About
Last week I wrote about adding a feature to our SPARQL Update’s INSERT OR REPLACE. With that feature it’s not needed to put a DELETE upfront the INSERT to clear a field. This makes our SPARQL-ish INSERT OR REPLACE in some ways more powerful than SQL’s UPDATE. Note, however, that all of INSERT OR REPLACE is non-standard in the SPARQL language. And this new null support certainly isn’t.
Support for null with INSERT OR REPLACE is now available in Tracker‘s master branch. How to use it is illustrated in the functional test. I’ll briefly explain the test.
For single value properties:
This is of course very simple.
INSERT { <subject> nie:title 'new test' }
INSERT OR REPLACE { <subject> nie:title null }
If you now select nie:title for <subject> then of course you’ll get that its nie:title field is unset.
For multi value properties:
Begin situation:
INSERT { <subject> a nie:DataObject, nie:InformationElement }
INSERT { <ds1> a nie:DataSource }
INSERT { <ds2> a nie:DataSource }
INSERT { <ds3> a nie:DataSource }
INSERT { <subject> nie:dataSource <ds1>, <ds2>, <ds3> }
This will be the test query I’ll use for all cases:
SELECT ?ds WHERE { <subject> nie:dataSource ?ds }
For the begin situation that of course gives us <ds1>, <ds2> and <ds3>.
With null upfront, reset of list, rewrite of new list:
INSERT OR REPLACE { <subject> nie:dataSource null, <ds1>, <ds2> }
This will give us <ds1> and <ds2> for the test query. The first null resets the existing list, then <ds1> and <ds2> are added. This is probably the most sensible one to use for multi value properties.
With null in the middle, rewrite of new list:
INSERT OR REPLACE { <subject> nie:dataSource <ds1>, null, <ds2>, <ds3> }
This also gives us <ds2> and <ds3>. First <ds1> is added, but the null that follows clears it again. Then <ds2> and <ds3> get added. So the <ds1> there doesn’t make much sense, indeed.
With null at the end:
INSERT OR REPLACE { <subject> nie:dataSource <ds1>, <ds2>, <ds3>, null }
This one doesn’t make much sense either. The <ds1>, <ds2> and <ds3> get cleared by the null at the end. So the query gives us zero results.
With null as only element:
INSERT OR REPLACE { <subject> nie:dataSource null }
This one makes sense, you can use it to clear a multi value property of a resource. The query gives us zero results.
Multiple nulls:
INSERT OR REPLACE { <subject> nie:dataSource null, <ds1>, null, <ds2>, <ds3> }
Again doesn’t make much sense. First the list is cleared, then <ds1> is added, then it’s again cleared, then <ds2> and <ds3> are added. So the query gives <ds2> and <ds3>.