SQL动态语句 where和if标签的使用

1.resultType返回的是封装的实体类。返回的字段必须跟实体类的字段相同，不然有的数据会报空。

  
    <!--根据类型查询数据  -->
    <select id="findDeviceStatus" resultType="com.kldhq.weight.entity.CarGpsAlarmAndCar">
        SELECT
            cga.id,
            cga.device_number AS deviceNumber,
            cga.gps_time AS gpsTime,
            cga.latitude,
            cga.longitude,
            cga.speed,
            cga.status,
            cga.mileage,
            cga.direction,
            cga.device_status,
            cga.is_read AS isRead,
            cga.alarm_type AS alarmType,
            c.id AS Carid,
            c.car_number AS carNumber,
            c.driver,
            c.phone,
            c.color,
            c.gps_device_number AS gpsDeviceNumber
        FROM
            car_gps_alarm AS cga INNER JOIN
            car AS c ON cga.device_number = c.gps_device_number
        <where>
                <if test="alarmType !=null and alarmType !=''">
                     and cga.alarm_type = #{alarmType}
                </if>
                <if test="gpsDeviceNumber !=null and gpsDeviceNumber !=''">
                     and cga.device_number = #{gpsDeviceNumber}
                </if>
        </where>
    
    </select>

2.实体类封装

public class CarGpsAlarmAndCar {
    private Integer id;
    private String deviceNumber;
    private Date gpsTime;
    private Double latitude;
    private Double longitude;
    private Integer speed;
    private String status;
    private Double mileage;
    private String direction;
    private String deviceStatus;
    private Integer isRead;
    private String alarmType;
    
    private Integer Carid;
    private String carNumber;
    private String driver;
    private String phone;
    private String color;
    private String gpsDeviceNumber;