If the name of the variables are not in a consequential way, you can use the following syntax:
data test;
input id $ citing_1 citing2a citing2b;
cards;
1 11 12 13
2 21 22 23
;
proc sql;
select id, citing_1 from work.test
union all
select id, citing2a from work.test
union all
select id, citing2b from work.test;