EJB Query Language (EJB-QL) and WebLogic Server

EJB Query Language (QL) is a portable query language that defines finder methods for 2.0 entity EJBs with container-managed persistence. Use this SQL-like language to select one or more entity EJB objects or fields in your query. You can create queries in the deployment descriptor for any finder method other than findByPrimaryKey(). findByPrimaryKey is automatically handled by the EJB container.

 

EJB QL Requirement for EJB 2.0 Beans

The deployment descriptors must define each finder query for EJB 2.0 entity beans by using an EJB QL query string. You cannot use WebLogic Query Language (WLQL) with EJB 2.0 entity beans. WLQL is intended for use with EJB 1.1 container-managed persistence. For more information on WLQL and EJB 1.1 container-managed persistence, see Using WebLogic Query Language (WLQL) for EJB 1.1 CMP.

 

Using the EJB 2.0 WebLogic QL Extension for EJB QL

WebLogic Server has an SQL-like language, called WebLogic QL, that extends the standard EJB QL. This language works with the finder expressions and is used to query EJB objects from the RDBMS. You define the query in the weblogic-cmp-jar.xml deployment descriptor using the weblogic-ql element.

There must be a query element in the ejb-jar.file that corresponds to the weblogic-ql element in the weblogic-cmp-jar.xml file. However, the value of the weblogic-cmp-jar.xml query element overrides the value of the ejb-jar.xml query element.

These topics provide guidelines for using the WebLogic QL extension to EJB 2.0 QL:

 

upper and lower Functions

The EJB WebLogic QL upper and lower extensions convert the case of arguments to allow finder methods to return results that match the characters in an search expression but not the case. The case change is transient, for the purpose of string matching, and is not persisted in database. The underlying database must also support upper and lower functions.

 

upper

The upper function converts characters in its arguments from any case to upper case before string matching is performed. Use the upper function with an upper-case expression in a query to return all items that match the expression, regardless of case. For example:

select name from products where upper(name)='DETERGENT';

 

lower

The lower function converts characters in its arguments from any case to lower case before string matching is performed. Use the lower function with an lower-case expression in a query to return all items that match the expression, regardless of case.

select type from products where lower(name)='domestic';

 

Using ORDERBY

The EJB WebLogic QL extension ORDERBY is a keyword that works with the Finder method to specify the CMP field selection sequence for your selections.

Figure 12-1 WebLogic QL ORDERBY extension showing order by id. 

ORDERBY

 SELECT OBJECT(A) from A for Account.Bean

  ORDERBY A.id

You can specify an ORDERBY with ascending [ASC] or descending [DESC] order for multiple fields as follows:.

Figure 12-2 WebLogic QL ORDERBY extension showing order by id. with ASC and DESC 

ORDERBY <field> [ASC|DESC], <field> [ASC|DESC]

 SELECT OBJECT(A) from A for Account.Bean, OBJECT(B) from B for Account.Bean

  ORDERBY A.id ASC; B.salary DESC

Note: ORDERBY defers all sorting to the DBMS. Thus, the order of the retrieved result depends on the particular DBMS installation on top of which the bean is running

 

Using Subqueries

WebLogic Server supports the use of the following features with subqueries in EJB QL:

The relationship between WebLogic QL and subqueries is similar to the relationship between SQL queries and subqueries. Use WebLogic QL subqueries in the WHERE clause of an outer WebLogic QL query. With a few exceptions, the syntax for a subquery is the same as a WebLogic QL query.

To specify WebLogic QL, see Using the EJB 2.0 WebLogic QL Extension for EJB QL. Use those instructions with a SELECT statement that specifies a subquery as shown the following sample.

The following query selects all above average students as determined by the provided grade number: 

SELECT OBJECT(s) FROM studentBean AS s WHERE s.grade > (SELECT AVG(s2.grade) FROM StudentBean AS s2)

Note that in the above query the subquery, (SELECT AVG(s2.grade) FROM StudentBean AS s2), has the same syntax as an EJB QL query.

You can create nested subqueries.The depth is limited by the underlying database's nesting capabilities.

In a WebLogic QL query, the identifiers declared in the FROM clauses of the main query and all of its subqueries must be unique. This means that a subquery may not re-declare a previously declared identifier for local use within that subquery.

For example, the following example is not legal because employee bean is being declared as emp in both the query and the subquery: 

SELECT OBJECT(emp)



 FROM EmployeeBean As emp



  WHERE emp.salary=(SELECT MAX(emp.salary) FROM



   EmployeeBean AS emp WHERE employee.state=MA)

Instead, this query should be written as follows: 

SELECT OBJECT(emp)



 FROM EmployeeBean As emp



  WHERE emp.salary=(SELECT MAX(emp2.salary) FROM



   EmployeeBean AS emp2 WHERE emp2.state=MA)

The above examples correctly declares the subquery's employee bean to have a different identifier from the main query's employee bean.

 

Subquery Return Types

The return type of a WebLogic QL subquery can be one of a number of different types, such as:

Single cmp-field Type Subqueries

WebLogic Server supports a return type consisting of a cmp-field. The results returned by the subquery may consists of a single value or collection of values. An example of a subquery that returns value(s) of the type cmp-field is as follows: 

SELECT emp.salary FROM EmployeeBean AS emp WHERE emp.dept = `finance'

This subquery selects all of the salaries of employees in the finance department.

Aggregate Functions

WebLogic Server supports a return type consisting of an aggregate of a cmp-field. As an aggregate always consist of a single value, the value returned by the aggregate is always a single value. An example of a subquery that return a value of the type aggregate (MAX) of a cmp-field is as follows: 

SELECT MAX(emp.salary) FROM EmployeeBean AS emp WHERE emp.state=MA

This subquery selects the single highest employee salary in Massachusetts.

For more information on aggregate functions, see Using Aggregate Functions.

Beans with Simple Primary Key

WebLogic Server supports a return type consisting of a cmp-bean with a simple primary key.

The following example illustrates a subquery that returns the value(s) of the type bean with a simple primary key: 

SELECT OBJECT(emp) FROM EMployeeBean As emp WHERE emp.department.budget>1,000,000

This subquery provides a list of all employee in departments with budgets greater than $1,000,000.

Note: Beans with compound primary keys are NOT supported. Attempts to designate the return type of a subquery to a bean with a compound primary key will fail when you compile the query.

 

Subqueries as Comparison Operands

Use subqueries as the operands of comparison operators and arithmetic operators. WebLogic QL supports subqueries as the operands of:

  • these comparison operators: [NOT]IN, [NOT]EXISTS

and

  • these arithmetic operators: <, >, <=, >=, =, <> with ANY and ALL

[NOT]IN

The [NOT]IN comparison operator tests whether the left-had operand is or is not a member of the subquery operand on the right-hand side.

An example of a subquery which is the right-hand operand of the NOT IN operator is as follows: 

SELECT OBJECT(item)



 FROM ItemBean AS item 



  WHERE item.itemId NOT IN



   (SELECT oItem2.item.itemID



     FROM OrderBean AS orders2, IN(orders2.orderItems)oIttem2

The subquery selects all items from all orders.

The main query's NOT IN operator selects all the items that are not in the set returned by the subquery. So the end result is that the main query selects all unordered items.

[NOT]EXISTS

The [NOT]EXISTS comparison operator tests whether the set returned by the subquery operand is or is not empty.

An example of a subquery which is the operand of the NOT EXISTS operand is as follows: 

SELECT (cust) FROM CustomerBean AS cust



 WHERE NOT EXISTS



  (SELECT order.cust_num FROM OrderBean AS order



   WHERE cust.num=order_num)

This is an example of a query with a correlated subquery. See Correlated and Uncorrelated Subqueries for more information. the following query returns all customers that have not placed an order.

SELECT (cust) FROM CustomerBean AS cust



 WHERE cust.num NOT IN



  (SELECT order.cust_um FROM OrderBean AS order



   WHERE cust.num=order_num)

Arithmetic Operators

Use arithmetic operators for comparison when the right-hand subquery operand returns a single value. If the right hand subquery instead returns multiple values, then the qualifiers ANY or ALL must precede the subquery.

An example of a subquery which uses the `=' operator is as follows: 

SELECT OBJECT (order)



 FROM OrderBean AS order, IN(order.orderItems)oItem



  WHERE oItem.quantityOrdered = 



   (SELECT MAX (subOItem.quantityOrdered)



    FROM Order ItemBean AS subOItem



     WHERE subOItem,item itemID = ?1) 



AND oItem.item.itemId = ?1

For a given itemId, the subquery returns the maximum quantity ordered of that item. Note that this aggregate returned by the subquery is a single value as required by the `=' operator.

For the same given itemId, the main query's `=' comparison operator checks which order's OrderItem.quantityOrdered equals the maximum quantity returned by the subquery. The end result is that the query returns the OrderBean that contains the maximum quantity of a given item that has been ordered.

Use arithmetic operators in conjunction with ANY or ALL, when the right-hand subquery operand may return multiple values.

An example of a subquery which uses ANY and ALL is as follows: 

SELECT OBJECT (order)



 FROM OrderBean AS order, IN(order.orderItems)oItem



  WHERE oItem.quantityOrdered > ALL



   (SELECT subOItem.quantityOrdered



    FROM OrderBean AS suborder IN (subOrder.orderItems)subOItem



     WHERE subOrder,orderId = ?1)

For a given orderId, the subquery returns the set of orderItem.quantityOrdered of each item ordered for that orderId. The main query's `>' ALL operator looks for all orders whose orderItem.quantityOrdered exceeds all values in the set returned by the subquery. The end result is that the main query returns all orders in which all orderItem.quantityOrdered exceeds every orderItem.quantityOrdered of the input order.

Note that since the subquery can return multi-valued results that they `>'ALL operator is used rather then the `>' operator.

 

Correlated and Uncorrelated Subqueries

WebLogic Server supports both correlated and Uncorrelated subqueries.

UnCorrelated Subqueries

Uncorrelated subqueries may be evaluated independently of the outer query. An example of an uncorrelated subquery is as follows: 

SELECT OBJECT(emp) FROM EmployeeBean AS emp



 WHERE emp.salary>



 (SELECT AVG(emp2.salary) FROM EmployeeBean AS emp2)

This example of a uncorrelated subquery selects the employees whose salaries are above average. This examples uses the `>' arithmetic operator.

Correlated

Correlated subqueries are subqueries in which values from the outer query are involved in the evaluation of the subquery. An example of a correlated subquery is as follows: 

SELECT OBJECT (mainOrder) FROM OrderBean AS mainOrder



 WHERE 10>



  (SELECT COUNT (DISTINCT subOrder.ship_date)



 FROM OrderBean AS subOrder



 WHERE subOrder.ship_date>mainOrder.ship_date



 AND mainOrder.ship_date IS NOT NULL

This example of a correlated subquery selects the last 10 shipped orders. This example uses the NOT IN operator.

Note: Keep in mind that correlated subqueries can involve more processing overhead the uncorrelated subqueries.

 

DISTINCT Clause with Subqueries

Use the DISTINCT clause in a subquery to enable an SQL SELECT DISTINCT in the subquery's generated SQL. Using a DISTINCT clause in a subquery is different from using one in a main query because the EJB container enforces the DISTINCT clause in a main query; whereas the DISTINCT clause in the subquery is enforced by the generated SQL SELECT DISTINCT. The following is an example of a DISTINCT clause in a subquery: 

SELECT OBJECT (mainOrder) FROM OrderBean AS mainOrder



 WHERE 10>



  (SELECT COUNT (DISTINCT subOrder.ship_date)



 FROM OrderBean AS subOrder



 WHERE subOrder.ship_date>mainOrder.ship_date



 AND mainOrder.ship_date IS NOT NULL

 

Using Aggregate Functions

WebLogic Server supports aggregate functions with WebLogic QL. You only use these functions as SELECT clause targets, not as other parts of a query, such as a WHERE clause. The aggregate functions behave like SQL functions. They are evaluated over the range of the beans returned by the WHERE conditions of the query

To specify WebLogic QL, see Using the EJB 2.0 WebLogic QL Extension for EJB QL. Use those instructions with a SELECT statement that specifies an aggregate function as shown in the samples shown in the following table.

A list of the supported functions and sample statements follow:

Aggregate
Function

Description

Sample Statement

MIN(x) Returns the minimum value of this field. SELECT MIN(t.price) FROM TireBean AS t WHERE t.size=?1
This statement selects the lowest price for a tire of a given input size.
MAX(x) Returns the maximum value of this field. SELECT MAX(s.customer_count) FROM SalesRepBean AS s WHERE s.city='Los Angeles'
This statement selects the maximum number of customers served by any single sales representative in Los Angeles.
AVG( [DISTINCT] x) Returns the average value of this field SELECT AVG(b.price) FROM BookBean AS b WHERE b.category='computer_science'
This statement selects the Average Price of a book in the Computer Science category.
SUM( [DISTINCT] x) Returns the sum of this field. SELECT SUM(s.customer_count) FROM SalesRepBean AS s WHERE s.city='Los Angeles'
This statement retrieves the total number of customers served by sales representatives in Los Angeles.
COUNT( [DISTINCT] x) Returns the number of occurrences of a field. SELECT COUNT(s.deal.amount) FROM SalesRepBean AS s, IN(deal)s WHERE s.deal.status='closed' AND s.deal.amount>=1000000
This statement retrieves the number of closed deals for at lease 1 million dollars.

You can return aggregate functions in ResultSets as described below.

 

Using Queries that Return ResultSets

WebLogic Server supports ejbSelect() queries that return the results of multi-column queries in the form of a java.sql.ResultSet. To support this feature, WebLogic Server now allows you to use the SELECT clause to specify a comma delimited list of target fields as shown in the following query: 

SELECT emmp.name, emp.zip FROM EmployeeBean AS emp

This query returns a java.sqlResultSet with rows whose columns are the values Employee's Name and Employee's Zip.

To specify WebLogic QL, see Using the EJB 2.0 WebLogic QL Extension for EJB QL. Use those instructions with a query specifying a ResultSet as shown in the above query to specify WebLogic QL, see Using the EJB 2.0 WebLogic QL Extension for EJB QL. Use those instructions with a SELECT statement that specifies an aggregate query like the samples shown in the following table.

.

ResultSets created in EJB QL can only return cmp-field values or aggregates of cmp-field values, they cannot return beans.

In addition, you can create powerful queries, as described in the following example, when you combine cmp-fields and aggregate functions.

The following rows (beans) show the salaries of employees in different locations:

Name

Location

Salary

Matt CA 110,000
Rob CA 100,000

CMP fields showing salaries of employees in Arizona

Name

Location

Salary

Dan AZ 120,000
Dave AZ 80,000

CMP fields showing salaries of employees in Texas

Name

Location

Salary

Curly TX 70,000
Larry TX 180,000
Moe TX 80,00

Note: Each row represents a bean.

The following SELECT statement shows a query that uses ResultSets and the aggregate function (AVG) along with a GROUP BY statement and an ORDER BY statement using a descending sort to retrieve results from a multi-column query. 

SELECT e.location, AVG(e.salary)



 FROM Finder EmployeeBean AS e



  GROUP BY e.location



   ORDER BY 2 DESC

The query shows the average salary in of employees at each location in descending order. The number, 2 means that the ORDERBY sort is on the second item in the SELECT statement. The GROUP BY clause specifies the AVERAGE salary of employees with a matching e.location attribute.

The ResultSet, in descending order is as follows:

Location

Average

TX 110,000
AZ 100,000
CA 105,000

Note: You can only use integers as ORDERBY arguments in queries that return ResultSets. WebLogic Server does not support the use of integers as ORDERBY arguments in any Finder or ejbselect() that returns beans.

 

Using Oracle SELECT HINTS

WebLogic Server supports an EJB QL extension that allows you to pass INDEX usage hints to the Oracle Query optimizer. With this extension, you can provide a hint to the database engine. For example, if you know that the database you are searching can benefit from an ORACLE_SELECT_HINT, you can define an ORACLE_SELECT_HINT clause that will take ANY string value and then insert that String value after the SQL SELECT statement as a hint to the database.

To use this option, declare a query that uses this feature in the weblogic-ql element in weblogic-cmp-jar.xml. The weblogic-ql element specifies a query that contains a WebLogic specific extension to the EJB-QL language.

The WebLogic QL keyword and usage is as follows: 

SELECT OBJECT(a) FROM BeanA AS a WHERE a.field > 2 ORDERBY a.field SELECT_HINT '/*+ INDEX_ASC(myindex) */'

This statement generates the following SQL with the optimizer hint for Oracle:

SELECT /*+ INDEX_ASC(myindex) */ column1 FROM ....

In the WebLogic QL ORACLE_SELECT_HINT clause, whatever is between the single quotes (' ') is what gets inserted after the SQL SELECT. It is the query writer's responsibility to make sure that the data within the quotes makes sense to the Oracle database."get" and "set" Method Restrictions

WebLogic Server uses a series of accessor methods. The names of these methods begin with set and get. WebLogic Server uses these methods to read and modify container-managed fields. These container-generated classes must begin with "get" or "set" and use the actual name of a persistent field defined in ejb-jar.xml. The methods are also declared as public, protected, and abstract.

 

Properties-Based Methods of the Query Interface

The Query interface contains both find and execute methods. The find methods work like standard EJB methods, in that they return EJBObjects. The execute methods work more like Select statements in that you can select individual fields.

The Query interface return type is a disconnected ResultSet, meaning you access the information from the returned object the same way you would access it from a ResultSet, except that the ResultSet does not hold open a database connection.

The Query interface's properties-based methods offer an alternate way of specifying settings particular to a query. The QueryProperties interface holds standard EJB query settings while the WLQueryProperties interface holds WebLogic-specific query settings.

Although the Query interface extends QueryProperties, the actual Query implementation extends WLQueryProperties so it can be safely casted, as in the example in Figure 12-3, which sets field group settings:

Figure 12-3 Setting Field Group Settings with WLQueryProperties 

Query query=qh.createQuery(); ((WLQueryProperties) query).setFieldGroupName("myGroup"); Collection results=query.find(ejbql);

or 

Query query=qh.createQuery(); Properties props = new Properties(); props.setProperty(WLQueryProperties.GROUP_NAME, "myGroup"); Collection results=query.find(ejbql, props);

 

Migrating from WLQL to EJB QL

If you have an existing application that uses EJB 1.1, your container-managed entity EJBs may use WLQL for finder methods. This section provides a quick reference to common WLQL operations. Use this table to map the WLQL syntax to EJB QL syntax.

Sample WLQL Syntax

Equivalent EJB QL Syntax

(= operand1 operand2) WHERE operand1 = operand2
(< operand1 operand2) WHERE operand1 < operand2
(> operand1 operand2) WHERE operand1 > operand2
(<= operand1 operand2) WHERE operand1 <= operand2
(>= operand1 operand2) WHERE operand1 >= operand2
(! operand) WHERE NOT operand
(& expression1 expression2) WHERE expression1 AND expression2
(| expression1 expression2) WHERE expression1 OR expression2
(like text_string%) WHERE operand LIKE `text_string%'
(isNull operand) WHERE operand IS NULL
(isNotNull operand) WHERE operand IS NOT NULL

 

Known Issue with Implied Cross Products

When an EJB QL query contains an implied cross product - as opposed to an explicit one - the EJB-QL query can return an empty result.

Consider this example query: 

SELECT OBJECT(e) FROM EmployeeBean AS e WHERE e.name LIKE 'Joe' OR e.acct.balance < 100

This query references AccountEJB, but AccountEJB is not listed in the FROM clause. The result of this query is identical to that of query with AccountEJB explicitly listed in the FROM clause.

 

EJB QL Error-Reporting

Compiler error messages in EJB QL provide a visual aid to identify which part of the query is in error and allow the reporting of more than one error per compilation.

 

Visual Indicator of Error in Query

When an error is reported, EJB QL indicates the location of the problem within these symbols: =>> <<=. These symbols are highlighted in red in the following sample compiler error report. 

ERROR: Error from appc: Error while reading 'META-INF/FinderEmployeeBeanRDBMS.xml'. The error was: 

Query: 

EJB Name: FinderEmployeeEJB 

Method Name: findThreeLowestSalaryEmployees 

Parameter Types: (java.lang.String) 

Input EJB Query: SELECT OBJECT(e) FROM FinderEmployeeBean e WHERE f.badField = '2' O 

R (e.testId = ?1) ORDERBY e.salary 

SELECT OBJECT(e ) FROM FinderEmployeeBean e 



WHERE =>> f.badField <<= = '2' OR ( e.testId = ?1 ) ORDERBY e.salary 

Invalid Identifier in EJB QL expression: 

Problem, the path expression/Identifier 'f.badField' starts with an identifier: 'f'. The identifier 'f', which can be either a range variable identifier or a collection member identifier, is required to be declared in the FROM clause of its query or in the FROM clause of a parent query. 

'f' is not defined in the FROM clause of either its query or in any parent query. 

Action, rewrite the query paying attention to the usage of 'f.badField'.

 

Multiple Errors Reported after a Single Compilation

If a query contains multiple errors, EJB QL is now capable of reporting more than one of these after a single compilation. Previously, the compiler could only report one error per compilation. Reporting of subsequent errors required recompilation.

The compiler is not guaranteed to report all errors after a single compilation.

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