What is domain constraints in DBMS?

Answer 1

each column contains same type of data thus when you select a data type for a particular domain then DBMS will not accept any value of other data type.

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Domain constraints are a user-defined data type which enforces the integrity of the standard data types. A domain consists of all values permitted in a column. In constructing your database you would first select a standard data type such as VARCHAR or INT. You could then constrain or limit the data using NOT NULL, UNIQUE, CHECK, PRIMARY KEY or FOREIGN KEY. For example the CHECK constraint could limit INT data within your domain [column] to numbers less than 5000. You would thus define a custom data type using domain constraints.

Answer 2

Constraints within a database are rules which control values allowed in columns and also enforce the integrity between columns and tables.

An example of a column constraint would be a 'CHECK' constraint on a column to limit the values allowed. For example, you could specify the datatype of a column to be tinyint, which can store values from 0-255, but then specify a CHECK constraint that limits values of 1-99, like this;

CREATE TABLE my_example (SomeColumn tinyint, CONSTRAINT

ChkValue CHECK (SomeColumn BETWEEN 1 and 99 ))

Another common constraint is UNIQUE, which ensures that the value in a column is unique within the table. This would be used on something like a customer code in a customer table, where you need to ensure that no two customers have the same code. This can also be done with a PRIMARY KEY constraint.

To enforce integrity, you can specify that a column has a FOREIGN KEY relationship with the values in another table, a concept at the heart of a relational database. An example of this would be a FK constraint on a 'sales order' table linking the order customer code back to the customer table. The FK serves two functions; (1) The code entered on the order must be valid and (2) the customer can't be deleted if there are any orders.

The simplest and most common constraint is the nullability of a column, specified with the NULL and NOT NULL operators when creating columns;

CREATE TABLE MyExample (ID int NOT NULL, Description varchar(50) NULL).