use of constraint key word and select record from table with condition

Use of constraint key word and selection of  record from table with condition 

--first create database………………………..

create database databasename

--select database …..

use databasename

--creating table with CONSTRAINT keyword record entry. SECOND COLUMN value will be  either ‘f’ or ‘m’ . if user enter exempt ‘f’ or ‘m’ the sql server generate error.  

create table stu_gen (roll int primary key IDENTITY(1,1), gen nvarchar(1) CONSTRAINT gen_CONSTRAINT CHECK (gen IN ('f','m')))

--INSERT RECORD IN STU_GEN TABLE

insert into stu_gen(gen) values('f')

insert into stu_gen(gen) values('m')

insert into stu_gen(gen) values('f')

insert into stu_gen(gen) values('m')

insert into stu_gen(gen) values('f')

insert into stu_gen(gen) values('f')

insert into stu_gen(gen) values('m')

insert into stu_gen(gen) values('f')

--RECORD SELECTION WITH CONDITION

 select roll,Gender=(select Gender= case when

stu_gen.gen='m' then 'Male'

else

'Female'

end)

from stu_gen