Select distinct entities by specific column

问题

I have a table with 4 columns : id, column_a and column_b

My initial query is basicaly

Session session = initialize my hibernate session;
Criteria criteria = session.createCriteria(MytableBean.class);
// add some constraints
return criteria.list()

And it returns some results like :

id  |  column_a  | column_b | 
1   |  A         | 1        | 
2   |  A         | 1        | 
3   |  A         | 2        | 
4   |  B         | 3        | 
5   |  B         | 3        | 

But I don't want to get several times the same couple (column_a, column_b), even if I need to have it many times in my database.

So, I'm looking for a query which returns a result similar to:

id  |  column_a  | column_b | 
    |  A         | 1        | 
    |  A         | 2        | 
    |  B         | 3        | 

In sql I would use SELECT DISTINCT column_a, column_b FROM ....

But I can't use sql (and either don't want to use hql, as far as possible).

回答1:

The following will accomplish this. A list of Object[] will be returned.

    Criteria criteria = session.createCriteria(MytableBean.class);
    ProjectionList projList = Projections.projectionList();
    projList.add(Projections.property("column_a"));
    projList.add(Projections.property("column_b"));
    projList.add(Projections.groupProperty("column_a"));
    projList.add(Projections.groupProperty("column_b"));

    criteria.setProjection(projList);
    return criteria.list();

来源：https://stackoverflow.com/questions/8354704/select-distinct-entities-by-specific-column