Handling NULL values

[atqnp]

Hi,

I'm sorry if this has been asked before.
How do I handle NULL values? For example:

SELECT * FROM db WHERE a IS NOT NULL

or

SELECT * FROM db WHERE a IS NULL

is it by just writing blanks when extracting data?
ex: role ==

[jedi1992]

I dont know how to create table(predicate) in logica with null values for the columns.
Tried different things like None, Null, NULL, NULL() etc but could not.

Assuming you are able to do it, then using IsNull(role) == true; works.

p.s IsNull is the camel case of ISNULL big query function

[atqnp]

Thank you very much for the reply!

I tried your suggestion but it needs a small tweak.
I am able to create a predicate using NULL. You have to make sure they are lowercases. I noticed it when I made a mistake when trying to handle a boolean value. Example: True will not work but true will.

Example of predicate with the answer (This is kind of a similar example in CoLab ):

%%logica NoRole

@Engine("sqlite");

Employee(name: "Alice", role: "Product Manager");
Employee(name: "Bob", role: "Engineer");
Employee(name: "Caroline", role: "Engineer");
Employee(name: "David", role: "Data Scientist");
Employee(name: "Eve", role: null);

NoRole(name:) :- Employee(name:, role:),
  IsNull(role);

for NOT NULL, I had to put a negation ~IsNull(role); This works but I don't know if there is a better way.

[EvgSkv]

@jedi1992 Thanks for answering this! @atqnp Great that you figured it out!

Just to confirm:

1. Boolean and null literals are indeed lowercase: true, false, null.
2. Checking for null is done with IsNull(x) function which compiles to SQL's x IS NULL.
3. Indeed for is not null we need to negate IsNull. It's preferred to negate it with ! operator rather than ~, however both works. I've extended section in the tutorial about it.