Found this on the blog of my friend Lars. I haven't got enough fingers to count the number of times I needed something like this, so I am putting on my own blog as well. :)
A typical cryptic DB2 error message contains something like the following.
Assignment of a NULL value to a NOT NULL column "TBSPACEID=3, TABLEID=14, COLNO=1" is not allowed.
Here is some SQL you can run to determine which table and column such an error is referring to:
SELECT tabschema, tabname, colname FROM syscat.columns WHERE colno = XXXX AND ( tabschema, tabname ) IN (SELECT tabschema, tabname FROM syscat.tables WHERE tbspaceid = yyyy AND tableid = ZZZZ )
Replace XXXX with the column number, YYYY with the table space id and ZZZZ with the table id.