Sql ALTER TABLE
ALTER command in SQL is used to modify column/constraint in a 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 the 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)
Alter Column
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 date to datetime and set default to current date.
Drop Constraint
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.
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