Chapter 7. EJB-QL: The Object Query Language
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## Chapter 7. EJB-QL: The Object Query Language

EJB3-QL has been heavily inspired by HQL, the native Hibernate Query Language. Both are therefore very close to SQL, but portable and independent of the database schema. People familiar with HQL shouldn't have any problem using EJB-QL. Actually, you use the same query API for EJB-QL and HQL queries. Portable EJB3 applications however should stick to EJB-QL or similar vendor extensions are needed.

## 7.1. Case Sensitivity

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

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

## 7.2. The from clause

The simplest possible EJB-QL query is of the form:

    select c from eg.Cat c

which simply returns all instances of the class `eg.Cat`. Unlike HQL, the select clause is not optional in EJB-QL. We don't usually need to qualify the class name, since the entity name defaults to the unqualified class name (`@Entity`). So we almost always just write:

    select c from Cat c

As you may have noticed you can assign aliases to classes, the `as` keywork is optional. An alias allows you to refer to `Cat` in other parts of the query.

    select cat from Cat as cat

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

    select form, param from Formula as form, Parameter as param

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

## 7.3. Associations and joins

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

    select cat, mate, kitten from Cat as cat
        inner join cat.mate as mate
        left outer join cat.kittens as kitten

    select cat from Cat as cat left join cat.mate.kittens as kittens

The supported join types are borrowed from ANSI SQL

- `inner join`

- `left outer join`

The `inner join`, `left outer join` constructs may be abbreviated.

    select cat, mate, kitten from 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 fetching options in the associations and collection mapping metadata. See the Performance chapter of the Hibernate reference guide for more information.

    select cat from Cat as cat
        inner join fetch cat.mate
        left join fetch cat.kittens

A fetch join does not usually need to assign an alias, because the associated objects should not be used in the `where` clause (or any other clause). Also, the associated objects are not returned directly in the query results. Instead, they may be accessed via the parent object. The only reason we might need an alias is if we are recursively join fetching a further collection:

    select cat from Cat as cat
        inner join fetch cat.mate
        left join fetch cat.kittens child
        left join fetch child.kittens

Note that the `fetch` construct may not be used in queries called using `scroll()` or `iterate()`. Nor should `fetch` be used together with `setMaxResults()` or `setFirstResult()`. It is possible to create a cartesian product by join fetching more than one collection in a query (as in the example above), be careful the result of this product isn't bigger than you expect. Join fetching multiple collection roles also sometimes gives unexpected results for bag mappings, so be careful about how you formulate your queries in this case.

TODO: The last statement is useless and typical developer thinking, please elaborate. The word "sometimes" should never appear in any technical documentation.

If you are using property-level lazy fetching (with bytecode instrumentation), it is possible to force Hibernate to fetch the lazy properties immediately (in the first query) using `fetch all properties`. This is Hibernate specific option:

    select doc from Document doc fetch all properties order by doc.name

    select doc from Document doc fetch all properties where lower(doc.name) like '%cats%'

## 7.4. The select clause

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

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

The query will select `mate`s of other `Cat`s. Actually, you may express this query more compactly as:

    select cat.mate from Cat cat

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

    select cat.name from 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 DomesticCat as mother
        inner join mother.mate as mate
        left outer join mother.kittens as offspr

or as a `List` (HQL specific feature)

    select new list(mother, offspr, mate.name)
    from DomesticCat as mother
        inner join mother.mate as mate
        left outer join mother.kittens as offspr

or as an actual typesafe Java object,

    select new Family(mother, mate, offspr)
    from DomesticCat as mother
        join mother.mate as mate
        left join mother.kittens as offspr

assuming that the class `Family` has an appropriate constructor.

You may assign aliases to selected expressions using `as`:

    select max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n
    from Cat cat

This is most useful when used together with `select new map` (HQL specific feature):

    select new map( max(bodyWeight) as max, min(bodyWeight) as min, count(*) as n )
    from Cat cat

This query returns a `Map` from aliases to selected values.

## 7.5. Aggregate functions

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 Cat cat

The supported aggregate functions are

- `avg(...), avg(distinct ...), sum(...), sum(distinct ...), min(...), max(...)`

- `count(*)`

- `count(...), count(distinct ...), count(all...)`

You may use arithmetic operators, concatenation, and recognized SQL functions in the select clause (dpending on configured dialect, HQL specific feature):

    select cat.weight + sum(kitten.weight)
    from Cat cat
        join cat.kittens kitten
    group by cat.id, cat.weight

    select firstName||' '||initial||' '||upper(lastName) from Person

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

    select distinct cat.name from Cat cat

    select count(distinct cat.name), count(cat) from Cat cat

## 7.6. Polymorphic queries

A query like:

    select cat from Cat as cat

returns instances not only of `Cat`, but also of subclasses like `DomesticCat`. Hibernate queries may name any Java class or interface in the `from` clause (portable EJB-QL queries should only name mapped entities). 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 java.lang.Object o // HQL only

The interface `Named` might be implemented by various persistent classes:

    from Named n, Named m where n.name = m.name // HQL only

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. (It also means you can't call these queries using `Query.scroll()`.)

## 7.7. The where clause

The `where` clause allows you to narrow the list of instances returned. If no alias exists, you may refer to properties by name:

    select cat from Cat cat where cat.name='Fritz'

returns instances of `Cat` named 'Fritz'.

    select foo
    from Foo foo, 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:

    select cat from 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

    select foo from 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:

    select cat, rival from Cat cat, Cat rival where cat.mate = rival.mate

    select cat, mate
    from Cat cat, 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 mapped identifer property name.). Note that this keyword is specific to HQL.

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

    select cat from 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`.

    select person from bank.Person person
    where person.id.country = 'AU'
        and person.id.medicareNumber = 123456

    select account 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 Java class name embedded in the where clause will be translated to its discriminator value. Once again, this is specific to HQL.

    select cat from Cat cat where cat.class = 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>`). `Any` is specific to Hibernate

    from AuditLog log, Payment payment
    where log.item.class = 'Payment' 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.

## 7.8. Expressions

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

- mathematical operators `+, -, *, /`

- binary comparison operators `=, >=, <=, <>, !=, like`

- logical operations `and, or, not`

- Parentheses `( )`, indicating grouping

- `in`, `not in`, `between`, `is null`, `is not null`, `is empty`, `is not empty`, `member of` and `not member of`

- "Simple" case, `case ... when ... then ... else ... end`, and "searched" case, `case when ... then ... else ... end (specific to HQL)`

- string concatenation `...||...` or `concat(...,...) (use concat() for portable EJB-QL queries)`

- `current_date()`, `current_time()`, `current_timestamp()`

- `second(...)`, `minute(...)`, `hour(...)`, `day(...)`, `month(...)`, `year(...)`, (specific to HQL)

- Any function or operator defined by EJB-QL 3.0: `substring(), trim(), lower(), upper(), length(), locate(), abs(), sqrt(), bit_length()`

- `coalesce()` and `nullif()`

- `cast(... as ...)`, where the second argument is the name of a Hibernate type, and `extract(... from ...)` if ANSI `cast()` and `extract()` is supported by the underlying database

- Any database-supported SQL scalar function like `sign()`, `trunc()`, `rtrim()`, `sin()`

- JDBC IN parameters `?`

- named parameters `:name`, `:start_date`, `:x1`

- SQL literals `'foo'`, `69`, `'1970-01-01 10:00:01.0'`

- Java `public static final` constants `eg.Color.TABBY`

`in` and `between` may be used as follows:

    select cat from DomesticCat cat where cat.name between 'A' and 'B'

    select cat from DomesticCat cat where cat.name in ( 'Foo', 'Bar', 'Baz' )

and the negated forms may be written

    select cat from DomesticCat cat where cat.name not between 'A' and 'B'

    select cat from 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 Hibernate configuration:

    hibernate.query.substitutions true 1, false 0

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

    select cat from Cat cat where cat.alive = true

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

    select cat from Cat cat where cat.kittens.size > 0

    select cat from Cat cat where size(cat.kittens) > 0

For indexed collections, you may refer to the minimum and maximum indices using `minindex` and `maxindex` functions. Similarly, you may refer to the minimum and maximum elements of a collection of basic type using the `minelement` and `maxelement` functions. These are HQL specific features.

    select cal from Calendar cal where maxelement(cal.holidays) > current date

    select order from Order order where maxindex(order.items) > 100

    select order 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 subquery (see below). While subqueries are supported by EJB-QL, `elements` and `indices` are specific HQL features.

    select mother from Cat as mother, Cat as kit
    where kit in elements(foo.kittens)

    select p from NameList list, Person p
    where p.name = some elements(list.names)

    select cat from Cat cat where exists elements(cat.kittens)

    select cat from Player p where 3 > all elements(p.scores)

    select cat from Show show where 'fizard' in indices(show.acts)

Note that these constructs - `size`, `elements`, `indices`, `minindex`, `maxindex`, `minelement`, `maxelement` - may only be used in the where clause in Hibernate3.

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

    select order 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

    select cat from 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
        )

## 7.9. The order by clause

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

    select cat from DomesticCat cat
    order by cat.name asc, cat.weight desc, cat.birthdate

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

## 7.10. The group by clause

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 Cat cat
    group by cat.color

    select foo.id, avg(name), max(name)
    from Foo foo join foo.names name
    group by foo.id

A `having` clause is also allowed.

    select cat.color, sum(cat.weight), count(cat)
    from 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 (eg. not in MySQL).

    select cat
    from Cat cat
        join cat.kittens kitten
    group by cat
    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.

## 7.11. Subqueries

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

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

    select cat from DomesticCat as cat
    where cat.name = some (
        select name.nickName from Name as name
    )

    select cat from Cat as cat
    where not exists (
        from Cat as mate where mate.mate = cat
    )

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

For subqueries with more than one expression in the select list, you can use a tuple constructor:

    select cat from Cat as cat
    where not ( cat.name, cat.color ) in (
        select cat.name, cat.color from DomesticCat cat
    )

Note that on some databases (but not Oracle or HSQLDB), you can use tuple constructors in other contexts, for example when querying components or composite user types:

    select cat from Person where name = ('Gavin', 'A', 'King')

Which is equivalent to the more verbose:

    select cat from Person where name.first = 'Gavin' and name.initial = 'A' and name.last = 'King')

There are two good reasons you might not want to do this kind of thing: first, it is not completely portable between database platforms; second, the query is now dependent upon the ordering of properties in the mapping document.

## 7.12. EJB-QL examples

Hibernate queries can be quite powerful and complex. In fact, the power of the query language is one of Hibernate's main selling points (and now EJB-QL). 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 subqueries, 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 `AWAITING_APPROVAL` 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.AWAITING_APPROVAL
        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.AWAITING_APPROVAL
        or payment.statusChanges[ maxIndex(payment.statusChanges) ].user <> :currentUser
    group by status.name, status.sortOrder
    order by status.sortOrder

However the query would have been HQL specific.

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
        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

## 7.13. Bulk UPDATE & DELETE Statements

Hibernate now supports UPDATE and DELETE statements in HQL/EJB-QL. See Section 6.1, “Bulk update/delete” for details.

## 7.14. Tips & Tricks

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 subselects, 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 subselects, use the following query:

    select usr.id, usr.name
    from User usr.name
        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

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