Question

I’m trying to perform some simple SQL (using sql server 2005) using QueryOver in NHibernate. The query is to count how many times a serial number is repeatedly used in a list of items and then to select the serial numbers with only 1 use. Note that I don’t want distinct serial numbers since I don’t want to use serial numbers that have >1 use.

The SQL query to do this is:

SELECT SERNUM, expr1
FROM (SELECT SERNUM, COUNT(SERNUM) AS expr1
      FROM ITEM
      GROUP BY SERNUM) AS derivedtbl_1
WHERE (expr1 = 1)

What I have done so far using QueryOver is:

var query = session.QueryOver<Item>()
                        .Select(Projections.ProjectionList()
                        .Add(Projections.Count<Item>(x => x.Sernum))
                        .Add(Projections.GroupProperty("Sernum"))).List();

This code generates SQL (though it does not like putting the Count() column to a List). I’m not sure how to access the column of data generated by Count() to say only return where it is 1. The SQL that it does generates is:

SELECT   count(this_.SERNUM) as y0_,
         this_.SERNUM        as y1_
FROM     ITEM this_
GROUP BY this_.SERNUM

I’m not sure if I’m approaching this the right way but hopefully someone can point me to it.

Answer 1

var serials = session.QueryOver<Item>()
    .Where(Restrictions.Eq(Projections.Count<Item>(i => i.Id), 1));
    .Select(Projections.GroupProperty<Item>(i => i.Sernum))
    .List();

should generate something like

SELECT SERNUM FROM ITEM GROUP BY SERNUM HAVING COUNT(*) = 1

to get the items, do something like

var subquery = QueryOver.Of<Item>()
    .Where(Restrictions.Eq(Projections.Count<Item>(i => i.Id), 1));
    .Select(Projections.GroupProperty<Item>(i => i.Sernum));

var items = QueryOver.Of<Item>()
    .WithSubquery.Where(i => i.Sernum).In(subquery);
    .List();