JPQL和Join Table

Question

My understanding of SQL and JPQL are not that great and I have been trying to create a JPQL query of the following sql statement:

select group.* from user, user_group, group 
where user_group.user_id = user.id 
and user_group.group_id = group.id 
and user.id = [userID to search]

edit: Woops I forgot to add the search by user id part to the query. I would like to get all groups that a user belongs in.

But I just cannot get the syntax correct. Any help would be greatly appreciated.

Relevant code snippets:

Group.java

@Table(name = "group")
@Entity
public class Group implements Serializable {

@Id
@GeneratedValue
@Column(name = "id")
private Integer id;

@JoinTable(name = "user_group", joinColumns = {
    @JoinColumn(name = "group_id", referencedColumnName = "id")}, inverseJoinColumns = {
    @JoinColumn(name = "user_id", referencedColumnName = "id")})
@ManyToMany
private Collection<User> userCollection;

}

User.java

@Table(name = "user")
@Entity
public class User implements Serializable {

@Id
@NotNull
@GeneratedValue
@Column(name = "id")
private Integer id;

@Column(name = "email", unique=true, nullable=false)
private String email;

@ManyToMany(mappedBy = "userCollection")
private Collection<Group> GroupCollection;
}

Answer 1

Using JPQL it would be:

TypedQuery<Group> query = em.createQuery(
    "SELECT DISTINCT g FROM User u LEFT JOIN u.groupCollection g " +
    "WHERE u = :user", Group.class);
query.setParameter("user", user);
List<Group> = query.getResultsList();

where em is your EntityManager and user is the instance of the User class for which to load group list. If you only have the user id, change with:

TypedQuery<Group> query = em.createQuery(
    "SELECT DISTINCT g FROM User u LEFT JOIN u.groupCollection g " +
    "WHERE u.id = :user", Group.class);
query.setParameter("user", userId);

It would be better to use a Set or SortedSet (or maybe a List if the user can be in the same group more than once) instead of a Collection .