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I saw JPQL queries using JOIN, LEFT JOIN and JOIN FETCH statement. What are the differences between these 3 join statements?
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You might know the JOIN and LEFT JOIN statements from SQL. It supports clauses with the same name and a very similar syntax. The JOIN FETCH clause is specific to JPA.
Let’s take a closer look at all 3 options.
In JPQL, you can define a JOIN statement based on a specified association between 2 entities. Your persistence provider, e.g., Hibernate, translates this into an SQL JOIN statement.
The SQL JOIN statement tells the database to combine the columns of 2 tables to a set which you can use within the same query. You can define a join condition that specifies which rows of each table shall be joined with each other. All rows which don’t fulfill the condition are not part of the set.
In most cases, this condition compares the values of a primary key column of one table with the values of a foreign key column of an associated table. But you can also define more complex conditions using multiple columns of both tables.
In JPQL, you need to define join statements based on association mappings. This mapping provides the names of the foreign key and primary key columns. That makes the definition of the join statement easier, but you can’t add any additional predicates.
Here’s an example of a JPQL query that returns all Author entities who’ve written a Book which title contains the word “Hibernate”. It joins the Author entity with the Book entity and uses the title attribute of the Book in the WHERE clause.
List<Author> authors = em.createQuery("SELECT a FROM Author a JOIN a.books b WHERE b.title LIKE '%Hibernate%'", Author.class).getResultList();
After you activated the logging of SQL statements, you can see that Hibernate generates the following statement for the JPQL query. It uses the defined many-to-many association to join the Author table with the association table BookAuthor. It then joins the association table with the Book table.
16:41:15,056 DEBUG [org.hibernate.SQL] - select author0_.id as id1_0_, author0_.firstName as firstNam2_0_, author0_.lastName as lastName3_0_, author0_.version as version4_0_ from Author author0_ inner join BookAuthor books1_ on author0_.id=books1_.authorId inner join Book book2_ on books1_.bookId=book2_.id where book2_.title like '%Hibernate%'
The LEFT JOIN statement is similar to the JOIN statement. The main difference is that a LEFT JOIN statement includes all rows of the entity or table referenced on the left side of the statement.
I use that in the following example to select all Authors with the lastName “Janssen” and their Books. If the database contains a Book for a specific Author, the query returns it as the second element in the Object. Otherwise, that array element is null.
A simple JOIN statement would only return the Authors who have written a Book. The second element of the Object would never be null.
List<Object> authors = em.createQuery("SELECT a, b FROM Author a LEFT JOIN a.books b WHERE a.lastName = 'Janssen'").getResultList();
Hibernate generates the following SQL statement for this query. It selects all columns mapped by the Author and Book entities and uses the defined association to create a left join between the Book and the Author tables.
16:54:10,510 DEBUG [org.hibernate.SQL] - select author0_.id as id1_0_0_, book2_.id as id1_1_1_, author0_.firstName as firstNam2_0_0_, author0_.lastName as lastName3_0_0_, author0_.version as version4_0_0_, book2_.publisherid as publishe5_1_1_, book2_.publishingDate as publishi2_1_1_, book2_.title as title3_1_1_, book2_.version as version4_1_1_ from Author author0_ left outer join BookAuthor books1_ on author0_.id=books1_.authorId left outer join Book book2_ on books1_.bookId=book2_.id where author0_.lastName='Janssen'
The FETCH keyword of the JOIN FETCH statement is JPA-specific. It tells the persistence provider to not only join the 2 database tables within the query but to also initialize the association on the returned entity. You can use it with a JOIN and a LEFT JOIN statement.
Let’s change the first example and replace the JOIN statement with a JOIN FETCH statement.
List<Author> authors = em.createQuery("SELECT a FROM Author a JOIN FETCH a.books b WHERE b.title LIKE '%Hibernate%'", Author.class).getResultList();
The JPQL query selects Author entities. But as you can see in the SQL statement, Hibernate now selects all columns mapped by the Author and the Book entity. Hibernate then maps the result set to Author and Book entities. It uses the Book entities to initialize the books attribute of each Author entity before it returns a List of Author entities.
16:57:53,384 DEBUG [org.hibernate.SQL] - select author0_.id as id1_0_0_, book2_.id as id1_1_1_, author0_.firstName as firstNam2_0_0_, author0_.lastName as lastName3_0_0_, author0_.version as version4_0_0_, book2_.publisherid as publishe5_1_1_, book2_.publishingDate as publishi2_1_1_, book2_.title as title3_1_1_, book2_.version as version4_1_1_, books1_.authorId as authorId2_2_0__, books1_.bookId as bookId1_2_0__ from Author author0_ inner join BookAuthor books1_ on author0_.id=books1_.authorId inner join Book book2_ on books1_.bookId=book2_.id where book2_.title like '%Hibernate%'
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JPQL is very similar to SQL and provides you with powerful query capabilities. You can learn more about it in my Ultimate Guide to JPQL Queries.
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