SQL set allowed values for a column
alter table ExampleTable
add (new_column varchar(20) default 'value1',
constraint ckExampleTable check (new_column in ('value1', 'value2', 'value3')));
You should actually do this as two different statements:
ALTER TABLE test
ADD new_column VARCHAR(20) DEFAULT 'value1'
ALTER TABLE test
ADD CONSTRAINT CK_exampleTable_newColumn CHECK (new_column IN ('value1','value2','value3'))