JPQL how to write multiple level join fetches? For example, Order - OrderItem - Product.
select o from Order o left join fetch o.orderItems join fetch o.orderItems.product
Join fetch can not have identification variable. How to write join fetch attribute product of orderItem in the example above?
Related
I prefer to indicate join types when I use it in database systems but when I switch to a new project, there is a single join is used. Generally I prefer to use LEFT JOIN or INNER JOIN according to my needs, but I have not found which JOIN type is considered when a single JOIN is used in PostgreSQL.
select p.uuid from Product s " +
join Category c on p.uuid = c.siteUuid
join Brand b on b.uuid = c.brandUuid
Inner Join is the default join when we use plain JOIN.
For better readablity of the queries, It is always preferred to write INNER JOIN
Reference:
https://www.postgresql.org/docs/current/queries-table-expressions.html#id-1.5.6.6.5.6.4.3.1.2
I want to get list of events using following query.
I am able to do it through normal query at database level . But I am strugulling when writing JPA query.
Request you all to help me in writing JPA query.
select e.id from event e
left outer join
(sub_portfolio s inner join
(portfolio P inner join ACADEMY a on p.academy_id=a.id) on s.portfolio_id=p.id)
on e.sub_portfolio_id=s.id
Supposing I have a Person entity that has #OneToMany relationship with Phone entity. If I want to eagerly fetch Phone entities associated with a Person, I got 2 options:
SELECT p FROM Person p JOIN p.phones
or
SELECT p FROM person p JOIN FETCH p.phones
So what is the difference between them?
Remember that, in the end, JOIN FETCH is always a JOIN.
By default #OneToMany relationships are lazy loaded.
Using JOIN FETCH you load the related entities in advance.
You can find more info reading FETCH JOIN is still a JOIN by Piotr Nowicki
I know what joins are and how to use them in plain SQL but i can't find any explanation of internet to query object with relations. I use entity,getList.size() to query objects that has oneToMany or ManyToMany associations with my entity. I'm wondering is there any way to query object with all his relations.
public Person
#OneToMany
List<Cat>
#ManyToMany
List<DormRoom>
what is do to get all object is;
Person p=PersonDAO.getWithId(1L);
p.getCats.size();
p.getsDormRooms.size();
Now i'm wondering the get fully evulated object with JPQL, CriteriaBuilder and maybe with QueryDSL.
It's called a fetch join:
select distinct p from Person p left join fetch p.cats left join fetch p.dormRooms
where p.id = :id
Beware though:
in Hibernate at least, that will only work if at most 1 of the collections is a bag (i.e. a list without any specified order column).
that will create a query returning the cartesian product of cats * dormRooms. So if a person has 100 cats and 100 dormRooms, 10,000 rows will be retrieved.
Can someone explain multiple left join logic?
For example i have 3 tables: Company, company_text, company_rank.
Company has 4 records (id's: 1,2,3,4), company_text has 4 records with company names (1-a,2-b,3-c, 4-d), company_rank has following (1-1st, 2-2nd, 3-3rd). Note that company_rank table is not having 4th record. Now i want records of all companies using LEFT join. In case if there is no rank, display as 'zzzz' and sort in the descending order of rank and when rank is null sort by descending order of id.
select * from company
LEFT JOIN company_text ON company.id = company_text.id
LEFT JOIN company_rank ON company_text.id = company_rank.id
order by isnull(company_rank.rank,'zzzz'), rank desc
Will this work?
Basically i am trying to understand how LEFT JOIN works if there are many left joins? This doc. has good info on joins: http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html but it don't have information on how multiple LEFT JOINS work? In case if multiple joins are present, how records data will be pulled?