Hibernate Query Language Essentials

From Clause

To retrieve all instances of a class with HQL, use the from clause.

 
List<Continent> continents = session.createQuery("from Continent").list();

The List (which uses generics to demonstrate the return type) is populated with an instance of the Continent class for each row returned by this SQL query:

select
  CONT_ID,
  CONT_NAME
from
  CONTINENT

If there are classes with the same name in different packages, use a fully qualified name to identify the correct one.

 
from levelup.world.hql.Continent

Where Clause

To return a subset of rows based on selection criteria, include the where clause. This is known as selection or restriction.

List<Continent> theAmericas = session.createQuery(
      "from Continent where name like '%America%'")
      .list();

This translates to the following SQL:

select
  CONT_ID,
  CONT_NAME
from
  CONTINENT
where
  CONT_NAME like '%America%'

Most of the common SQL expressions can also be found in HQL. For example, this HQL

List<Country> countries = session.createQuery(
      "from Country " +
      "where (populationUpdatedOn is not null " +
      "or lower(name) in ('gabon', 'gambia')) " +
      "and area between 100000 AND 100000000")
      .list();

results in this SQL

select
  CTRY_ID,
  AREA,
  CONT_ID,
  CURRENCY,
  CTRY_NAME,
  POP,
  POP_UPD_ON
from
  COUNTRY
where
  (
    pop_upd_on is not null
    or lower(ctry_name) in ('gabon' , 'gambia')
  )
  and (area between 100000 and 100000000)

Associations

We can also access entity associations in our query.

List<Country> africanCountries = session.createQuery(
      "from Country where continent.name = 'Africa'")
      .list();

The CONTINENT and COUNTRY tables are joined to produce the results.

select
  COUNTRY.CTRY_ID,
  COUNTRY.AREA,
  COUNTRY.CONT_ID,
  COUNTRY.CURRENCY,
  COUNTRY.CTRY_NAME,
  COUNTRY.POP,
  COUNTRY.POP_UPD_ON
from
  COUNTRY,
  CONTINENT
where
  COUNTRY.CONT_ID=CONTINENT.CONT_ID
  and CONT_NAME='Africa'

The HQL query in this example uses implicit association joining (i.e. the join is not specified in full) which is available for many-to-one and one-to-one associations. This syntax is convenient but can be confusing if many associations are accessed in the query, as the underlying joins are not obvious. The preferred method for more complex queries is to use explicit join syntax.

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