【MyBatis】mybatis的增删查改和namespace属性

C : create增

R : retrieve检索

U : update改

D : delete删

1. insert增加操作:

• JDBC中的占位符是? , 而在mybatis中的占位符是#{}

1.1 使用map集合传参:

• java程序中使用Map可以给SQL语句中的占位符传值!

• #{这里面写什么?} , 大括号中写map集合的key.

• 如果大括号中的key不存在, 那么获取到的就是null

• map的key建议写的见名知意, 建议写成表的字段名.

// CarMapper.xml
<insert id="insertCar">
    //  #{k1} -> 底层会调用map.get("k1")
    insert into 
    car(id,car_num,brand,guide-price,produce_time,car-type) 
    values
    (null, #{car_num}, #{brand}, #{guide_price}, #{produce_time}, #{car_type})
</insert>

// @test
public class main(String[] args){
    SqlSession sqlSession = SqlSessionUtil.openSession();
    Map<String,Object> map = new HashMap<>();

    map.put("car_num","..");
    map.put("brand","..");
    map.put("guide_price","..");
    map.put("produce_time","..");
    map.put("car_type","..");

    // insert方法的参数: 
    //    第一个参数: sqlId, 从CarMapper.xml中复制过来
    //    第二个参数: 封装数据的对象
    int count = sqlSession.insert("insertCar",map);
    sqlsession.commit();
    sqlsession.close();
} 

1.2 使用POJO类传参:

• Mapper.xml中的#{}, 大括号里面写上pojo类的getXyz()方法的xyz

• mybatis在底层给#{}传值的时候, 需要先获取值. 通过反射机制调用getXxxx()方法获取到值

• getUserName() ---> #{userName}

// CarMapper.xml
<insert id="insertCar">
    insert into car
    (id,car_num,brand,guide_price,produce_time,car_type) 
    values
    (null, #{carNum}, #{brand}, #{guidePrice}, #{produceTime}, #{carType})
</insert>

/**
 * 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;
}

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    // 封装数据
    Car car = new Car(null,"666","大奔",67.6,"2023-1-1","燃油");
    // 执行sql
    int count = sqlSession.insert("insertCar",car);
    System.out.println(count);
    sqlSession.commit();
    sqlSession.close();
}

2. delete删除操作:

// CarMapper.xml
<delete id="deleteById">
    <!--  因为只有一个占位符, 所以 {里面写什么都行, 但是不能不写} -->
    delete from car where id=#{id}
</delete>

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    int count = sqlSession.delete("deleteById",67);
    System.out.println(count);
    sqlSession.commit();
    sqlSession.close();
}

3. update修改操作：

// CarMapper.xml
<update id="updateById">
    update car set
        car_num=#{carNum},
        brand=#{brand},
        guide_price=#{guidePrice},
        produce_time=#{produceTime},
        car_type=#{carType}
    where 
        id=#{id}
</update>

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    Car car = new Car(6L,"999","宝马",66,"2021-1-1","新能源");
    int count = sqlSession.update("updateById",car);
    System.out.println(count);
    sqlSession.commit();
    sqlSession.close();

}

4. select查询:

4.1 查询一个数据:

• 列名要和pojo类中的属性名要对的上, 否则查出来为null

• mybatis执行了select之后, 一定会返回一个结果集对象 ResultSet

• 接下来就是mybatis应该从ResultSet中取出数据, 封装java对象

// CarMapper.xml
// 必须要指定返回结果的类型resultType
<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>

// @test
public static void main(String[] args) {
    SqlSession sqlSession = SqlSessionUtil.openSession();
    // mybatis执行了select之后, 一定会返回一个结果集对象 ResultSet
    // 接下来就是mybatis应该从ResultSet中取出数据, 封装java对象
    Object car = sqlSession.selectOne("selectById",12);
    System.out.println(car);
    sqlSession.close();
}

4.2 查询所有数据:

// CarMapper.xml
// resultType还是指定要封装的结果集的类型, 不是指定List类型, 是指定List集合中元素的类型.
<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>

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

5. 关于Mapper文件的namespace:

• 在sql mapper.xml 文件中有一个namespace, 这个属性是用来指定命名空间的, 用来防止id重复.

• 实际上, mybatis中的sqlid的完整写法是: namespace.id

• <mapper namespace="...">

// CarMapper.xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybati
s.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="...">
    <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>
</mapper>