`nullManagement`

The library nullManagement implements support functions for null management.

isnull

returns true if X is a marked null, false otherwise

ifnull

returns A if X is null, B otherwise

coalesce

returns X if X is not null OR if both X and Y are null; returns Y otherwise

`isnull`

Returns true if X is a marked null, false otherwise.

isnull(X)

Where:

X is the value to be checked if it is a marked null.

Example

@library("null:", "nullManagement").
p(1).
p1(A,B) :- p(A). 
v1(X):- p1(A,B), X = null:isnull(B).
@output("v1").

Expected results

v1(true)
v1(false)

`ifnull`

Returns A if X is null, B otherwise.

ifnull(X, A, B)

Where:

X is the value to be checked if it is null.
A is the value to return if X is null.
B is the value to return if X is not null.

Example

@library("null:", "nullManagement").
v3(Residence,Gender,Education,Labor,Weight,RowId,X) :-
v1(Residence,Gender,Education,Labor,Weight,RowId),
X=null:ifnull(Labor,Residence,Education).
@output("v3").

Expected results

v3("Res1", "Male", "Edu1", null, 70, 1, "Res1")
v3("Res2", "Female", "Edu2", "Labor2", 65, 2, "Labor2")

`coalesce`

Returns X if X is not null OR if both X and Y are null; returns Y otherwise.

coalesce(X, Y)

Where:

X is the first value to check.
Y is the second value to check.

Example

@library("null:", "nullManagement").
p(1).
p1(A,B) :- p(A).
v1(X):- p1(A,B), X = null:coalesce(B, A).
@output("v1").

Expected results

v1(1)
v1(A)