The null value

Predicates in relational databases use three-valued logic (3VL), where there are three truth values: true, false, and null,. In a relational database, the third value, null, can be interpreted in many ways, such as unknown data, missing data, not applicable, or will be loaded later. The 3VL is used to remove ambiguity. For example, no two null values are equal.

In the next chapter, you will learn how to connect to the database and run queries. Now, I would like to show how a logical OR/AND truth table can be generated by the SQL language:

\pset null null
WITH data (v) as (VALUES (true), (false),(null)) 
    SELECT DISTINCT 
        first.v::TEXT as a, 
        second.v::TEXT as b, 
        (first.v AND second.v)::TEXT AS "a and b", 
        (first.v OR second.v)::TEXT as "a or b" 
    FROM 
        data as first cross join 
        data as second 
    ORDER BY a DESC nulls last, b DESC nulls last;
   a   | b     | a and b | a or b 
-------+-------+---------+--------
 true  | true  | true    | true
 true  | false | false   | true
 true  | null  | null    | true
 false | true  | false   | true
 false | false | false   | false
 false | null  | false   | null
 null  | true  | null    | true
 null  | false | false   | null
 null  | null  | null    | null
(9 rows)

The following table, which is generated by SQL, shows the NOT truth operator:

WITH data (v) as (VALUES (true), (false),(null)) SELECT v::text as a, (NOT v)::text as "NOT a" FROM data order by a desc nulls last;
   a   | NOT a 
-------+-------
 true  | false
 false | true
 null  | null
(3 rows)