jpa 的sql怎么取空

在 JPA 中，可以使用 `IS NULL` 条件来查询空值。以下是一个示例代码：

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    List<User> findByEmailIsNull();
}

上述代码中的 `findByEmailIsNull` 方法将返回所有 `email` 字段为空的用户列表。你可以根据需要修改方法名和条件来查询其他空值字段。

相关问题

jpa sql in

JPA (Java Persistence API) is a Java specification for object-relational mapping (ORM) that allows developers to work with databases using Java objects. JPA provides a set of annotations that can be used to map Java classes to database tables, and it also provides an API for querying and manipulating data.

When working with JPA, you can use JPQL (Java Persistence Query Language) to write queries that are database-agnostic and can be translated to SQL by the JPA provider. JPQL is similar to SQL, but instead of operating on tables and columns, it operates on entities and their attributes.

Here's an example of using JPQL to query for all employees with a salary greater than 50000:

TypedQuery<Employee> query = entityManager.createQuery(
    "SELECT e FROM Employee e WHERE e.salary > 50000", Employee.class);
List<Employee> employees = query.getResultList();

This JPQL query selects all `Employee` entities where the `salary` attribute is greater than 50000. The `entityManager` object is an instance of `javax.persistence.EntityManager`, which is responsible for managing JPA entities and transactions.

When the query is executed, the JPA provider (such as Hibernate or EclipseLink) will translate the JPQL query to SQL and execute it against the database. The result is returned as a list of `Employee` objects.

jpa 手写sql

JPA是Java Persistence API的缩写，通常情况下我们使用JPA来进行数据库操作，因为它可以使用对象来进行数据库操作，从而简化了我们的代码量。但是，在某些特殊情况下，我们可能需要手写SQL语句来进行数据库操作。

下面是一个使用JPA手写SQL的例子：

@Repository
public class UserRepositoryImpl implements UserRepository {
    
    @PersistenceContext
    private EntityManager entityManager;

    @Override
    public List<User> findUsersByAgeGreaterThan(int age) {
        String sql = "SELECT * FROM user WHERE age > :age";
        Query query = entityManager.createNativeQuery(sql, User.class);
        query.setParameter("age", age);
        List<User> userList = query.getResultList();
        return userList;
    }

}

在上面的例子中，我们使用了`createNativeQuery`方法来创建一个Native SQL Query对象，然后使用`setParameter`方法来设置参数，最后使用`getResultList`方法来获取查询结果。

需要注意的是，手写SQL语句可能会导致一些潜在的问题，比如SQL注入等安全问题，因此在使用时需要格外小心。