Vergleich in einer errechneten Spalte

test=# create table tomon(datum1 timestamp, datum2 timestamp);
CREATE TABLE
test=*# insert into tomon values (now(), now()+'1minute'::interval);
INSERT 0 1
test=*# insert into tomon values (now(), now()+'100minute'::interval);
INSERT 0 1
test=*# select * from tomon ;
           datum1           |           datum2           
----------------------------+----------------------------
 2019-07-04 14:33:18.408628 | 2019-07-04 14:34:18.408628
 2019-07-04 14:33:18.408628 | 2019-07-04 16:13:18.408628
(2 rows)

test=*# select *, case when datum2-datum1 > '3minutes'::interval then 'x' else null end as "more than 3 minutes" from tomon ;
           datum1           |           datum2           | more than 3 minutes
----------------------------+----------------------------+---------------------
 2019-07-04 14:33:18.408628 | 2019-07-04 14:34:18.408628 |
 2019-07-04 14:33:18.408628 | 2019-07-04 16:13:18.408628 | x
(2 rows)

test=*#