What is EJBQL stands for

1
What is EJB-QL stands for?

• Enterprise Java Beans
• QUERY LANGUAGE

2
Why an EJB-Query Language ?

• No standard way to define queries with
  container-managed persistence.
• EJB container providers have their own
  DB-systems and query syntax.
• So, portability optimization problems between
  containers.

3
EJB Query Language is

• an SQL like language(EJB QL select_clause
  from_clause where_clause)
• a portable query language for EJB 2.0
• a specification which can be compiled into the
  target platforms language.

4
How to use EJB-QL ?

• How to define Relationships among Beans?
• Defined Keywords.
• How to write QUERY ?

5
Example system
6
STUDENT to TAKE_COURSE relation in deployment
descriptor

• ltrelationshipsgt ltejb-relationgt  ltejb-relatio
  n-namegtTAKE_COURSE-STUDENTlt/ejb-relation-namegt  lt
  ejb-relationship-rolegt    ltejb-relationship-role-
  namegt course-takenBy-students  lt/ejb-relationship-
  role-namegt    ltmultiplicitygtManylt/multiplicitygt 
     ltrelationship-role-sourcegt      ltejb-namegtTake
  _CourseEJBlt/ejb-namegt    lt/relationship-role-sour
  cegt    ltcmr-fieldgt      ltcmr-field-namegt
  studentslt/cmr-field-namegt      ltcmr-field-typegt
  Student  lt/cmr-field-typegt    lt/cmr-fieldgt  lt/ej
  b-relationship-rolegtlt/ejb-relationgtlt/relation
  shipsgt

7

• ltrelationshipsgt ltejb-relationgt  ltejb-relatio
  n-namegtSTUDENT-TAKE_COURSElt/ejb-relation-namegt  lt
  ejb-relationship-rolegt    ltejb-relationship-role-
  namegt student-take-course  lt/ejb-relationship-role
  -namegt    ltmultiplicitygtOnelt/multiplicitygt    ltr
  elationship-role-sourcegt      ltejb-namegtStudentEJ
  Blt/ejb-namegt    lt/relationship-role-sourcegt    lt
  cmr-fieldgt      ltcmr-field-namegt
  takenCourseslt/cmr-field-namegt      ltcmr-field-typ
  egt Take_Course  lt/cmr-field-typegt    lt/cmr-fieldgt
    lt/ejb-relationship-rolegtlt/ejb-relationgtlt/r
  elationshipsgt

8
Available query keywords
EJB-QL covers a subset of SQL92.
9
Query Example 1

• SELECT OBJECT(s)
• FROM STUDENT s
• Method findall()

10
Query Example 2

• SELECT DISTINCT OBJECT(s)
• FROM STUDENT s
• WHERE s.department ?1
• Method findByDepartment(String department)

11
Query Example 3

• SELECT DISTINCT c.name
• FROM STUDENT s, IN (s.courses) AS c
• WHERE s ?1
• Method ejbSelectCourseNames
• (LocalStudent student)

12
Query Example 4

• SELECT DISTINCT OBJECT(s)
• FROM STUDENT s, IN (s.courses) AS c
• WHERE c.instructor ?1
• Method findByInstructor(String instructor)

13
Query 4 in deployment descriptor

• ltquerygtltdescriptiongtMethod to find students who
  takes course from a given instructorlt/descriptiongt
    ltquery-methodgt    ltmethod-namegtfindByInstructo
  rlt/method-namegt    ltmethod-paramsgt      ltmethod-
  paramgtStringlt/method-paramgt    lt/method-paramsgt  
    lt/query-methodgtltresult-type-mappinggtLocalStude
  ntlt/result-type-mappinggt  ltejb-qlgt
• SELECT DISTINCT OBJECT(s)
• FROM STUDENT s, IN (s.courses) AS c
• WHERE c.instructor ?1 lt/ejb-qlgt
• lt/querygt

14
Query 4 in StudentEJB

• public abstract class StudentBean implements
  javax.ejb.EntityBean
• ...
• public abstract java.util.Collection
  selectByInstructor
• (String instructor) throws FinderException

15
Restrictions of EJB-QL

• No comments allowed.
• Date and time in Java long format. (but
  java.util.Calendar class can convert to long)
• Inheritance not supported.
• Not all SQL features are supported.(Yet)(example
  GROUP BY, ORDER BY)

16
Questions ?

• Kerem Basol
• Ö. Bilal Orhan