How to append to a existing record in SQL?

in Oracle, the string concatenation goes like this:

field1 = field1 || '12345'

Your statement should work as long as field1 is not null or the data to be appended is not null.

Something like this could help in the case where field1 is null.

update table1 set field1 = ISNULL(field1, '') + '123456' where field2 = '12'

Your question is a bit confusing because you are saying append but your example is really just a set operation:

update table1 set field1 = '123456', field2 = '' where field2 = '12'

if you were actually appending it would depend on your database but lookup string concatenation for reference.

update table set field1 = concat(field2, '3456') where field2 = '12'