Seriously?
How come the godlike Java dev in this community do not know that thing?
I am shocked ! 
Now for a bit of context
Now as for what JPQL is, it stands for Java Persistance Query Language and is related to JPA standard. This is a language to query over objects (AFAIK because I have 3 days of training on JPA, no more). It is very close to SQL. Simply, instead of working with SQL entities, we work with Java objects.
One example from this afternoon.
I did prepare my JPQL with SQL solution ahead so it drove how I wrote my queries.
One of them would look like this:
SELECT p FROM Product p
JOIN p.order o
JOIN o.client cl
WHERE cl.zipCode IN(12345,54321)
AND o.quantity > 3
could have been written like this because:
Product type has an order fieldOrder type has a client fieldClient type has a zipCode field
SELECT p FROM Product p
WHERE p.order.client.zipCode IN(12345,54321)
AND o.quantity > 3
I actually have a Discord server with people that were on a previous training with me and part of the class took Java (was my initial pick) and the rest took .NET/C# (where I landed in the end by reconsidering my choice).
In C# we have LINQ (Language INtegrated Query) which splits into sub-techs (LINQ to XML, LINQ to SQL, LINQ to Objects…).
So I asked the Java mates if there was such thing as LINQ in Java. I was told there isn’t. In my own experience since I started my current training, my guess would be a bit like this (though some differences may apply) :
| .NET / C# |
Java (Stack) |
| LINQ to Objects |
Streams |
| LINQ Query Syntax |
JPQL |
As of LINQ, it needs a bit of practice of course.
We have a convenience tool called LINQPad which allows us to practice outside of an IDE, without having to compile anything.
With it, we can query databases in different fashion (for the most part):
- SQL
- C# Expression
- C# Statement
- C# Program
It can also give advanced information such as
IL (Intermediate Language → equivalent to byte code if I’m not mistaken)
Or Expression Tree
In the following picture, we can see on the left a panel showing the available connections and tables. The main usage for LINQPad is trying out queries.
I wrote the same query in different ways as sample.
SQL
SELECT * from Orders
WHERE ShippedDate IS NOT NULL
AND OrderDate BETWEEN '1997-09-25' AND '1997-10-10'
ORDER BY OrderDate;
LINQ Query Syntax
from o in Orders
where o.ShippedDate != null
&& o.OrderDate >= new DateTime(1997,09,25)
&& o.OrderDate <= new DateTime(1997,10,10)
orderby o.OrderDate
select o
LINQ Method/Operator Syntax
For that one there is a Dump() extension method available to actually
display the result. It does not appear in C# code.
Orders.Where(o => o.ShippedDate != null
&& o.OrderDate >= new DateTime(1997,09,25)
&& o.OrderDate <= new DateTime(1997,10,10))
.OrderBy(o=> o.OrderDate).Dump();
A “JPQLPad” software would definitely be a nice bonus. At least for querying with JPQL.
