ALTER TABLE "table_name" ADD [CONSTRAINT_NAME] [CONSTRAINT_TYPE] [CONSTRAINT_CONDITION];
ALTER TABLE Customer ADD CONSTRAINT Con_First UNIQUE (Address);
ALTER TABLE Customer drop CONSTRAINT Con_First

The ALTER TABLE statement is used to add, delete, or modify columns in an existing table. To add a column in a table, use the following syntax:
ALTER TABLE table_name ADD column_name datatype

To add or remove constraint in a table, use the following syntax:
ALTER TABLE Person ADD UNIQUE (P_Id)
ALTER TABLE Person DROP CONSTRAINT uc_PersonID
ALTER TABLE Person ADD PRIMARY KEY (P_Id)
ALTER TABLE Person DROP CONSTRAINT pk_PersonID
ALTER TABLE Order ADD FOREIGN KEY (P_Id) REFERENCES Persons(P_Id)
ALTER TABLE Order DROP CONSTRAINT fk_PerOrders
ALTER TABLE Person ADD CHECK (P_Id>0)
ALTER TABLE Person DROP CONSTRAINT chk_Person
ALTER TABLE Person ALTER COLUMN City SET DEFAULT 'SANDNES'
ALTER TABLE Person ALTER COLUMN City DROP DEFAULT

To delete a column in a table, use the following syntax
ALTER TABLE table_name DROP COLUMN column_name

Note: In MS SQL, it is not possible to rename the field name.