The Java EE 5 Tutorial

Return Types

The return type of the SELECT clause is defined by the result types of the select expressions contained within it. If multiple expressions are used, the result of the query is an Object[], and the elements in the array correspond to the order of the expressions in the SELECT clause, and in type to the result types of each expression.

A SELECT clause cannot specify a collection-valued expression. For example, the SELECT clause p.teams is invalid because teams is a collection. However, the clause in the following query is valid because the t is a single element of the teams collection:

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

The following query is an example of a query with multiple expressions in the select clause:

SELECT c.name, c.country.name
 FROM customer c
 WHERE c.lastname = ’Coss’ AND c.firstname = ’Roxane’

It returns a list of Object[] elements where the first array element is a string denoting the customer name and the second array element is a string denoting the name of the customer’s country.

Aggregate Functions in the SELECT Clause

The result of a query may be the result of an aggregate function, listed in Table 27–10.

Table 27–10 Aggregate Functions in Select Statements

Name 

Return Type 

Description 

AVG

Double

Returns the mean average of the fields. 

COUNT

Long

Returns the total number of results. 

MAX

the type of the field 

Returns the highest value in the result set. 

MIN

the type of the field 

Returns the lowest value in the result set. 

SUM

Long (for integral fields)Double (for floating point fields)BigInteger (for BigInteger fields)BigDecimal (for BigDecimal fields)

Returns the sum of all the values in the result set. 

For select method queries with an aggregate function (AVG, COUNT, MAX, MIN, or SUM) in the SELECT clause, the following rules apply:

The following example returns the average order quantity:

SELECT AVG(o.quantity)
 FROM Order o

The following example returns the total cost of the items ordered by Roxane Coss:

SELECT SUM(l.price)
FROM Order o JOIN o.lineItems l JOIN o.customer c
WHERE c.lastname = ’Coss’ AND c.firstname = ’Roxane’

The following example returns the total number of orders:

SELECT COUNT(o)
FROM Order o

The following example returns the total number of items in Hal Incandenza’s order that have prices:

SELECT COUNT(l.price)
FROM Order o JOIN o.lineItems l JOIN o.customer c
WHERE c.lastname = ’Incandenza’ AND c.firstname = ’Hal’