SQLServer ALTER TABLE
Add Column(s)
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.
Drop Column
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).
Add Primary Key
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)