What is the meaning of constraints

Answer 1

constraint - kənˈstreɪnt/ noun
Definition: a limitation or restriction.

E.g. You use constraints when you want a specific attribute in your entity to contain only unique values, or not to contain any null values, or to check if the inserted value is valid based on some condition, etc.

Answer 2

they are basically limitations,rules

Answer 3

Constraints are used to prevent invalid data entry into relational database.These can be added at the time of table creation or can be added later by using the "ALTER" keyword.