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Spring Repository without #Query

I am working on a desktop application built using spring framework and one of the part of the application is not working. I found that the repository class does not have any queries with #Query annotation. I haven't encountered it before.
When I try to open the form that uses this, I get an error that the application is not able to connect to the database. The application has 3 databases specified in the application.properties. I have the following questions:
1) How does the following code work without a query specified with #Query annotation. Or where is the query written.
#Repository
public interface AccountRepository extends JpaRepository<Account, Long> {
List<Account> findAccountsByActiveIsTrueAndAccountTypeEquals(String accountType);
List<Account> findAccountsByAccountTypeLike(String type);
}
2) How do we specify which of the database to search for. For example: I have 3 mysql databases currently connected to my application. I wish to access data from DB1 through my Spring boot application through the usual flow of
UI model-> BE Controller/ Service layer -> Repository(Interface) which (usually) has the query written with #Query. How we specify which database this query goes for ?

For your first question I can answer that the JpaRepository has an internal system that analyses the method name you have written and then generates the query that has to be executed to the database.
The #Query annotation is used when the method name and the generated query is not returning the result you wanted to so you specifically tell the compiler which query should be executed.
As mentioned here: https://docs.spring.io/spring-data/jpa/docs/1.5.0.RELEASE/reference/html/jpa.repositories.html
2.3.1 Query lookup strategies.
The JPA module supports defining a query manually as String or have it being derived from the method name.
Declared queries
Although getting a query derived from the method name is quite convenient, one might face the situation in which either the method name parser does not support the keyword one wants to use or the method name would get unnecessarily ugly. So you can either use JPA named queries through a naming convention (see Section 2.3.3, “Using JPA NamedQueries” for more information) or rather annotate your query method with #Query (see Section 2.3.4, “Using #Query” for details).
So basically using a naming convention will do the magic.
Also an interesting question and perfect answer can be found here:
How are Spring Data repositories actually implemented?
For your second question you can refer to this example:
https://www.baeldung.com/spring-data-jpa-multiple-databases
It might be a bit complicated in the beginning but eventually it will work.

He use JPA, JpaRepository has CRUD methodes
https://docs.spring.io/spring-data/jpa/docs/current/reference/html/#reference
In your application.properties, you can put your mysql DB info

Why this works without #Query?
Because you are using JpaRepository which provides an easy way to get data based on your entity and it's fields.
Here your Account will have active, accountType etc fields. You can use JPA's query creation keywords such as AND, OR, Equals, Like and many more.
Derived queries with the predicates IsStartingWith, StartingWith, StartsWith, IsEndingWith", EndingWith, EndsWith, IsNotContaining, NotContaining, NotContains, IsContaining, Containing, Contains the respective arguments for these queries will get sanitized. This means if the arguments actually contain characters recognized by LIKE as wildcards these will get escaped so they match only as literals. The escape character used can be configured by setting the escapeCharacter of the #EnableJpaRepositories annotation.
How do we specify which of the database to search?
You can create configuration classes based on your databases and define data sources based on that using #PropertySource.
For more details see example here
#Configuration
#PropertySource({ "classpath:persistence-multiple-db.properties" })
#EnableJpaRepositories(
basePackages = "com.baeldung.multipledb.dao.product",
entityManagerFactoryRef = "productEntityManager",
transactionManagerRef = "productTransactionManager"
)

Is there a way to dynamically generate Spring Data Jpa queries?

I'm writing an app using Spring Boot, Spring Data. And I'm trying to implement a filtering feature based on different filter parameters.
Using Spring Data queries we can define quite complex logic, e.g.:
#Query("SELECT u FROM User u WHERE u.status = 1")
Collection<User> findAllActiveUsers();
But what if the number of where clauses, order, limit, number of different parameters are unknown till we make an actual filter request which can be quite complex.
Right now filter params are send in a json object which I parse and retrieve them and the result sql query can be something like this:
SELECT * FROM table
WHERE field1 != `value1` and (field1 != ` value2 `OR (field1 = `value3` AND filed2 < 3))
AND field2 != 99
Is it possible to generate dynamically complex queries with undefined (till the actual filter request, during runtime) number of params, where clauses and other stuff?

I use this active project RSQL for JPA
https://github.com/perplexhub/rsql-jpa-specification

Sometime back I wrote an article on Spring Data JPA Query with Dynamic Where Clause. In this example you can send a Where Clause and Map of parameters for that. You can make use of this and modify it a bit suit your needs.

I would suggest using Spring JPA Specification
ref : https://spring.io/blog/2011/04/26/advanced-spring-data-jpa-specifications-and-querydsl/

To build it in a type safe manner, you can use FluentJPA.

spring data JPA: How to execute aggregate functions

I am using Spring Data JPA and I have a requirement where I need to execute the SQL query as below:
select key_1, sum(key_2), sum(key_3) from table_1
where key_1 in ('1', '2') and {dynamic_columns} group by key_1;
For dynamic columns, I am using Spring Data JPA Specifications but I am not sure how can I write the aggregate functions like sum() on multiple columns.

With Spring Data JPA you have a few options in a spring repository.
References:
Baeldung - Spring data JPA
Spring.io - Spring data JPA
Specify a native query
#Query(value="select key_1, sum(key_2) ...", nativeQuery=true)
Optional<Object[]> getAggregates;
Use the providers (HQL for hibernate etc. query language)
#Query(value="select e.key_1, sum(e.key_2) from entity e ...")
Optional<Object[]> getAggregates;
You'll use objects here because you are not returning a specific entity you are adding custom (aggregate) columns. If you were returning a specific entity with a JPA repository you could return that entity instead of Object[]. Each item inside the object array will correspond to a columnof data, if you had multiple rows here you would use:
Optional<List<Object[]>> getAggregates;
Finally, if you have not used optionals before you will get your object array by:
if(objectsOptional.isPresent()) {
Objects[] objects = objectsOptional.get();
...
}
If this isn't what you were looking for i'll need more information to help you out.

How to get the newest record for every user using spring data mongodb?

I am struggling with a mongo query. I need to find a collection of documents in single query. The collection should contain document with newest date (field createdAt) for every user in single query.
There is a test case in Spock to demonstrate what I am trying to acheive:
def 'should filter the newest location for every user'() {
given:
List locationsInDb = [
buildLocation(USERNAME_1, '2017-02-03T10:37:30.00Z'),
buildLocation(USERNAME_1, '2017-03-04T10:37:30.00Z'),
buildLocation(USERNAME_2, '2017-02-05T10:37:30.00Z'),
buildLocation(USERNAME_2, '2017-03-06T10:37:30.00Z')
]
insertToMongo(locationsInDb)
when:
List filteredLocations = locationRepository.findLastForEveryUser()
then:
filteredLocations == [locationsInDb.get(1), locationsInDb.get(3)]
}
I found that distinct methods are a part of 2.1.0.M1 version so they are not available yet.
I was also trying with #Query annotation but the documentation (link below) does not specify how to create a query like mine.
https://docs.spring.io/spring-data/data-document/docs/current/reference/html/#d0e3309
Thanks for your help.

There are no means to express the query you are looking for via a derived query in Spring Data, nor using the MongoDB native query operators. Distinct as well will not do the job as it just extracts distinct values of a single field into an array.
Please consider using an Aggregation. Spring Data specifics can be found in the reference documentation.

How to make dynamic queries at run-time in Spring Boot and Data?

I am new to Java and started with Spring Boot and Spring Data JPA, so I know 2 ways on how to fetch data:
by Repository layer, with Literal method naming: FindOneByCity(String city);
by custom repo, with #Query annotation: #Query('select * from table where city like ?');
Both ways are statical designed.
How should I do to get data of a query that I have to build at run time?
What I am trying to achieve is the possibility to create dynamic reports without touching the code. A table would have records of reports with names and SQl queries with default parameters like begin_date, end_date etc, but with a variety of bodies. Example:
"Sales report by payment method" | select * from sales where met_pay = %pay_method% and date is between %begin_date% and %end_date%;

The Criteria API is mainly designed for that.
It provides an alternative way to define JPA queries.
With it you could build dynamic queries according to data provided at runtime.
To use it, you will need to create a custom repository implementation ant not only an interface.
You will indeed need to inject an EntityManager to create needed objects to create and execute the CriteriaQuery.
You will of course have to write boiler plate code to build the query and execute it.
This section explains how to create a custom repository with Spring Boot.
About your edit :
What I am trying to achieve is the possibility to create dynamic
reports without touching the code. A table would have records of
reports with names and SQl queries with default parameters like
begin_date, end_date etc, but with a variety of bodies.
If the queries are written at the hand in a plain text file, Criteria will not be the best choice as JPQL/SQL query and Criteria query are really not written in the same way.
In the Java code, mapping the JPQL/SQL queries defined in a plain text file to a Map<String, String> structure would be more adapted.
But I have some doubts on the feasibility of what you want to do.
Queries may have specific parameters, for some cases, you would not other choice than modifying the code. Specificities in parameters will do query maintainability very hard and error prone. Personally, I would implement the need by allowing the client to select for each field if a condition should be applied.
Then from the implementation side, I would use this user information to build my CriteriaQuery.
And there Criteria will do an excellent job : less code duplication, more adaptability for the query building and in addition more type-checks at compile type.

Spring-data repositories use EntityManager beneath. Repository classes are just another layer for the user not to worry about the details. But if a user wants to get his hands dirty, then of course spring wouldn't mind.
That is when you can use EntityManager directly.
Let us assume you have a Repository Class like AbcRepository
interface AbcRepository extends JpaRepository<Abc, String> {
}
You can create a custom repository like
interface CustomizedAbcRepository {
void someCustomMethod(User user);
}
The implementation class looks like
class CustomizedAbcRepositoryImpl implements CustomizedAbcRepository {
#Autowired
EntityManager entityManager;
public void someCustomMethod(User user) {
// You can build your custom query using Criteria or Criteria Builder
// and then use that in entityManager methods
}
}
Just a word of caution, the naming of the Customized interface and Customized implementating class is very important

In last versions of Spring Data was added ability to use JPA Criteria API. For more information see blog post https://jverhoelen.github.io/spring-data-queries-jpa-criteria-api/ .