ALTER TABLE Employees ADD StartingDate date NOT NULL DEFAULT GetDate(), DateOfBirth date NULL
The above statement would add columns named
StartingDate which cannot be NULL with default value as current date and
DateOfBirth which can be NULL in Employees table.
ALTER TABLE Employees DROP COLUMN salary;
This will not only delete information from that column, but will drop the column salary from table employees(the column will no more exist).
ALTER TABLE Employees DROP CONSTRAINT DefaultSalary
This Drops a constraint called DefaultSalary from the employees table definition.
Note:- Ensure that constraints of the column are dropped before dropping a column.
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
To learn more about constraints, see the Oracle documentation.
ALTER TABLE Employees ALTER COLUMN StartingDate DATETIME NOT NULL DEFAULT (GETDATE())
This query will alter the column datatype of
StartingDate and change it from simple
datetime and set default to current date.
ALTER TABLE EMPLOYEES ADD pk_EmployeeID PRIMARY KEY (ID)
This will add a Primary key to the table Employees on the field
ID. Including more than one column name in the parentheses along with ID will create a Composite Primary Key. When adding more than one column, the column names must be separated by commas.
ALTER TABLE EMPLOYEES ADD pk_EmployeeID PRIMARY KEY (ID, FName)