> Note that the truth value unknown is indistinguishable from the null for the Boolean type. Otherwise, the Boolean type would have four logical values.
Although they are very closely related, I believe the general `null` value (for any data type) and three-valued logic (null for Boolean) should be separated (that's why I have two articles).
> “Every [SQL] data type includes a special value, called the null value,” “that is used to indicate the absence of any data value”.
(http://modern-sql.com/concept/null)
For the Boolean type, null = unknown:
> Note that the truth value unknown is indistinguishable from the null for the Boolean type. Otherwise, the Boolean type would have four logical values.
(http://modern-sql.com/concept/three-valued-logic)
Although they are very closely related, I believe the general `null` value (for any data type) and three-valued logic (null for Boolean) should be separated (that's why I have two articles).