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Aggregate Functions in EJB-QL
Selecting a ResultSet
When more than one cmp-field is to be returned by a single query method, the return
type must be of type ResultSet. This allows you to select multiple cmp-fields from the
same or multiple EJBs in the same query method. You then write code to extract the
desired data from the ResultSet. This feature is a Borland extension of the CMP 2.x
specification.
Aggregate Functions in EJB-QL
Aggregate functions are MIN, MAX, SUM, AVG, and COUNT. For the aggregate
functions MIN, MAX, SUM, and AVG, the path expression that forms the argument for
the function must terminate in a cmp-field. Also, database queries for MAX, MIN, SUM,
and AVG will return a null value if there are no rows corresponding to the argument to
the aggregate function. If the return type is an object-type, then null is returned. If the
return type is a primitive type, then the container will throw a ObjectNotFoundException
(a sub-class of FinderException) if there is no value in the query result.
The path expression to the COUNT functions may terminate in either a cmp-field or
cmr-field, or may be an identification variable.
For example, the following EJB-QL aggregate function terminates in a CMP field:
ejbSelectMaxLineItemId
java.lang.String
Local
SELECT MAX(l.lineItemId) FROM Account AS a, IN (a.accountLineItem) l
WHERE l.accountId=?1
The following restrictions must be observed for aggregate functions:
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■
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Arguments to the SUM and AVG functions must be numeric (Integer, Byte, Long,
Short, Double, Float, and BigDecimal).
Arguments to the MAX and MIN functions must correspond to orderable cmp-field
types (numeric, string, character, and dates).
The path expression that forms the argument for the COUNT function can terminate
in either a cmp-field or a cmr-field. Application performance is greatly enhanced
when the COUNT function is used to determine the size of a collection of cmr-fields.
Data Type Returns for Aggregate Functions
The following table shows the data types that can be arguments for the various
aggregate functions in EJB-QL selecting a single object, and what data types will be
returned.
An aggregate function that selects multiple objects returns a collection of the
wrappered Java data type that is returned.
Table 17.1
Aggregate functions return types
Aggregate Function Argument data type Expected return type
MIN, MAX, SUM java.lang.Integer java.lang.Integer
AVG java.lang.Integer java.lang.Double
154 BES Developer’s Guide