Chapter 10. JPA Query
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## Chapter 10. JPA Query

The `javax.persistence.Query` interface is the mechanism for issuing queries in JPA. The primary query language used is the Java Persistence Query Language, or `JPQL`. JPQL is syntactically very similar to SQL, but is object-oriented rather than table-oriented.

The API for executing JPQL queries will be discussed in Section 10.1, “JPQL API”, and a full language reference will be covered in Section 10.2, “JPQL Language Reference”.

## 10.1. JPQL API

### 10.1.1. Query Basics

    SELECT x FROM Magazine x

The preceding is a simple JPQL query for all `Magazine` entities.

    public Query createQuery (String jpql);

The `EntityManager.createQuery` method creates a `Query` instance from a given JPQL string.

    public List getResultList ();

Invoking `Query.getResultList` executes the query and returns a `List` containing the matching objects. The following example executes our `Magazine` query above:

    EntityManager em = ...
    Query q = em.createQuery ("SELECT x FROM Magazine x");
    List<Magazine> results = (List<Magazine>) q.getResultList ();

A JPQL query has an internal namespace declared in the `from` clause of the query. Arbitrary identifiers are assigned to entities so that they can be referenced elsewhere in the query. In the query example above, the identifier `x` is assigned to the entity `Magazine`.

| Note
The `as` keyword can optionally be used when declaring identifiers in the `from` clause. `SELECT x FROM Magazine x` and `SELECT x FROM Magazine AS x` are synonymous.

Following the `select` clause of the query is the object or objects that the query returns. In the case of the query above, the query's result list will contain instances of the `Magazine` class.

| Note
When selecting entities, you can optionally use the keyword `object`. The clauses `select x` and `SELECT OBJECT(x)` are synonymous.

The optional `where` clause places criteria on matching results. For example:

    SELECT x FROM Magazine x WHERE x.title = 'JDJ'

Keywords in JPQL expressions are case-insensitive, but entity, identifier, and member names are not. For example, the expression above could also be expressed as:

    SELECT x FROM Magazine x WHERE x.title = 'JDJ'

But it could not be expressed as:

    SELECT x FROM Magazine x WHERE x.TITLE = 'JDJ'

As with the `select` clause, alias names in the `where` clause are resolved to the entity declared in the `from` clause. The query above could be described in English as "for all `Magazine` instances `x`, return a list of every `x` such that `x`'s `title` field is equal to 'JDJ'".

JPQL uses SQL-like syntax for query criteria. The `and` and `or` logical operators chain multiple criteria together:

    SELECT x FROM Magazine x WHERE x.title = 'JDJ' OR x.title = 'JavaPro'

The `=` operator tests for equality. `<>` tests for inequality. JPQL also supports the following arithmetic operators for numeric comparisons: `>, >=, <, <=`. For example:

    SELECT x FROM Magazine x WHERE x.price > 3.00 AND x.price <= 5.00

This query returns all magazines whose price is greater than 3.00 and less than or equal to 5.00.

    SELECT x FROM Magazine x WHERE x.price <> 3.00

This query returns all Magazines whose price is not equals to 3.00.

You can group expressions together using parentheses in order to specify how they are evaluated. This is similar to how parentheses are used in Java. For example:

    SELECT x FROM Magazine x WHERE (x.price > 3.00 AND x.price <= 5.00) OR x.price = 7.00

This expression would match magazines whose price is 4.00, 5.00, or 7.00, but not 6.00. Alternately:

    SELECT x FROM Magazine x WHERE x.price > 3.00 AND (x.price <= 5.00 OR x.price = 7.00)

This expression will magazines whose price is 5.00 or 7.00, but not 4.00 or 6.00.

JPQL also includes the following conditionals:

- `[NOT] BETWEEN`: Shorthand for expressing that a value falls between two other values. The following two statements are synonymous:

      SELECT x FROM Magazine x WHERE x.price >= 3.00 AND x.price <= 5.00

      SELECT x FROM Magazine x WHERE x.price BETWEEN 3.00 AND 5.00

- `[NOT] LIKE`: Performs a string comparison with wildcard support. The special character '_' in the parameter means to match any single character, and the special character '%' means to match any sequence of characters. The following statement matches title fields "JDJ" and "JavaPro", but not "IT Insider":

      SELECT x FROM Magazine x WHERE x.title LIKE 'J%'

  The following statement matches the title field "JDJ" but not "JavaPro":

      SELECT x FROM Magazine x WHERE x.title LIKE 'J__'

- `[NOT] IN`: Specifies that the member must be equal to one element of the provided list. The following two statements are synonymous:

      SELECT x FROM Magazine x WHERE x.title IN ('JDJ', 'JavaPro', 'IT Insider')

      SELECT x FROM Magazine x WHERE x.title = 'JDJ' OR x.title = 'JavaPro' OR x.title = 'IT Insider'

- `IS [NOT] EMPTY`: Specifies that the collection field holds no elements. For example:

      SELECT x FROM Magazine x WHERE x.articles is empty

  This statement will return all magazines whose `articles` member contains no elements.

- `IS [NOT] NULL`: Specifies that the field is equal to null. For example:

      SELECT x FROM Magazine x WHERE x.publisher is null

  This statement will return all Magazine instances whose "publisher" field is set to `null`.

- `NOT`: Negates the contained expression. For example, the following two statements are synonymous:

      SELECT x FROM Magazine x WHERE NOT(x.price = 10.0)

      SELECT x FROM Magazine x WHERE x.price <> 10.0

### 10.1.2. Relation Traversal

Relations between objects can be traversed using Java-like syntax. For example, if the Magazine class has a field named "publisher" or type Company, that relation can be queried as follows:

    SELECT x FROM Magazine x WHERE x.publisher.name = 'Random House'

This query returns all `Magazine` instances whose `publisher` field is set to a `Company` instance whose name is "Random House".

Single-valued relation traversal implies that the relation is not null. In SQL terms, this is known as an inner join. If you want to also include relations that are null, you can specify:

    SELECT x FROM Magazine x WHERE x.publisher.name = 'Random House' or x.publisher is null

You can also traverse collection fields in queries, but you must declare each traversal in the `from` clause. Consider:

    SELECT x FROM Magazine x, IN(x.articles) y WHERE y.authorName = 'John Doe'

This query says that for each `Magazine` `x`, traverse the `articles` relation and check each `Article` `y`, and pass the filter if `y`'s `authorName` field is equal to "John Doe". In short, this query will return all magazines that have any articles written by John Doe.

| Note
The `IN()` syntax can also be expressed with the keywords `inner join`. The statements `SELECT x FROM Magazine x, IN(x.articles) y WHERE y.authorName = 'John Doe'` and `SELECT x FROM Magazine x inner join x.articles y WHERE y.authorName = 'John Doe'` are synonymous.

### 10.1.3. Fetch Joins

JPQL queries may specify one or more `join fetch` declarations, which allow the query to specify which fields in the returned instances will be pre-fetched.

    SELECT x FROM Magazine x join fetch x.articles WHERE x.title = 'JDJ'

The query above returns `Magazine` instances and guarantees that the `articles` field will already be fetched in the returned instances.

Multiple fields may be specified in separate `join fetch` declarations:

    SELECT x FROM Magazine x join fetch x.articles join fetch x.authors WHERE x.title = 'JDJ'

| Note
Specifying the `join fetch` declaration is functionally equivalent to adding the fields to the Query's `FetchConfiguration`. See Section 5.6, “Fetch Groups”.

### 10.1.4. JPQL Functions

As well as supporting direct field and relation comparisons, JPQL supports a pre-defined set of functions that you can apply.

- `CONCAT(string1, string2)`: Concatenates two string fields or literals. For example:

      SELECT x FROM Magazine x WHERE CONCAT(x.title, 's') = 'JDJs'

- `SUBSTRING(string, startIndex, length)`: Returns the part of the `string` argument starting at `startIndex` (1-based) and ending at `length` characters past `startIndex`.

      SELECT x FROM Magazine x WHERE SUBSTRING(x.title, 1, 1) = 'J'

- `TRIM([LEADING | TRAILING | BOTH] [character FROM] string`: Trims the specified character from either the beginning (`LEADING`), the ending (`TRAILING`), or both (`BOTH`) the beginning and ending of the string argument. If no trim character is specified, the space character will be trimmed.

      SELECT x FROM Magazine x WHERE TRIM(BOTH 'J' FROM x.title) = 'D'

- `LOWER(string)`: Returns the lower-case of the specified string argument.

      SELECT x FROM Magazine x WHERE LOWER(x.title) = 'jdj'

- `UPPER(string)`: Returns the upper-case of the specified string argument.

      SELECT x FROM Magazine x WHERE UPPER(x.title) = 'JAVAPRO'

- `LENGTH(string)`: Returns the number of characters in the specified string argument.

      SELECT x FROM Magazine x WHERE LENGTH(x.title) = 3

- `LOCATE(searchString, candidateString [, startIndex])`: Returns the first index of `searchString` in `candidateString`. Positions are 1-based. If the string is not found, returns 0.

      SELECT x FROM Magazine x WHERE LOCATE('D', x.title) = 2

- `ABS(number)`: Returns the absolute value of the argument.

      SELECT x FROM Magazine x WHERE ABS(x.price) >= 5.00

- `SQRT(number)`: Returns the square root of the argument.

      SELECT x FROM Magazine x WHERE SQRT(x.price) >= 1.00

- `MOD(number, divisor)`: Returns the modulo of `number` and `divisor`.

      SELECT x FROM Magazine x WHERE MOD(x.price, 10) = 0

- `CURRENT_DATE`: Returns the current date.

- `CURRENT_TIME`: Returns the current time.

- `CURRENT_TIMESTAMP`: Returns the current timestamp.

### 10.1.5. Polymorphic Queries

All JPQL queries are polymorphic, which means the `from` clause of a query includes not only instances of the specific entity class to which it refers, but all subclasses of that class as well. The instances returned by a query include instances of the subclasses that satisfy the query conditions. For example, the following query may return instances of `Magazine`, as well as `Tabloid` and `Digest` instances, where `Tabloid` and `Digest` are `Magazine` subclasses.

    SELECT x FROM Magazine x WHERE x.price < 5

### 10.1.6. Query Parameters

JPQL provides support for parameterized queries. Either named parameters or positional parameters may be specified in the query string. Parameters allow you to re-use query templates where only the input parameters vary. A single query can declare either named parameters or positional parameters, but is not allowed to declare both named and positional parameters.

    public Query setParameter (int pos, Object value);

Specify positional parameters in your JPQL string using an integer prefixed by a question mark. You can then populate the `Query` object with positional parameter values via calls to the `setParameter` method above. The method returns the `Query` instance for optional method chaining.

    EntityManager em = ...
    Query q = em.createQuery ("SELECT x FROM Magazine x WHERE x.title = ?1 and x.price > ?2");
    q.setParameter (1, "JDJ").setParameter (2, 5.0);
    List<Magazine> results = (List<Magazine>) q.getResultList ();

This code will substitute `JDJ` for the `?1` parameter and `5.0` for the `?2` parameter, then execute the query with those values.

    public Query setParameter (String name, Object value);

Named parameter are denoted by prefixing an arbitrary name with a colon in your JPQL string. You can then populate the `Query` object with parameter values using the method above. Like the positional parameter method, this method returns the `Query` instance for optional method chaining.

    EntityManager em = ...
    Query q = em.createQuery ("SELECT x FROM Magazine x WHERE x.title = :titleParam and x.price > :priceParam");
    q.setParameter ("titleParam", "JDJ").setParameter ("priceParam", 5.0);
    List<Magazine> results = (List<Magazine>) q.getResultList ();

This code substitutes `JDJ` for the `:titleParam` parameter and `5.0` for the `:priceParam` parameter, then executes the query with those values.

### 10.1.7. Ordering

JPQL queries may optionally contain an `order by` clause which specifies one or more fields to order by when returning query results. You may follow the `order by field` clause with the `asc` or `desc` keywords, which indicate that ordering should be ascending or descending, respectively. If the direction is omitted, ordering is ascending by default.

    SELECT x FROM Magazine x order by x.title asc, x.price desc

The query above returns `Magazine` instances sorted by their title in ascending order. In cases where the titles of two or more magazines are the same, those instances will be sorted by price in descending order.

### 10.1.8. Aggregates

JPQL queries can select aggregate data as well as objects. JPQL includes the `min`, `max`, `avg`, and `count` aggregates. These functions can be used for reporting and summary queries.

The following query will return the average of all the prices of all the magazines:

    EntityManager em = ...
    Query q = em.createQuery ("SELECT AVG(x.price) FROM Magazine x");
    Number result = (Number) q.getSingleResult ();

The following query will return the highest price of all the magazines titled "JDJ":

    EntityManager em = ...
    Query q = em.createQuery ("SELECT MAX(x.price) FROM Magazine x WHERE x.title = 'JDJ'");
    Number result = (Number) q.getSingleResult ();

### 10.1.9. Named Queries

Query templates can be statically declared using the `NamedQuery` and `NamedQueries` annotations. For example:

    @Entity
    @NamedQueries({
        @NamedQuery(name="magsOverPrice",
            query="SELECT x FROM Magazine x WHERE x.price > ?1"),
        @NamedQuery(name="magsByTitle",
            query="SELECT x FROM Magazine x WHERE x.title = :titleParam")
    })
    public class Magazine
    {
        ...
    }

These declarations will define two named queries called `magsOverPrice` and `magsByTitle`.

    public Query createNamedQuery (String name);

You retrieve named queries with the above `EntityManager` method. For example:

    EntityManager em = ...
    Query q = em.createNamedQuery ("magsOverPrice");
    q.setParameter (1, 5.0f);
    List<Magazine> results = (List<Magazine>) q.getResultList ();

    EntityManager em = ...
    Query q = em.createNamedQuery ("magsByTitle");
    q.setParameter ("titleParam", "JDJ");
    List<Magazine> results = (List<Magazine>) q.getResultList ();

### 10.1.10. Delete By Query

Queries are useful not only for finding objects, but for efficiently deleting them as well. For example, you might delete all records created before a certain date. Rather than bringing these objects into memory and delete them individually, JPA allows you to perform a single bulk delete based on JPQL criteria.

Delete by query uses the same JPQL syntax as normal queries, with one exception: begin your query string with the `delete` keyword instead of the `select` keyword. To then execute the delete, you call the following `Query` method:

    public int executeUpdate ();

This method returns the number of objects deleted. The following example deletes all subscriptions whose expiration date has passed.

Example 10.1. Delete by Query

    Query q = em.createQuery ("DELETE s FROM Subscription s WHERE s.subscriptionDate < :today");
    q.setParameter ("today", new Date ());
    int deleted = q.executeUpdate ();

### 10.1.11. Update By Query

Similar to bulk deletes, it is sometimes necessary to perform updates against a large number of queries in a single operation, without having to bring all the instances down to the client. Rather than bring these objects into memory and modifying them individually, JPA allows you to perform a single bulk update based on JPQL criteria.

Update by query uses the same JPQL syntax as normal queries, except that the query string begins with the `update` keyword instead of `select`. To execute the update, you call the following `Query` method:

    public int executeUpdate ();

This method returns the number of objects updated. The following example updates all subscriptions whose expiration date has passed to have the "paid" field set to true.

Example 10.2. Update by Query

    Query q = em.createQuery ("UPDATE Subscription s SET s.paid = :paid WHERE s.subscriptionDate < :today");
    q.setParameter ("today", new Date ());
    q.setParameter ("paid", true);
    int updated = q.executeUpdate ();

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