feat(influx): 添加相邻时间点差值计算功能

- 在InfluxDbSqlConstant中新增DIFFERENCE常量定义
- 在InfluxQueryWrapper中添加difference方法实现差值计算功能
- 实现相邻时间点数据差值查询的SQL片段构建逻辑
- 支持通过字段名进行差值计算并返回查询包装器实例

src/main/java/com/njcn/influx/constant/InfluxDbSqlConstant.java

@@ -110,6 +110,7 @@ public interface InfluxDbSqlConstant {
     String REGULAR_PREFIX="~/^";
     String REGULAR_SUFFIX="$/";
     String REGULAR_MIDDLE="|";
+    String DIFFERENCE = "DIFFERENCE";
 
     /**
      * “ tz('Asia/Shanghai')”

src/main/java/com/njcn/influx/query/InfluxQueryWrapper.java

@@ -13,7 +13,10 @@ import org.influxdb.annotation.Column;
 import org.influxdb.annotation.Measurement;
 
 import java.lang.reflect.Field;
-import java.util.*;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Objects;
 
 /**
  * @author hongawen
@@ -595,6 +598,25 @@ public class InfluxQueryWrapper {
         return this;
     }
 
+    /***
+     * 计算相邻时间点之间的差值
+     * @author xy
+     * @param columnName 表字段名
+     * @return InfluxQueryWrapper
+     * DIFFERENCE("columnName")
+     */
+    public <T, R> InfluxQueryWrapper difference(String columnName) {
+        String selectFragment = InfluxDbSqlConstant.DIFFERENCE +
+                InfluxDbSqlConstant.LBK +
+                InfluxDbSqlConstant.DQM +
+                columnName +
+                InfluxDbSqlConstant.DQM +
+                InfluxDbSqlConstant.RBK +
+                InfluxDbSqlConstant.AS_VALUE;
+        selectColumns.add(selectFragment);
+        return this;
+    }
+
     /***
      * 注：该函数还需调研，暂时不要用
      * 统计指定字段邻近值的变化率