Union in JPA query - from the same table

Question

I have a requirement where I need to restrict number of records returned from a table for a particular flag and all records for the other flag value.

For example:

Answer 1

JPA does not support UNION, but if you use EclipseLink, UNION is supported in JPQL. Also the COLUMN operator can be used to access special columns like rownum.

See, http://java-persistence-performance.blogspot.com/2012/05/jpql-vs-sql-have-both-with-eclipselink.html

Answer 2

No, this is not possible with JPQL, because it does not have UNION (ALL). Additionally there is no way to limit amount of rows returned in query string itself with rownum, but that is done via setMaxResults.

In many situations

• executing two queries,
• limiting number of results in first one with setMaxResults, and
• discarding duplicates and combining results of both queries in Java

is viable solution.