Hibernate Query Language or HQL for short is extremely
powerful query language. HQL is much like SQL and are case-insensitive,
except for the names of the Java Classes and properties. Hibernate Query
Language is used to execute queries against database. Hibernate automatically
generates the sql query and execute it against underlying database if HQL is
used in the application. HQL is based on the relational object models and makes
the SQL object oriented. Hibernate Query Language uses Classes and properties
instead of tables and columns. Hibernate Query Language is extremely powerful
and it supports Polymorphism, Associations, Much less verbose than SQL.
There are other options that can be used while using
Hibernate. These are Query By Criteria (QBC) and Query BY Example (QBE)
using Criteria API and the Native SQL queries. In this lesson we will
understand HQL in detail.
Why to use HQL?
- Full support for relational operations: HQL
allows representing SQL queries in the form of objects. Hibernate Query
Language uses Classes and properties instead of tables and columns.
- Return result as Object: The HQL queries
return the query result(s) in the form of object(s), which is easy to use.
This elemenates the need of creating the object and populate the data from
- Polymorphic Queries: HQL fully supports polymorphic
queries. Polymorphic queries results the query results along with all
the child objects if any.
- Easy to Learn: Hibernate Queries are easy to
learn and it can be easily implemented in the applications.
- Support for Advance features: HQL contains
many advance features such as pagination, fetch join with dynamic profiling,
Inner/outer/full joins, Cartesian products. It also supports Projection,
Aggregation (max, avg) and grouping, Ordering, Sub queries and SQL function
- Database independent: Queries written in HQL
are database independent (If database supports the underlying feature).
Understanding HQL Syntax
Any Hibernate Query Language may consist of following elements:
- Aggregate functions
Clauses in the HQL are:
- order by
- group by
Aggregate functions are:
- avg(...), sum(...),
count(...), count(distinct ...), count(all...)
Subqueries are nothing but its a query within another query. Hibernate supports
Subqueries if the underlying database supports it.