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NULL fields are used to represent "missing information and inapplicable information." The use of NULL allows a table to distinguish between an unknown value and a value of 0 or FALSE.
To properly handle NULL, the table must contain the $NULFLD$ field, a hidden field generated by c-treeDB at creation time. Tables created with the c-treeDB Interface (used under FairCom DB SQL) have a hidden field, $NULFLD$, which is used to determine if each user-created field in the record buffer has a NULL value. FairCom DB SQL requires this capability to implement constraints.
c-treeDB and FairCom DB SQL will access tables without the $NULFLD$ field, but the table’s fields will always return a non-NULL status.
The ctdbClearRecord() function handles NULL fields when $NULFLD$ support is available.
