SQL to find Unit whose value equals and not in

Say, we have table_1 like - 

unit | req_type

A     |   1

A     |   2

B     |   2

B     |   3

D     |   2

E     |   2

E     |   4

We have to write a SQL such that, 

1. we can select Unit with Req_Type=2
2. Also, that Unit should not have any other Req_Type in list. Ex - 1,3

Solution - 

We can add another column such that -  case when  req_type in (1,3) then 1 else 0 end col1

unit | req_type | col1

A     |   1          |   1

A     |   2          |   0

B     |   2          |   0

B     |   3          |   1

D     |   2          |   0

E     |   2          |   0

E     |   4          |   0

Then we can do a group by and take count(col1)

unit | c

A     | 1

B     | 1

D     | 0

E     | 0

Then we can select all rows which have count = 0

Compete SQL - 

select unit from (select unit, sum(col1) c  from (select unit, req_type, case when  req_type in (1,3) then 1 else 0 end col1 from  table_1) group by unit) where c=0