I have an Oracle background where a decode instruction was never far away. Nowadays I use MySQL for lots of stuff and when I needed a decode, I was stuck for a few minutes.
The query below changes a table which previously had both a 'note' and a 'description' field. These were combined with the following query in a new field 'descr'. After the update, the old fields could both be dropped from the table.
update tscu set descr = ( case
when length(description) = 0 and length(note) > 0 then note
when length(note) = 0 then description
when length(description) > 0 and length(note) > 0 then concat(description, ' Note: ', note) end )