JPA之JPQL基础查询

曾经终败给现在 2022-05-21 07:44 370阅读 0赞

@NamedQuery(name="test_jpql",query="SELECT t FROM TestJPQL t")
    @Entity
    @Table(name="TEST_JPQL")
    public class TestJPQL { 
    
        private Integer id;
        private String name;
        private String address;
    
        public TestJPQL(String name, String address) {
            super();
            this.name = name;
            this.address = address;
        }
    
        public TestJPQL() {
            super();
        }
    
        @Id
        @GeneratedValue
        public Integer getId() {
            return id;
        }
    
        public void setId(Integer id) {
            this.id = id;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public String getAddress() {
            return address;
        }
    
        public void setAddress(String address) {
            this.address = address;
        }
    
        @Override
        public String toString() {
            return "TestJPQL [id=" + id + ", name=" + name + ", address=" + address + "]";
        }
    }

**`createQuery查询基础`**

Query createQuery = entityManager.createQuery("SELECT t FROM TestJPQL t");
    
    @SuppressWarnings("unchecked")
    List<TestJPQL> resultList = createQuery.getResultList();
    System.out.println(resultList);

代码中通过对象`TestJPQL`查询数据，调用`getResultList`返回一个List结果集

**`createQuery之new对象代码`**

Query createQuery = entityManager.createQuery("SELECT new TestJPQL(t.name,t.address) FROM TestJPQL t");
    
    @SuppressWarnings("unchecked")
    List<TestJPQL> resultList = createQuery.getResultList();
    System.out.println(resultList);

通过在语句中`new`一个对象，将对应元素以对应参数传入查询。`注意: 此时需要对实体类添加对应的有参构造器和和无参构造器`。

**`createNamedQuery代码`**

Query createQuery = entityManager.createNamedQuery("test_jpql");
    
    @SuppressWarnings("unchecked")
    List<TestJPQL> resultList = createQuery.getResultList();
    System.out.println(resultList);

在实体类需要添加`@NamedQuery`注解，`name`为`createNamedQuery`方法传入的参数，`query`为JPQL语句，可以有select部分，也没有没有。

**`createNativeQuery`**

Query createQuery = entityManager.createNativeQuery("SELECT id FROM test_jpql WHERE id = ? ");
    createQuery.setParameter(1, 1);
    Object singleResult = createQuery.getSingleResult();
    System.out.println(singleResult);

通过`createNativeQuery`方法，传入SQL查询。且只能返回一个元素\[即只能查询指定行的某列数据\],  
所以通过`getSingleResult`查询出对应列的数据。