Thanks for replying

dwain.c I hadn't thought of that

Jeff There are no pre-existing triggers.

I tried the periodical job but it takes ages to run. I'd expected first time to run to take ages as many rows had non blank column values. I suspect even the second run would take a long time: The columns are not part of an index and some of the columns are TEXT datatype

and default to '' which I can leave in.

so

UPDATE tbl###

SET Fld1= NULL, Fld2=NULL, Fld3=NULL

WHERE Fld1 IS NOT NULL OR Fld2 IS NOT NULL OR datalength(Fld3) <> 0

I use

datalength(Fld3) <> 0

because I don't mind if the TEXT field Fld3 = ''.