Example Queries

The following queries are from the Player entity of the roster application, which is documented in Chapter 26.

Simple Queries

If you are unfamiliar with the query language, these simple queries are a good place to start.

A Basic Select Query

   SELECT p    FROM Player p

Data retrieved: All players.

Description: The FROM clause declares an identification variable named p, omitting the optional keyword AS. If the AS keyword were included, the clause would be written as follows:

   FROM Player AS p

The Player element is the abstract schema name of the Player entity.

See also: Identification Variables (page 877)

Eliminating Duplicate Values

   SELECT DISTINCT p    FROM Player p    WHERE p.position = ?1

Data retrieved: The players with the position specified by the query's parameter.

Description: The DISTINCT keyword eliminates duplicate values.

The WHERE clause restricts the players retrieved by checking their position, a persistent field of the Player entity. The ?1 element denotes the input parameter of the query.

See also: Input Parameters (page 883), The DISTINCT Keyword (page 894)

Using Named Parameters

   SELECT DISTINCT p    FROM Player p    WHERE p.position = :position AND p.name = :name

Data retrieved: The players having the specified positions and names.

Description: The position and name elements are persistent fields of the Player entity. The WHERE clause compares the values of these fields with the named parameters of the query, set using the Query.setNamedParameter method. The query language denotes a named input parameter using colon (:) followed by an identifier. The first input parameter is :position, the second is :name.

Queries That Navigate to Related Entities

In the query language, an expression can traverse (or navigate) to related entities. These expressions are the primary difference between the Java Persistence query language and SQL. Queries navigates to related entities, whereas SQL joins tables.

A Simple Query with Relationships

   SELECT DISTINCT p    FROM Player p, IN(p.teams) t

Data retrieved: All players who belong to a team.

Description: The FROM clause declares two identification variables: p and t. The p variable represents the Player entity, and the t variable represents the related Team entity. The declaration for t references the previously declared p variable. The IN keyword signifies that teams is a collection of related entities. The p.teams expression navigates from a Player to its related Team. The period in the p.teams expression is the navigation operator.

You may also use the JOIN statement to write the same query:

   SELECT DISTINCT p    FROM Player p JOIN p.teams t

This query could also be rewritten as:


Navigating to Single-Valued Relationship Fields

Use the JOIN clause statement to navigate to a single-valued relationship field:

   SELECT t    FROM Team t JOIN t.league l    WHERE l.sport = 'soccer' OR l.sport ='football'

In this example, the query will return all teams that are in either soccer or football leagues.

Traversing Relationships with an Input Parameter

   SELECT DISTINCT p    FROM Player p, IN (p.teams) AS t    WHERE t.city = :city

Data retrieved: The players whose teams belong to the specified city.

Description: This query is similar to the previous example, but it adds an input parameter. The AS keyword in the FROM clause is optional. In the WHERE clause, the period preceding the persistent variable city is a delimiter, not a navigation operator. Strictly speaking, expressions can navigate to relationship fields (related entities), but not to persistent fields. To access a persistent field, an expression uses the period as a delimiter.

Expressions cannot navigate beyond (or further qualify) relationship fields that are collections. In the syntax of an expression, a collection-valued field is a terminal symbol. Because the teams field is a collection, the WHERE clause cannot specify p.teams.cityan illegal expression.

See also: Path Expressions (page 880)

Traversing Multiple Relationships

   SELECT DISTINCT p    FROM Player p, IN (p.teams) t    WHERE t.league = :league

Data retrieved: The players that belong to the specified league.

Description: The expressions in this query navigate over two relationships. The p.teams expression navigates the Player-Team relationship, and the t.league expression navigates the Team-League relationship.

In the other examples, the input parameters are String objects, but in this example the parameter is an object whose type is a League. This type matches the league relationship field in the comparison expression of the WHERE clause.

Navigating According to Related Fields

   SELECT DISTINCT p    FROM Player p, IN (p.teams) t    WHERE t.league.sport = :sport

Data retrieved: The players who participate in the specified sport.

Description: The sport persistent field belongs to the League entity. To reach the sport field, the query must first navigate from the Player entity to Team (p.teams) and then from Team to the League entity (t.league). Because the league relationship field is not a collection, it can be followed by the sport persistent field.

Queries with Other Conditional Expressions

Every WHERE clause must specify a conditional expression, of which there are several kinds. In the previous examples, the conditional expressions are comparison expressions that test for equality. The following examples demonstrate some of the other kinds of conditional expressions. For descriptions of all conditional expressions, see the section WHERE Clause (page 881).

The LIKE Expression

   SELECT p    FROM Player p    WHERE p.name LIKE 'Mich%'

Data retrieved: All players whose names begin with "Mich."

Description: The LIKE expression uses wildcard characters to search for strings that match the wildcard pattern. In this case, the query uses the LIKE expression and the % wildcard to find all players whose names begin with the string "Mich." For example, "Michael" and "Michelle" both match the wildcard pattern.

See also: LIKE Expressions (page 885)

The IS NULL Expression

   SELECT t    FROM Team t    WHERE t.league IS NULL

Data retrieved: All teams not associated with a league.

Description: The IS NULL expression can be used to check if a relationship has been set between two entities. In this case, the query checks to see if the teams are associated with any leagues, and returns the teams that do not have a league.

See also: NULL Comparison Expressions (page 886), NULL Values (page 890)

The IS EMPTY Expression

   SELECT p    FROM Player p    WHERE p.teams IS EMPTY

Data retrieved All players who do not belong to a team.

Description The teams relationship field of the Player entity is a collection. If a player does not belong to a team, then the teams collection is empty and the conditional expression is TRUE.

See also: Empty Collection Comparison Expressions (page 886)

The BETWEEN Expression

   SELECT DISTINCT p    FROM Player p    WHERE p.salary BETWEEN :lowerSalary AND :higherSalary

Data retrieved The players whose salaries fall within the range of the specified salaries.

Description This BETWEEN expression has three arithmetic expressions: a persistent field (p.salary) and the two input parameters (:lowerSalary and :higherSalary). The following expression is equivalent to the BETWEEN expression:

   p.salary >= :lowerSalary AND p.salary <= :higherSalary

See also BETWEEN Expressions (page 884)

Comparison Operators

   SELECT DISTINCT p1    FROM Player p1, Player p2    WHERE p1.salary > p2.salary AND p2.name = :name

Data retrieved All players whose salaries are higher than the salary of the player with the specified name.

Description The FROM clause declares two identification variables (p1 and p2) of the same type (Player). Two identification variables are needed because the WHERE clause compares the salary of one player (p2) with that of the other players (p1).

See also Identification Variables (page 877)

Bulk Updates and Deletes

The following examples show how to use the UPDATE and DELETE expressions in queries. UPDATE and DELETE operate on multiple entities according to the condition or conditions set in the WHERE clause. The WHERE clause in UPDATE and DELETE queries follows the same rules as SELECT queries.

Update Queries

   UPDATE Player p    SET p.status = 'inactive'    WHERE p.lastPlayed < :inactiveThresholdDate

Description: This query sets the status of a set of players to inactive if the player's last game was longer than the date specified in inactiveThresholdDate.

Delete Queries

   DELETE    FROM Player p    WHERE p.status = 'inactive'    AND p.teams IS EMPTY

Description: This query deletes all inactive players who are not on a team.

The JavaT EE 5 Tutorial
The JavaT EE 5 Tutorial
Year: 2004
Pages: 309

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