Domain Constraint

It shows that each attribute in a relation has to contain an atomic value only from the corresponding domains. The data types associated with commercial RDBMS domains include:

1) Real numbers (double precision floats, float,)

2) Standard numeric data types for integer (such as integer, short- integer,  long integer)

3)  Characters

4)  Fixed length strings and variable length strings.

Therefore, domain constraint shows the condition that we want to put on every instance of the relation. So the values that show in each column must be drawn from the domain associated with that column.

For instance, consider the relation:

STUDENT

In the relation above, AGE of the relation STUDENT always relates to the integer domain within a specified range (if any), and not to strings or any other domain. Within a domain non-atomic values should be avoided. This much time cannot be checked by domain constraints. For instance, a database which has area code and phone numbers as two dissimilar fields will take phone numbers as-

A non-atomic value in this case for a phone can be 1129534466, Though, this value can be accepted by the Phone field.