1. A computer implemented method, comprising: holding NULL values in an attribute of a body of records in a database; wherein metadata of said database defines: a constraint on said attribute restricting values in said attribute to non-NULL values, and a default value for said attribute; to compute a query, generating a value for said attribute for a certain record of said body of records that holds a NULL value in said attribute, wherein generating a value includes generating said default value as said value for said attribute.