Spring Data JPA 和 Exists 查询

我正在使用 Spring Data JPA (使用 Hibernate 作为 JPA 提供者) ，并希望定义一个带有 HQL 查询的 exists方法:

public interface MyEntityRepository extends CrudRepository<MyEntity, String> {


@Query("select count(e) from MyEntity e where ...")
public boolean existsIfBlaBla(@Param("id") String id);


}

当我运行这个查询时，我得到一个 java.lang.ClassCastException: java.lang.Long cannot be cast to java.lang.Boolean。

HQL 查询必须看起来如何才能实现这一点？我知道我可以简单地返回一个 Long 值，然后在 count > 0中检入我的 Java 代码，但是这种变通方法应该是不必要的，对吗？

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最佳答案

I think you can simply change the query to return boolean as

@Query("select count(e)>0 from MyEntity e where ...")

If you are checking exists based on Primary key value CrudRepository already have exists(id) method.

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in my case it didn't work like following 

@Query("select count(e)>0 from MyEntity e where ...")

You can return it as boolean value with following

@Query(value = "SELECT CASE  WHEN count(pl)> 0 THEN true ELSE false END FROM PostboxLabel pl ...")
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Since Spring data 1.12 you can use the query by Example functionnality by extending the QueryByExampleExecutor interface (The JpaRepositoryalready extends it).
Then you can use this query (among others) : 

<S extends T> boolean exists(Example<S> example);

Consider an entity MyEntity which as a property name, you want to know if an entity with that name exists, ignoring case, then the call to this method can look like this : 

//The ExampleMatcher is immutable and can be static I think
ExampleMatcher NAME_MATCHER = ExampleMatcher.matching()
.withMatcher("name", GenericPropertyMatchers.ignoreCase());
Example<MyEntity> example = Example.<MyEntity>of(new MyEntity("example name"), NAME_MATCHER);
boolean exists = myEntityRepository.exists(example);
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Apart from the accepted answer, I'm suggesting another alternative. Use QueryDSL, create a predicate and use the exists() method that accepts a predicate and returns Boolean.

One advantage with QueryDSL is you can use the predicate for complicated where clauses.

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Spring Data JPA 1.11 now supports the exists projection in repository query derivation.

See documentation here.

In your case the following will work:

public interface MyEntityRepository extends CrudRepository<MyEntity, String> {
boolean existsByFoo(String foo);
}
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You can use Case expression for returning a boolean in your select query like below.

@Query("SELECT CASE WHEN count(e) > 0 THEN true ELSE false END FROM MyEntity e where e.my_column = ?1")
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You can use .exists (return boolean) in jpaRepository.

if(commercialRuleMsisdnRepo.exists(commercialRuleMsisdn.getRuleId())!=true){


jsRespon.setStatusDescription("SUCCESS ADD TO DB");
}else{
jsRespon.setStatusCode("ID already exists is database");
}
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It's gotten a lot easier these days!

@Repository
public interface PageRepository extends JpaRepository<Page, UUID> {


Boolean existsByName(String name); //Checks if there are any records by name
Boolean existsBy(); // Checks if there are any records whatsoever


}
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Spring data provides method for checking the existence of a row using field: example: boolean existsByEmployeeIdAndEmployeeName(String employeeId, String employeeName);


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