How to specify TTL(Time to Live) for a Column?

cqlsh:ks> // Create table with Primary Key
cqlsh:ks> drop table student;
cqlsh:ks> create table student(
      ...    name text,
      ...    class text,
      ...    primary key(name));
cqlsh:ks>
cqlsh:ks> // By default TTL value of a Column is null #64
cqlsh:ks> insert into student(name, class) values('Bob', '1st');
cqlsh:ks> select class, TTL(class) from student;

 class | ttl(class)
-------+------------
   1st |       null

(1 rows)
cqlsh:ks>
cqlsh:ks> // TTL Cannot be retrieved for Primary Key
cqlsh:ks> select name, TTL(name) from student;
InvalidRequest: Error from server: code=2200 [Invalid query] message="Cannot use selection function ttl on PRIMARY KEY part name"
cqlsh:ks>
cqlsh:ks> // Specify TTL(for 15 seconds) for a Column
cqlsh:ks> update student using TTL 15 set class='2nd' where name='Bob';
cqlsh:ks> select name, TTL(class) from student;

 name | ttl(class)
------+------------
  Bob |         14

(1 rows)
cqlsh:ks> select name, TTL(class) from student;

 name | ttl(class)
------+------------
  Bob |       null

(1 rows)
// Note : We do not have any mechanism to provide TTL at row level currently