**Note: **This is taken from the Chicken Wiki, where a more recent version could be available.

The sql-null extension implements the following interface.

(sql-null)

Return an object, corresponding to a SQL NULL value. The object is guaranteed to be of a type disjoint from all of the R5RS' standard types. It's unspecified whether the values returned by this function will be `eq?' to each other:

(eq? (sql-null) (sql-null)) => unspecified.

(sql-null? OBJECT)

Return #t if OBJECT is a SQL NULL object. Return #f otherwise.

(sql-not OBJECT)

Return OBJECT if OBJECT is a SQL NULL object. Return the value of `(not OBJECT)' otherwise.

(sql-not (sql-null)) => SQL NULL; (sql-not 'a) => #f; (sql-not #f) => #t;

(let ((null (sql-null))) (eq? null (sql-not null))) => #t.

Syntax: (sql-and TEST-1 ...)

The TEST expressions are evaluated from left to right, and the value of the first expression that evaluates to a false value is returned, and any remaining TESTs are not evaluated. If there were no expressions to evaluate to a false value, the value of any of the expressions to evaluate to a SQL NULL is returned. If there were no such expressions as well, #t is returned.

(sql-and 1) => 1; (sql-and #t (sql-null)) => SQL NULL; (sql-and #f (sql-null)) => #f.

In the absence of the expressions that evaluate to SQL NULL values, the semantics is the same as for `(and test-1 ...)'.

Syntax: (sql-or test-1 ...)

The TEST expressions are evaluated from left to right, and the value of the first expression that evaluates to a value, other than SQL NULL and a false value (a ``SQL true'' value), is returned, and any remaining TESTs are not evaluated. If there were no expressions to evaluate to a SQL true value, the value of any of the expressions to evaluate to a SQL NULL is returned. If there were no such expressions as well, #f is returned. In the absence of the expressions that evaluate to SQL NULL values, the semantics is the same as for `(or test-1 ...)'.