What is a default constraint?
Ankur Jain
The default constraint is used to insert a default value into a column. The default value will be added to all new records, if no other value is specified, including NULL.
Default constraints are a special case of column defaults. A column default is some value or function that the column will take when an INSERT statement doesn't explicitly assign a particular value. In other words, the column default is what the column will get as a value by default.
A default constraint is a value that SQL Server automatically places in a particular field in a table.