Queries are case-insensitive, except for names of .NET classes and properties. So SeLeCT is the same as sELEct is the same as SELECT but Eg.FOO is not Eg.Foo and foo.barSet is not foo.BARSET.

This manual uses lowercase HQL keywords. Some users find queries with uppercase keywords more readable, but we find this convention ugly when embedded in C# code.

The simplest possible NHibernate query is of the form:

from Eg.Cat

which simply returns all instances of the class Eg.Cat.

Most of the time, you will need to assign an alias, since you will want to refer to the Cat in other parts of the query.

from Eg.Cat as cat

This query assigns the alias cat to Cat instances, so we could use that alias later in the query. The as keyword is optional; we could also write:

from Eg.Cat cat

Multiple classes may appear, resulting in a cartesian product or "cross" join.

from Formula, Parameter

from Formula as form, Parameter as param

It is considered good practice to name query aliases using an initial lowercase, consistent with naming standards for local variables (eg. domesticCat).

We may also assign aliases to associated entities, or even to elements of a collection of values, using a join.

from Eg.Cat as cat 
    inner join cat.Mate as mate
    left outer join cat.Kittens as kitten

from Eg.Cat as cat left join cat.Mate.Kittens as kittens

from Formula form full join form.Parameter param

The supported join types are borrowed from ANSI SQL

The inner join, left outer join and right outer join constructs may be abbreviated.

from Eg.Cat as cat 
    join cat.Mate as mate
    left join cat.Kittens as kitten

In addition, a "fetch" join allows associations or collections of values to be initialized along with their parent objects, using a single select. This is particularly useful in the case of a collection. It effectively overrides the outer join and lazy declarations of the mapping file for associations and collections. See Section 21.1, “Fetching strategies” for more information.

from Eg.Cat as cat 
    inner join fetch cat.Mate
    left join fetch cat.Kittens

The associated objects are not returned directly in the query results. Instead, they may be accessed via the parent object.

It is possible to create a cartesian product by join fetching more than one collection in a query, so take care in this case. Join fetching multiple collection roles is also disabled for bag mappings. Note also that the fetch construct may not be used in queries called using Enumerable(). Finally, note that full join fetch and right join fetch are not meaningful.

The select clause picks which objects and properties to return in the query result set. Consider:

select mate 
from Eg.Cat as cat 
    inner join cat.Mate as mate

The query will select Mates of other Cats. Actually, you may express this query more compactly as:

select cat.Mate from Eg.Cat cat

You may even select collection elements, using the special elements function. The following query returns all kittens of any cat.

select elements(cat.Kittens) from Eg.Cat cat

Queries may return properties of any value type including properties of component type:

select cat.Name from Eg.DomesticCat cat
where cat.Name like 'fri%'

select cust.Name.FirstName from Customer as cust

Queries may return multiple objects and/or properties as an array of type object[]

select mother, offspr, mate.Name 
from Eg.DomesticCat as mother
    inner join mother.Mate as mate
    left outer join mother.Kittens as offspr

or as an actual type-safe object

select new Family(mother, mate, offspr)
from Eg.DomesticCat as mother
    join mother.Mate as mate
    left join mother.Kittens as offspr

assuming that the class Family has an appropriate constructor.

HQL queries may even return the results of aggregate functions on properties:

select avg(cat.Weight), sum(cat.Weight), max(cat.Weight), count(cat)
from Eg.Cat cat

Collections may also appear inside aggregate functions in the select clause.

select cat, count( elements(cat.Kittens) ) 
from Eg.Cat cat group by cat.Id, cat.Weight, ...

The supported aggregate functions are

The distinct and all keywords may be used and have the same semantics as in SQL.

select distinct cat.Name from Eg.Cat cat

select count(distinct cat.Name), count(cat) from Eg.Cat cat

A query like:

from Eg.Cat as cat

returns instances not only of Cat, but also of subclasses like DomesticCat. NHibernate queries may name any .NET class or interface in the from clause. The query will return instances of all persistent classes that extend that class or implement the interface. The following query would return all persistent objects:

from System.Object o

The interface INamed might be implemented by various persistent classes:

from Eg.Named n, Eg.Named m where n.Name = m.Name

Note that these last two queries will require more than one SQL SELECT. This means that the order by clause does not correctly order the whole result set.

In order to use non-mapped base classes or interfaces in HQL queries, they have to be imported. See Section 5.1.23, “import” for more information.

The where clause allows you to narrow the list of instances returned.

from Eg.Cat as cat where cat.Name='Fritz'

returns instances of Cat named 'Fritz'.

select foo 
from Eg.Foo foo, Eg.Bar bar
where foo.StartDate = bar.Date

will return all instances of Foo for which there exists an instance of Bar with a Date property equal to the StartDate property of the Foo. Compound path expressions make the where clause extremely powerful. Consider:

from Eg.Cat cat where cat.Mate.Name is not null

This query translates to an SQL query with a table (inner) join. If you were to write something like

from Eg.Foo foo  
where foo.Bar.Baz.Customer.Address.City is not null

you would end up with a query that would require four table joins in SQL.

The = operator may be used to compare not only properties, but also instances:

from Eg.Cat cat, Eg.Cat rival where cat.Mate = rival.Mate

select cat, mate 
from Eg.Cat cat, Eg.Cat mate
where cat.Mate = mate

The special property (lowercase) id may be used to reference the unique identifier of an object. (You may also use its property name.)

from Eg.Cat as cat where cat.id = 123

from Eg.Cat as cat where cat.Mate.id = 69

The second query is efficient. No table join is required!

Properties of composite identifiers may also be used. Suppose Person has a composite identifier consisting of Country and MedicareNumber.

from Bank.Person person
where person.id.Country = 'AU' 
    and person.id.MedicareNumber = 123456

from Bank.Account account
where account.Owner.id.Country = 'AU' 
    and account.Owner.id.MedicareNumber = 123456

Once again, the second query requires no table join.

Likewise, the special property class accesses the discriminator value of an instance in the case of polymorphic persistence. A .Net class name embedded in the where clause will be translated to its discriminator value.

from Eg.Cat cat where cat.class = Eg.DomesticCat

You may also specify properties of components or composite user types (and of components of components, etc). Never try to use a path-expression that ends in a property of component type (as opposed to a property of a component). For example, if store.Owner is an entity with a component Address

store.Owner.Address.City    // okay
store.Owner.Address         // error!

An "any" type has the special properties id and class, allowing us to express a join in the following way (where AuditLog.Item is a property mapped with <any>).

from Eg.AuditLog log, Eg.Payment payment 
where log.Item.class = 'Eg.Payment, Eg, Version=...' and log.Item.id = payment.id

Notice that log.Item.class and payment.class would refer to the values of completely different database columns in the above query.

Expressions allowed in the where clause include most of the kind of things you could write in SQL:

in and between may be used as follows:

from Eg.DomesticCat cat where cat.Name between 'A' and 'B'

from Eg.DomesticCat cat where cat.Name in ( 'Foo', 'Bar', 'Baz' )

and the negated forms may be written

from Eg.DomesticCat cat where cat.Name not between 'A' and 'B'

from Eg.DomesticCat cat where cat.Name not in ( 'Foo', 'Bar', 'Baz' )

Likewise, is null and is not null may be used to test for null values.

Booleans may be easily used in expressions by declaring HQL query substitutions in NHibernate configuration:

<property name="query.substitutions">true 1, false 0</property>

This will replace the keywords true and false with the literals 1 and 0 in the translated SQL from this HQL:

from Eg.Cat cat where cat.Alive = true

You may test the size of a collection with the special property size, or the special size() function.

from Eg.Cat cat where cat.Kittens.size > 0

from Eg.Cat cat where size(cat.Kittens) > 0

For indexed collections, you may refer to the minimum and maximum indices using minIndex and maxIndex. Similarly, you may refer to the minimum and maximum elements of a collection of basic type using minElement and maxElement.

from Calendar cal where cal.Holidays.maxElement > current date

There are also functional forms (which, unlike the constructs above, are not case sensitive):

from Order order where maxindex(order.Items) > 100

from Order order where minelement(order.Items) > 10000

The SQL functions any, some, all, exists, in are supported when passed the element or index set of a collection (elements and indices functions) or the result of a sub-query (see below).

select mother from Eg.Cat as mother, Eg.Cat as kit
where kit in elements(mother.Kittens)

select p from Eg.NameList list, Eg.Person p
where p.Name = some elements(list.Names)

from Eg.Cat cat where exists elements(cat.Kittens)

from Eg.Player p where 3 > all elements(p.Scores)

from Eg.Show show where 'fizard' in indices(show.Acts)

Note that these constructs - size, elements, indices, minIndex, maxIndex, minElement, maxElement - have certain usage restrictions:

Elements of indexed collections (arrays, lists, maps) may be referred to by index (in a where clause only):

from Order order where order.Items[0].id = 1234

select person from Person person, Calendar calendar
where calendar.Holidays['national day'] = person.BirthDay
    and person.Nationality.Calendar = calendar

select item from Item item, Order order
where order.Items[ order.DeliveredItemIndices[0] ] = item and order.id = 11

select item from Item item, Order order
where order.Items[ maxindex(order.items) ] = item and order.id = 11

The expression inside [] may even be an arithmetic expression.

select item from Item item, Order order
where order.Items[ size(order.Items) - 1 ] = item

HQL also provides the built-in index() function, for elements of a one-to-many association or collection of values.

select item, index(item) from Order order 
    join order.Items item
where index(item) < 5

Scalar SQL functions supported by the underlying database may be used

from Eg.DomesticCat cat where upper(cat.Name) like 'FRI%'

If you are not yet convinced by all this, think how much longer and less readable the following query would be in SQL:

select cust
from Product prod,
    Store store
    inner join store.Customers cust
where prod.Name = 'widget'
    and store.Location.Name in ( 'Melbourne', 'Sydney' )
    and prod = all elements(cust.CurrentOrder.LineItems)

Hint: something like

SELECT cust.name, cust.address, cust.phone, cust.id, cust.current_order
FROM customers cust,
    stores store,
    locations loc,
    store_customers sc,
    product prod
WHERE prod.name = 'widget'
    AND store.loc_id = loc.id
    AND loc.name IN ( 'Melbourne', 'Sydney' )
    AND sc.store_id = store.id
    AND sc.cust_id = cust.id
    AND prod.id = ALL(
        SELECT item.prod_id
        FROM line_items item, orders o
        WHERE item.order_id = o.id
            AND cust.current_order = o.id
    )

The list returned by a query may be ordered by any property of a returned class or components:

from Eg.DomesticCat cat
order by cat.Name asc, cat.Weight desc, cat.Birthdate

The optional asc or desc indicate ascending or descending order respectively.

A query that returns aggregate values may be grouped by any property of a returned class or components:

select cat.Color, sum(cat.Weight), count(cat) 
from Eg.Cat cat
group by cat.Color

select foo.id, avg( elements(foo.Names) ), max( indices(foo.Names) ) 
from Eg.Foo foo
group by foo.id

Note: You may use the elements and indices constructs inside a select clause, even on databases with no sub-selects.

A having clause is also allowed.

select cat.color, sum(cat.Weight), count(cat) 
from Eg.Cat cat
group by cat.Color 
having cat.Color in (Eg.Color.Tabby, Eg.Color.Black)

SQL functions and aggregate functions are allowed in the having and order by clauses, if supported by the underlying database (ie. not in MySQL).

select cat
from Eg.Cat cat
    join cat.Kittens kitten
group by cat.Id, cat.Name, cat.Other, cat.Properties
having avg(kitten.Weight) > 100
order by count(kitten) asc, sum(kitten.Weight) desc

Note that neither the group by clause nor the order by clause may contain arithmetic expressions. Also note that NHibernate currently does not expand a grouped entity, so you can't write group by cat if all properties of cat are non-aggregated. You have to list all non-aggregated properties explicitly.

For databases that support sub-selects, NHibernate supports sub-queries within queries. A sub-query must be surrounded by parentheses (often by an SQL aggregate function call). Even correlated sub-queries (sub-queries that refer to an alias in the outer query) are allowed.

from Eg.Cat as fatcat 
where fatcat.Weight > ( 
    select avg(cat.Weight) from Eg.DomesticCat cat 
)

from Eg.DomesticCat as cat 
where cat.Name = some ( 
    select name.NickName from Eg.Name as name 
)
    
from Eg.Cat as cat 
where not exists ( 
    from eg.Cat as mate where mate.Mate = cat 
)

from Eg.DomesticCat as cat 
where cat.Name not in ( 
    select name.NickName from Eg.Name as name 
)

NHibernate queries can be quite powerful and complex. In fact, the power of the query language is one of NHibernate's main selling points. Here are some example queries very similar to queries that I used on a recent project. Note that most queries you will write are much simpler than these!

The following query returns the order id, number of items and total value of the order for all unpaid orders for a particular customer and given minimum total value, ordering the results by total value. In determining the prices, it uses the current catalog. The resulting SQL query, against the ORDER, ORDER_LINE, PRODUCT, CATALOG and PRICE tables has four inner joins and an (uncorrelated) subselect.

select order.id, sum(price.Amount), count(item)
from Order as order
    join order.LineItems as item
    join item.Product as product,
    Catalog as catalog
    join catalog.Prices as price
where order.Paid = false
    and order.Customer = :customer
    and price.Product = product
    and catalog.EffectiveDate < sysdate
    and catalog.EffectiveDate >= all (
        select cat.EffectiveDate 
        from Catalog as cat
        where cat.EffectiveDate < sysdate
    )
group by order
having sum(price.Amount) > :minAmount
order by sum(price.Amount) desc

What a monster! Actually, in real life, I'm not very keen on sub-queries, so my query was really more like this:

select order.id, sum(price.amount), count(item)
from Order as order
    join order.LineItems as item
    join item.Product as product,
    Catalog as catalog
    join catalog.Prices as price
where order.Paid = false
    and order.Customer = :customer
    and price.Product = product
    and catalog = :currentCatalog
group by order
having sum(price.Amount) > :minAmount
order by sum(price.Amount) desc

The next query counts the number of payments in each status, excluding all payments in the AwaitingApproval status where the most recent status change was made by the current user. It translates to an SQL query with two inner joins and a correlated subselect against the PAYMENT, PAYMENT_STATUS and PAYMENT_STATUS_CHANGE tables.

select count(payment), status.Name 
from Payment as payment 
    join payment.CurrentStatus as status
    join payment.StatusChanges as statusChange
where payment.Status.Name <> PaymentStatus.AwaitingApproval
    or (
        statusChange.TimeStamp = ( 
            select max(change.TimeStamp) 
            from PaymentStatusChange change 
            where change.Payment = payment
        )
        and statusChange.User <> :currentUser
    )
group by status.Name, status.SortOrder
order by status.SortOrder

If I would have mapped the StatusChanges collection as a list, instead of a set, the query would have been much simpler to write.

select count(payment), status.Name 
from Payment as payment
    join payment.CurrentStatus as status
where payment.Status.Name <> PaymentStatus.AwaitingApproval
    or payment.StatusChanges[ maxIndex(payment.StatusChanges) ].User <> :currentUser
group by status.Name, status.SortOrder
order by status.SortOrder

The next query uses the MS SQL Server isNull() function to return all the accounts and unpaid payments for the organization to which the current user belongs. It translates to an SQL query with three inner joins, an outer join and a subselect against the ACCOUNT, PAYMENT, PAYMENT_STATUS, ACCOUNT_TYPE, ORGANIZATION and ORG_USER tables.

select account, payment
from Account as account
    left outer join account.Payments as payment
where :currentUser in elements(account.Holder.Users)
    and PaymentStatus.Unpaid = isNull(payment.CurrentStatus.Name, PaymentStatus.Unpaid)
order by account.Type.SortOrder, account.AccountNumber, payment.DueDate

For some databases, we would need to do away with the (correlated) subselect.

select account, payment
from Account as account
    join account.Holder.Users as user
    left outer join account.Payments as payment
where :currentUser = user
    and PaymentStatus.Unpaid = isNull(payment.CurrentStatus.Name, PaymentStatus.Unpaid)
order by account.Type.SortOrder, account.AccountNumber, payment.DueDate

You can count the number of query results without actually returning them:

var count = session.CreateQuery("select count(*) from ....").UniqueResult<long>();

To order a result by the size of a collection, use the following query:

select usr.id, usr.Name
from User as usr 
    left join usr.Messages as msg
group by usr.id, usr.Name
order by count(msg)

If your database supports sub-selects, you can place a condition upon selection size in the where clause of your query:

from User usr where size(usr.Messages) >= 1

If your database doesn't support sub-selects, use the following query:

select usr.id, usr.Name
from User usr
    join usr.Messages msg
group by usr.id, usr.Name
having count(msg) >= 1

As this solution can't return a User with zero messages because of the inner join, the following form is also useful:

select usr.id, usr.Name
from User as usr
    left join usr.Messages as msg
group by usr.id, usr.Name
having count(msg) = 0

Properties of an object can be bound to named query parameters:

IQuery q =
    s.CreateQuery("from foo in class Foo where foo.Name=:Name and foo.Size=:Size");
q.SetProperties(fooBean); // fooBean has properties Name and Size
var foos = q.List<Foo>();

Collections are pageable by using the IQuery interface with a filter:

IQuery q = s.CreateFilter( collection, "" ); // the trivial filter
q.setMaxResults(PageSize);
q.setFirstResult(PageSize * pageNumber);
var page = q.List<Cat>();

Collection elements may be ordered or grouped using a query filter:

var orderedCollection = s
    .CreateFilter(collection, "order by this.Amount")
    .List<Cat>();
var counts = s
    .CreateFilter(collection,
        "select this.Type, count(this) group by this.Type")
    .List<object[]>();