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SELECT DISTINCT + ORDER BY in JPA 2 Criteria API

I've a class Lawsuit, that contains a List<Hearing>, each one with a Date attribute.
I need to select all the Lawsuits ordered by the date of their Hearings
I've a CriteriaQuery like
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Lawsuit> cq = cb.createQuery(Lawsuit.class);
Root<Lawsuit> root = cq.from(Lawsuit.class);
I use distinct to flatten the results:
cq.select(root).distinct(true);
I then join Lawsuit with Hearing
Join<Lawsuit, Hearing> hearing = root.join("hearings", JoinType.INNER);
to create Predicates
predicateList.add(cb.isNotNull(hearing.<Date>get("date")));
and Orders:
orderList.add(cb.asc(hearing.<Date>get("date")));
Everything works fine if I avoid distinct, but if I use it, it complains about not being able to order based on fields that are not in the SELECT:
Caused by: org.postgresql.util.PSQLException: ERROR: for SELECT DISTINCT, ORDER BY expressions must appear in select list
The List<Hearing> is already accessible through the Lawsuit classes returned, so I'm confused: how should I add them to the select list ?

I've discovered the source of the problem somewhere else, and solving it has made unnecessary to do what asked in the question;
as described in other answers, it should be unnecessary to perform the distinct here.
The duplicate rows were originated by erroneous left joins that were performed on collections (attributes of the root object) even if the predicates were not been used:
Join<Lawsuit, Witness> witnesses = root.join("witnesses", JoinType.LEFT);
if (witnessToFilterWith!=null) {
predicateList.add(cb.equal(witnesses.<Long>get("id"),witnessToFilterWith.getId()));
}
The join should obviously be performed as inner and only if needed:
if (witnessToFilterWith!=null) {
Join<Lawsuit, Witness> witnesses = root.join("witnesses", JoinType.INNER);
predicateList.add(cb.equal(witnesses.<Long>get("id"),witnessToFilterWith.getId()));
}
So, if you're here because you're getting the same problem, search the problem in the joins.

You can also de-duplicate via group by based on primary key column of root table:
cq.groupBy(root.get("id")); // Assuming that Lawsuite.id is primary key column

JPA CriteriaQuery with LEAST and GREATEST functions

I am have a problem where i need to join two tables using the LEAST and GREATEST functions, but using JPA CriteriaQuery. Here is the SQL that i am trying to duplicate...
select * from TABLE_A a
inner join TABLE_X x on
(
a.COL_1 = least(x.COL_Y, x.COL_Z)
and
a.COL_2 = greatest(x.COL_Y, x.COL_Z)
);
I have looked at CriteriaBuilder.least(..) and greatest(..), but am having a difficult time trying to understand how to create the Expression<T> to pass to either function.

The simplest way to compare two columns and get the least/greatest value is to use the CASE statement.
In JPQL, the query would look like
select a from EntityA a join a.entityXList x
where a.numValueA=CASE WHEN x.numValueY <= x.numValueZ THEN x.numValueY ELSE x.numValueZ END
and a.numValueB=CASE WHEN x.numValueY >= x.numValueZ THEN x.numValueY ELSE x.numValueZ END
You can code the equivalent using CriteriaBuilder.selectCase() but I've never been a big fan of CriteriaBuilder. If requirements forces you to use CriteriaBuilder then please let me know and I can try to code the equivalent.
CriteriaBuilder least/greatest is meant to get the min/max value of all the entries in one column. Let's say you want to get the Entity that had the alphabetically greatest String name. The code would look like
CriteriaBuilder cb = entityManager.getCriteriaBuilder();
CriteriaQuery query = cb.createQuery(EntityX.class);
Root<EntityX> root = query.from(EntityX.class);
Subquery<String> maxSubQuery = query.subquery(String.class);
Root<EntityX> fromEntityX = maxSubQuery.from(EntityX.class);
maxSubQuery.select(cb.greatest(fromEntityX.get(EntityX_.nameX)));
query.where(cb.equal(root.get(EntityX_.nameX), maxSubQuery));
I created a sample Spring Data JPA app that demonstrates these JPA examples at
https://github.com/juttayaya/stackoverflow/tree/master/JpaQueryTest

It turns out that CriteriaBuilder does support calling LEAST and GREATEST as non-aggregate functions, and can be accessed by using the CriteriaBuilder.function(..), as shown here:
Predicate greatestPred = cb.equal(pathA.get(TableA_.col2),
cb.function("greatest", String.class,
pathX.get(TableX_.colY), pathX.get(TableX_.colZ)));

How to resolve Oracle's 'rownum' pseudocolumn with JPA Criteria API?

I would like to get first row from database using JPA Criteria. I use JPA, Hibernate 4.2.7.
In SQL, statement looks like:
SELECT * FROM houses WHERE rownum = 1;
My Java code to achive that looks like:
CriteriaBuilder builder = entityManager.getCriteriaBuilder();
CriteriaQuery<House> query = builder.createQuery(House.class);
Root<House> root = query.from(House.class);
query.select(root).where(builder.equal(root.get("rownum"), 1));
TypedQuery<House> tQuery = entityManager.createQuery(query);
House house = tQuery.getSingleResult();
But 'rownum' pseudocolumn cannot be resolved, I get exception:
java.lang.IllegalArgumentException: Unable to resolve attribute [rownum] against path
at org.hibernate.ejb.criteria.path.AbstractPathImpl.unknownAttribute(AbstractPathImpl.java:120)
at org.hibernate.ejb.criteria.path.AbstractPathImpl.locateAttribute(AbstractPathImpl.java:229)
at org.hibernate.ejb.criteria.path.AbstractPathImpl.get(AbstractPathImpl.java:200)
Is is possible, and if so, how to get 'rownum' pseudocolumn with Criteria API? Thanks for any suggestions.

You can accomplish this using setFirstResult and setMaxResults.
session.createCriteria(Foo.class)
.setFirstResult(0)
.setMaxResults(1);

you can set rownum or limits in property of query;
Query q =entityManager.createQuery("select * from employee_table");
q.setFirstResult(0);
q.setMaxResults(30);

Need help creating JPA criteria query

I'm building my first Java EE web application using Glassfish and JSF. I'm fairly new to the criteria query and I have a query I need to perform but the javaee6 tutorial seems a little thin on examples. Anyway, I'm having a hard time creating the query.
Goal: I want to pull the company with the most documents stored.
Companies have a OneToMany relationship with Documents.
Documents has a ManyToOne relationship with several tables, the "usertype" column distinguishes them.
MySQL query:
SELECT USERID, COUNT(USERID) AS CNT
FROM DOCUMENTS
WHERE USERTYPE="COMPANY"
GROUP BY USERID
ORDER BY CNT DESC
Thanks
--update--
Based on user feedback, here is what I have so far:
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Documents> cqry = cb.createQuery(Documents.class);
//Intersting Stuff
Root<Documents> root = cqry.from(Documents.class);
Expression userid = root.get("userID");
Expression usertype = root.get("userType");
Expression count = cb.count(userid);
cqry.multiselect(userid, count);
Predicate userType = cb.equal(usertype, "COMPANY");
cqry.where(userType);
cqry.groupBy(userid);
cqry.orderBy(cb.desc(count));
//more boilerplate
Query qry = em.createQuery(cqry);
List<Documents> results = qry.getResultList();
The error I get is:
Exception Description: Partial object queries are not allowed to maintain the cache or be edited. You must use dontMaintainCache().
Typical error, means nothing to me!

Your query doesn't return a complete entity object as you're only selecting two fields of the given table (this is why you're getting an error that says yadayadapartialyadayada).
Your solution is almost right, here's what you need to change to make it work—making it partial.
Instead of a plain CriteriaQuery<...> you have to create a tuple CriteriaQuery<..> by calling CriteriaBuilder.createTupleQuery(). (Basically, you can call CriteriaBuilder.createQuery(...) and pass Tuple.class to it as an argument. Tuple is a sort of wildcard entity class.)
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Tuple> cq= cb.createTupleQuery();
Root<Documents> root = cq.from(Documents.class);
Expression<Integer> userId = root.get("USERID");
Expression<String> userType = root.get("USERTYPE");
Expression<Long> count = cb.count(userId);
cq.multiselect(userId.alias("USERID"), count.alias("CNT"));
cq.where(cb.equal(userType, "COMPANY");
cq.groupBy(userId);
cq.orderBy(cb.desc(count));
TypedQuery<Tuple> tq = em.createQuery(cq);
for (Tuple t : tq.getResultsList()) {
System.out.println(t.get("USERID"));
System.out.println(t.get("CNT"));
}
(Accessing fields of a Tuple gave me an error if I didn't use aliases for them (in multiselect(...)). This is why I've used aliases, but this can be tackled more cleanly by using JPA 2's Metamodel API, which is described in the specification quite thoroughly. )
The documentation for CriteriaQuery.multiselect(...) describes the behaviour of queries using Tuple objects more deeply.

If you are using Hibernate, this should work:
ProjectionList pl = Projections.projectionList()
.add(Projections.groupProperty("userid"))
.add(Projections.property("userid"))
.add(Projections.count("userid"));
Criteria criteria = session.createCriteria(Document.class)
.add(Restrictions.eq("usertype",usertype))
.setProjection(pl)
.addOrder(Order.desc("cnt"));
Hope it helps!

Take a look into this easy tutorial. It uses JPA2 and Criteria
http://www.jumpingbean.co.za/blogs/jpa2-criteria-api
Regards!

You need to add a constructor to Documents with only userid and count because you will need it on:
cqry.multiselect(userid, count);

Referring to an earlier aliased field in a criteria query

In this query:
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<Tuple> q = cb.createTupleQuery();
// FROM GamePlayedEvent gpe
Root<GamePlayedEvent> gpe = q.from(GamePlayedEvent.class);
// SELECT gameId, COUNT(*) AS count, AVG(duration)
// AS avDur, AVG(rewardCurrency) AS avCur, AVG(rewardXP) avXp
q.select(cb.tuple(
gpe.<String>get("gameId"),
cb.count(gpe).alias("count"),
cb.avg(gpe.<Double>get("duration")).alias("avDur"),
cb.avg(gpe.<Integer>get("rewardCurrency")).alias("avCur"),
cb.avg(gpe.<Integer>get("rewardXp")).alias("avXp")
));
// WHERE loginTime BETWEEN ...
q.where(cb.between(gpe.<Date>get("time"), fromTime, toTime));
// GROUP BY gameId
q.groupBy(gpe.<String>get("gameId"));
// ORDER BY count DESC
q.orderBy(cb.desc(???));
How can I add the ORDER BY count DESC, referring to the "count" defined in the SELECT clause?

What if you just captured the count expression, and used it directly?
Expression event_count = cb.count(gpe);
q.select(cb.tuple(
gpe.<String>get("gameId"),
event_count,
...
));
q.orderBy(cb.desc(event_count));

I came across the same problem today but none of the suggested solutions worked for me because I needed to reuse the expression not only in the order by clause but also in the group by clause.
One obvious solution would be to create a view on the database level but this is a bit clumsy, creates an unnecessary subquery and even not possible if the db user isn't granted enough privileges. A better option which I ended up implementing is to write something like this
q.select(cb.tuple(
gpe.<String>get("gameId"),
cb.count(gpe),
...
)).groupBy(cb.literal(2)).orderBy(cb.literal(2));
The first downside of this approach is that the code is errorprone. The other drawback is that the generated sql query contains ordinal position notation, which works on some databases (like postgresql or mariadb) but doesn't work on others (like sql server). In my case, however, I found this to be the best option.
Tested on jpa 2.1 with hibernate 5.2.3 as a provider and postgresql 9.6.

Even though the Pro JPA 2 book describes that the alias method can be used to generate a sql query alias (on page 251) I have had no success with making it work with neither EclipseLink or Hibernate. For your question I would say that your orderBy line should read:
q.orderBy(cb.desc(cb.count(gpe));
if it was supported by the different vendors.
As far as my research goes it seams that the alias method is only used for naming elements in the tuble used in the select (so only for projection).
I have one question though. Why would you want to use the JPA Criteria API for this query. It (the query) seams to be static in nature so why not use JPQL where you can define your query aliases directly.

Have you tried setting up a projection with an alias?
criteria.setProjection(Projections.projectionList()
.add(Projections.count("item.id"), "countItems"));
criteria.addOrder(Order.desc("countItems"));

For a sum aggregation field I have the following code which worked for me:
CriteriaBuilder cb = em.getCriteriaBuilder();
CriteriaQuery<T> cq = cb.createQuery(entity);
Root<T> root = cq.from(entity);
cq.orderBy(cb.desc(cb.sum(root.get(orderByString))));
// orderByString is string entity field that is being aggregated and which we want to put in orderby clause as well.