using foreign keys.
2. Foreign key: A foreign key is a column or set of columns in a table that references the primary key of another table. A foreign key is used to enforce relationships between tables and ensure data integrity.
3. Unique key: A unique key is a column or set of columns that must be unique within a table, but can be NULL. This means that no two rows can have the same unique key value, but a row can have a NULL value for the unique key.
4. Not null: A not null constraint specifies that a column must contain a non-NULL value. This ensures that every row in the table has a value for the specified column.

By using key constraints, a DBMS can ensure that data is organized and stored in a consistent and meaningful way, and that relationships between data are maintained. This helps to prevent errors and inconsistencies in the data and enables the DBMS to enforce business rules and constraints.