JCR SQL2 query for an empty multivalued field - magnolia

Question

In our scenario we have a (multivalued) category field on JCR:node and we want to query all nodes that do not have a current selection. In the JCR viewer the fields value is [] but I can't find any query to select nodes with this condition. We have tries:
SELECT * FROM [mgnl:page] as p WHERE p.[categories]=''
or
SELECT * FROM [mgnl:page] as p WHERE p.[categories]=[]
or
SELECT * FROM [mgnl:page] as p WHERE p.[categories] is null
But they aren't working or don't select the proper result. How can we write a query selecting these nodes?

Answer 1

From the JSR 283:
5.10.3 Value Length
The length of a value in a single-value property, as defined in §3.6.7 Length of a Value, is returned by long Property.getLength()
Similarly, the method long[] Property.getLengths() is used to get an array of the lengths of all the values of a multi-value property.
From the JavaDocs:
/**
* Returns an array holding the lengths of the values of this (multi-value)
* property in bytes where each is individually calculated as described in
* {#link #getLength()}.
* <p>
* Returns a <code>-1</code> in the appropriate position if the
* implementation cannot determine the length of a value.
*
* #return an array of lengths
* #throws ValueFormatException if this property is single-valued.
* #throws RepositoryException if another error occurs.
*/
public long[] getLengths() throws ValueFormatException, RepositoryException;
Unfortunately, this does not make it clear what the result of LENGTH([multivaluedProperty)] is in a SQL2 query.
Though, after some manual testing, it seems that the LENGTH operand returns some number smaller than 0. Therefore, you could try
select * from [nt:base] where LENGTH([multivaluedProperty]) < 0
Let me know whether this works for you :)