update column in table with select from same table

[jmodic]

can you explain what are you trying to do? Is TXN_KEY primary key (or unique) in this table?

IF TXN_KEY is PK you don't need any additional select, just do:

update alpha_transactions
set purch_datetime= to_date(lpad(day,2,'0')||'/'||lpad(month,2,'0')||'/'||year||' '||lpad(hours,2,'0')||':'||lpad(minutes,2,'0'),'DD/MM/YYYY HH24:MI')
where....