[MyBatis] Summary of query statements

Define a Car class:

/**
 * 封装汽车相关信息的 pojo类
 */
public class Car {
    // 数据库表当中的字段应该和pojo类的属性一一对应
    // 建议使用包装类, 这样可以防止null的问题
    private Long id;
    private String carNum;
    private String brand;
    private Double guidePrice;
    private String produceTime;
    private String carType;
    public Car() {
    }
    public Car(Long id, String carNum, String brand, Double guidePrice, String produceTime, String carType) {
        this.id = id;
        this.carNum = carNum;
        this.brand = brand;
        this.guidePrice = guidePrice;
        this.produceTime = produceTime;
        this.carType = carType;
    }
    public Long getId() {return id;}
    public void setId(Long id) {this.id = id;}
    @Override
    public String toString() {
        return "Car{" +
                "id=" + id +
                ", carNum='" + carNum + '\'' +
                ", brand='" + brand + '\'' +
                ", guidePrice=" + guidePrice +
                ", produceTime='" + produceTime + '\'' +
                ", carType='" + carType + '\'' +
                '}';
    }
    public String getCarNum() {return carNum;}
    public void setCarNum(String carNum) {this.carNum = carNum;}
    public String getBrand() {return brand;}
    public void setBrand(String brand) {this.brand = brand;}
    public Double getGuidePrice() {return guidePrice;}
    public void setGuidePrice(Double guidePrice) {this.guidePrice = guidePrice;}
    public String getProduceTime() {return produceTime;}
    public void setProduceTime(String produceTime) {this.produceTime = produceTime;}
    public String getCarType() {return carType;}
    public void setCarType(String carType) {this.carType = carType;}
}

1. The returned Car of the query:

// CarMapper.xml
// 必须要指定返回结果的类型
<select id="selectById" resultType="Car">
    <!-- select * from car where id = #{id}-->
    // 列名要和pojo类中的属性名要对的上, 否则查出来为null
    select
        id, car_num as carNum, brand, guide_price as guidePrice,
        produce_time as produceTimme,
        car_type as catType
    from
        car
    where
        id = #{id}
</select>

// 接口
public interface CarMapper{
    // 根据id查询Car信息
    Car selectById(Long id);
}

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    CarMapper mapper = sqlSession.getMapper(CarMapper.class);
    
    Car car = mapper.selectById(16L);
    System.out.println(car);
    
    sqlSession.close();
}

2. The query returns multiple Cars:

// CarMapper.xml
// 必须要指定返回结果的类型
<select id="selectAll" resultType="Car">
    <!-- select * from car where id = #{id}-->
    // 列名要和pojo类中的属性名要对的上, 否则查出来为null
    select
        id, car_num as carNum, brand, guide_price as guidePrice,
        produce_time as produceTimme,
        car_type as catType
    from
        car
</select>

// 接口
public interface CarMapper{
    // 查询所有Car信息
    List<Car> selectAll();
}

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    CarMapper mapper = sqlSession.getMapper(CarMapper.class);
    
    List<Car> cars = mapper.selectAll();
    cars.forEach(car -> System.out.println(car));
    
    sqlSession.close();
}

3. The returned Map of the query:

• The key of the map is the column name of the database

// CarMapper.xml
// resultType="java.util.Map" 有别名: "map"
<select id="selectByIdRetMap" resultType="map">
    // 返回的类型是个map, 不需要和Car类属性匹配了
    select * from car where id = #{id}
</select>

// 接口
public interface CarMapper{
    // 根据id获取汽车信息, 将信息放到Map集合中
    Map<String, Object> selectByIdRetMap(Long id);
}

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    CarMapper mapper = sqlSession.getMapper(CarMapper.class);
    
    Map<String, Object> car = mapper.selectByIdRetMap(16L);
    System.out.println(car);
    sqlSession.close();
}

4. The query returns multiple Maps:

// CarMapper.xml
// resultType="map", 不能写"list"
<select id="selectAllRetListMap" resultType="map">
    // 返回的类型是个map, 不需要和Car类属性匹配了
    select * from car 
</select>

// 接口
public interface CarMapper{
    // 查询所有的car信息, 返回一个存放Map集合的List集合
    List<Map<String, Object>> selectAllRetListMap();
}

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    CarMapper mapper = sqlSession.getMapper(CarMapper.class);
    
    List<Map<String, Object>> maps = mapper.selectAllRetListMap();
    maps.forEach(map -> System.out.println(map));
    sqlSession.close();
}

5. The query returns Map<String,Map>:

• Use the id of the Car as the key, and it will be more convenient to take out the corresponding Map collection in the future

• Need to use @MapKey annotation

// CarMapper.xml
// resultType="map", 不能写"list"
<select id="selectAllRetMap" resultType="map">
    // 返回的类型是个map, 不需要和Car类属性匹配了
    select * from car 
</select>

// 接口
public interface CarMapper{
    // 查询所有的Car, 返回一个Map集合
    // Map集合的key是每条记录的主键值
    // Map集合的value是每条记录
    @MapKey("id") // 将查询结果的id值作为一个大map集合的key
    Map<Long,Map<String, Object>> selectAllRetMap();
}

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    CarMapper mapper = sqlSession.getMapper(CarMapper.class);
    
    Map<Long,Map<String, Object>> maps = mapper.selectAllRetMap();
    System.out.println(maps);
    sqlSession.close();
}

6. resultMap result mapping:

What should I do if the column name of the query result does not correspond to the attribute name of the Java object?

1. as to alias the column

// CarMapper.xml
// 必须要指定返回结果的类型
<select id="selectAll" resultType="Car">
    select
       id, 
        car_num as carNum, 
        brand, 
        guide_price as guidePrice,
        produce_time as produceTimme,
        car_type as catType
    from
        car
</select>

// 接口
public interface CarMapper{
    // 查询所有Car信息
    List<Car> selectAll();
}

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    CarMapper mapper = sqlSession.getMapper(CarMapper.class);
    
    List<Car> cars = mapper.selectAll();
    cars.forEach(car -> System.out.println(car));
    
    sqlSession.close();
}

2. Use resultMap for result mapping

// CarMapper.xml

<resultMap id="carResultMap" type="Car">
    // 如果数据库表中有主键, 一般都是有主键的, 否则不符合数据库设计第一范式
    // 如果有主键, 建议这里面配置一个id标签, 这样是为了提高效率
    <id property ="id" column="id"/>
    // property后面填写的是POJO类的属性名
    // column后面填写的是数据库的字段名
    <result property="carNum" column="car_num"/>
    // 如果属性名和字段名相同, 可以不配置
    <result property="brand" column="brand"/>    
    <result property="guidePrice" column="guide_price"/>
    <result property="produceTime" column="produce_time"/>
    <result property="carType" column="car_type"/>
</resultMap>

// resultMap属性用来指定使用哪个结果映射, resultMap后面的值是resultMap的id
<select id="selectAllByResultMap" resultMap="carResultMap">
    select * from car 
</select>

// 接口
public interface CarMapper{
    // 查询所有Car信息, 使用resultMap标签
    List<Car> selectAllByResultMap();
}

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    CarMapper mapper = sqlSession.getMapper(CarMapper.class);
    
    List<Car> cars = mapper.selectAllByResultMap();
    cars.forEach(car -> System.out.println(car));
    
    sqlSession.close();
}

3. Whether to enable automatic mapping of camel case naming (configuration settings)

• The premise of using this method is that the attributes follow the Java naming convention, and the column names of the database follow the SQL naming convention

• Java naming convention: the first letter is lowercase, and the first letter of each subsequent word is capitalized, following the hump naming method

• SQL naming convention: all lowercase, words are separated by underscores

如何启用该功能呢? 需要在mybatis-config.xml文件中配置
    
// 放在properties标签后面
<settings>
    <setting name="mapUnderscoreToCameCase" value="true">
</settings>

// CarMapper.xml
<select id="selectAllBymapUnderscoreToCameCase" resultType="Car">
    select * from car
</select>

// 接口
public interface CarMapper{
    // 查询所有Car信息, 启用驼峰命名自动映射机制
    List<Car> selectAllBymapUnderscoreToCameCase();
}

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    CarMapper mapper = sqlSession.getMapper(CarMapper.class);
    
    List<Car> cars = mapper.selectAllBymapUnderscoreToCameCase();
    cars.forEach(car -> System.out.println(car));
    
    sqlSession.close();
}

7. The total number of records returned by the query:

// CarMapper.xml
// resultType后面也可以写上java.lang.Long
<select id="selectTotal" resultType="long">
    select count(*) from car
</select>

// 接口
public interface CarMapper{
    // 获取Car的总记录条数
    Long selectTotal();
}

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    CarMapper mapper = sqlSession.getMapper(CarMapper.class);
    
    Long nums = mapper.selectTotal();
    System.out.println(nums);
    
    sqlSession.close();
}