the partial key with NULLs is considered an actual row key. One mode might be easily implementable with the new index method, if we want partial foreign keys to be able to reference partial keys in the referred table, i.e. One can imagine a further branching point in behaviors for foreign keys and I'm not sure how MATCH PARTIAL will be defined. As far as I know, the MATCH PARTIAL case is not implemented, but I think it might complement this new UNIQUE NULLS NOT DISTINCT behavior?
The MATCH FULL behavior is almost a style checker to say that a compound foreign key should be all null or all non-null, while still ignoring incomplete foreign keys. An incomplete foreign key is essentially ignored and not subject to integrity checks. The default MATCH SIMPLE behavior is consistent with the default UNIQUE constraint behavior. There is also a similar issue with FOREIGN KEY constraints. The new optional behavior makes more sense if you think that the uniqueness constraint enforces distinct records even with partial keys, just as the GROUP BY clause will determine distinct groups. The check is effectively deferred, and the partial or incomplete tuple might become valid and unique once its missing part(s) are filled in with real values. The prior (new default) UNIQUE constraint behavior made sense when you think of NULL as this bottom value and meaning "unknown", while also thinking that a uniqueness constraint is only applied to complete keys. It just returns a group for each distinct value. The existing GROUP BY behavior is different than WHERE because it does not have a special rule for handling true values. SELECT 'hidden by false result' WHERE NOT (CAST(NULL AS boolean) IS NULL) SELECT 'shown by true result' WHERE CAST(NULL AS boolean) IS NULL SELECT 'also hidden by null result' WHERE NOT CAST(NULL AS boolean) SELECT 'hidden by null result' WHERE CAST(NULL AS boolean) , because like equality the NOT operator will return NULL for a NULL input (so NULL is not like false and does not negate to true). Not understanding this distinction is also where people write buggy queries using WHERE NOT. This treats both false and null cases as the other possible outcome, when rows are not returned. It is the `WHERE` clause semantics, which are defined as returning rows when the boolean expression is true. It's not the `=` operator that is special or significant here.
NULL adds an extra state to every type much like the "bottom" type/value in some functional programming languages or type systems. It’s too late to worry about it now, just get on with your life.I think that this topic gets tripped up with people wanting to assume boolean means two possible states. For example, “Don’t cry over spilled milk” means “Don’t get upset over something that has already been done. The literal meaning of a proverb such as “Don’t cry over spilled milk” does makes sense on its own, but it’s not until you apply this meaning to a broader set of situations that you understand the real point of the proverb. The literal meaning of an idiom usually doesn’t make sense, and idioms can be almost impossible to understand unless you have learned or heard them before. Like idioms, proverbs often have a meaning that is greater than the meaning of the individual words put together, but in a different way than idioms.
“She passed the test by the skin of her teeth” means she almost didn’t pass.Ī proverb is a short popular saying that gives advice about how people should behave or that expresses a belief that is generally thought to be true. by the skin of your teeth means that something was successful, but only just barely.to rub someone the wrong way means to irritate someone.to be fed up with means to be tired and annoyed with something that has been happening for too long.What is the difference between idioms and proverbs? -Saurabh Singh, IndiaĪn idiom is a phrase that has a meaning of its own that cannot be understood from the meanings of its individual words.