Data types accepting NULL

Any typed data in YQL, including table columns, can be either non-nullable (guaranteed value) or nullable (empty value denoted as NULL). Data types that can include NULL values are called optional or, in SQL terms, nullable.

Optional data types in the text format use the question mark at the end (for example, String?) or the notation Optional<...>.
The following operations are most often performed on optional data types:

• IS NULL: Matching an empty value
• COALESCE: Leave the filled values unchanged and replace NULL with the default value that follows
• UNWRAP: Extract the value of the original type from the optional data type, T?. is converted to T
• JUST: Add optionality to the current type, T is converted to T?.
• NOTHING: Create an empty value with the specified type.

Optional (nullable) isn't a property of a data type or column, but a container type where containers can be arbitrarily nested into each other. For example, a column with the type Optional<Optional<Boolean>> can accept 4 values: NULL of the whole container, NULL of the inner container, TRUE, and FALSE. The above-declared type differs from List<List<Boolean>>, because it uses NULL as an empty list, and you can't put more than one non-null element in it. In addition, Optional<Optional<T>> type values are returned as results when searching by the key in the Dict(k,v) dictionary with Optional<T> type values. Using this type of result data, you can distinguish between a NULL value in the dictionary and a situation when the key is missing.

Example

$dict = {"a":1, "b":null};
$found = $dict["b"];
select if($found is not null, unwrap($found), -1);

Result:

# column0
null

Logical and arithmetic operations with NULL

The NULL literal has a separate singular Null type and can be implicitly converted to any optional type (for example, the nested type Optional<Optional<...Optional<T>...>>). In ANSI SQL, NULL means "an unknown value", that's why logical and arithmetic operations involving NULL or empty Optional have certain specifics.

Examples

SELECT
    True OR NULL,        -- Just(True) (works the same way as True OR <unknown value of type Bool>)
    False AND NULL,      -- Just(False)
    True AND NULL,       -- NULL   (more precise than Nothing<Bool?> – <unknown value of type Bool>)
    NULL OR NOT NULL,    -- NULL   (all NULLs are "different")
    1 + NULL,            -- NULL   (Nothing<Int32?>) - the result of adding 1 together with
                         --         unknown value of type Int)
    1 == NULL,           -- NULL   (the result of adding 1 together with unknown value of type Int)
    (1, NULL) == (1, 2), -- NULL   (composite elements are compared by component
                         --         through `AND`)
    (2, NULL) == (1, 3), -- Just(False) (expression is equivalent to 2 == 1 AND NULL == 3)