ALTER TABLE Employees
ADD CONSTRAINT DefaultSalary DEFAULT ((100)) FOR [Salary]
This adds a constraint called DefaultSalary which specifies a default of 100 for the Salary column.
A constraint can be added at the table level.
Types of constraints
- Primary Key - prevents a duplicate record in the table
- Foreign Key - points to a primary key from another table
- Not Null - prevents null values from being entered into a column
- Unique - uniquely identifies each record in the table
- Default - specifies a default value
- Check - limits the ranges of values that can be placed in a column
To learn more about constraints, see the Oracle documentation.