Any typed data in YQL, including table columns, can be either non-nullable (guaranteed value) or nullable (empty value denoted as
NULL). These values are called "optional" (or "nullable", in SQL terms).
The most common operation for such data types is COALESCE. COALESCE lets you leave the filled values unchanged while replacing
NULL with the default value that follows.
When you declare such data types in the text format, use the question mark at the end (for example,
String?) or the notation
Optional (nullable) isn't a property of a data type or column, but a container type, where containers can be arbitrarily nested in each other. For example, a column with the type
Optional<Optional<Boolean>> can accept 4 values:
NULL of the overarching container,
NULL of the inner container,
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. You can also use
Optional<Optional<T>> as a key lookup in the dictionary (
Optional<T> values. Using this type of result data, you can distinguish between a
NULL value in the dictionary and a missing key.