一、函数写法

函数名(参数) OVER (PARTITION BY子句 ORDER BY子句 ROWS/RANGE子句)

由三部分组成： 函数名：如sum、max、min、count、avg等聚合函数以及lead、lag行比较函数等； over：
关键字，表示前面的函数是分析函数，不是普通的集合函数； 分组子句：over关键字后面挂号内的内容；

分析子句又由下面三部分组成： PARTITION BY ：分组子句，表示分析函数的计算范围，不同的组互不相干； ORDER BY：
排序子句，表示分组后，组内的排序方式； ROWS/RANGE：窗口子句，是在分组(PARTITION
BY)后，组内的子分组（也称窗口），此时分析函数的计算范围窗口，而不是PARTITON。窗口有两种，ROWS和RANGE；

二、开窗的窗口范围ROWS与RANGE

1.范围限定用法 CURRENT ROW: 当前行
UNBOUNDED：不受控制的，无限的
UNBOUNDED PRECEDING: 区间的第一行
UNBOUNDED FOLLOWING：区间的最后一行
UNBOUNDED PRECEDING AND UNBOUNED FOLLOWING：针对当前所有记录的前一条、后一条记录，分组中的所有记录
PRECEDING：在…之前， N PRECEDING:当前行之前的N行，可以是数字用于RANGE数据范围限定，也可以是一个能计算出数字的表达式
FOLLOWING：在…之后，N FOLLOWING：当前行之后的N行，可以是数字用于RANGE数据范围限定，也可以是一个能计算出数字的表达式 ROWS BETWEEN
UNBOUNDED PRECEDING AND CURRENT ROW ：指第一行至当前行的数据
ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING ：指当前行到最后一行的汇总
ROWS BETWEEN 1 PRECEDING AND CURRENT ROW ：指当前行的上一行(ROWNUM-1)到当前行的数据
ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING ：指当前行的上一行(ROWNUM-1)到当前行的下一行(ROWNUM+1)的数据
RANGE BETWEEN CURRENT ROW AND 350 FOLLOWING：指当前行到当前行数据+350的范围内的数据
RANGE BETWEEN 5 PRECEDING AND 5 FOLLOWING：指当前行数据幅度减5加5后的范围内的数据

三、练习：

```bash
DROP TABLE IF EXISTS `test`;
CREATE TABLE `test`  (
  `video_id` int(0) NOT NULL COMMENT '视频ID',
  `dt` date NULL DEFAULT NULL,
  `if_follow` tinyint(0) NULL DEFAULT NULL COMMENT '是否关注'
) ENGINE = InnoDB CHARACTER SET = utf8mb4 COLLATE = utf8mb4_0900_ai_ci ROW_FORMAT = Dynamic;
 
-- ----------------------------
-- Records of test
-- ----------------------------
INSERT INTO `test` VALUES (2001, '2021-09-24', 1);
INSERT INTO `test` VALUES (2001, '2021-10-03', 9);
INSERT INTO `test` VALUES (2001, '2021-10-02', 2);
INSERT INTO `test` VALUES (2001, '2021-10-01', 6);
INSERT INTO `test` VALUES (2002, '2021-09-25', 1);
INSERT INTO `test` VALUES (2002, '2021-09-25', 1);
INSERT INTO `test` VALUES (2002, '2021-09-26', 6);
INSERT INTO `test` VALUES (2002, '2021-09-27', 1);
INSERT INTO `test` VALUES (2002, '2021-09-28', 1);
INSERT INTO `test` VALUES (2002, '2021-09-29', 8);
INSERT INTO `test` VALUES (2002, '2021-09-30', 7);
INSERT INTO `test` VALUES (2002, '2021-10-01', 1);
INSERT INTO `test` VALUES (2002, '2021-10-02', 9);
INSERT INTO `test` VALUES (2002, '2021-10-03', 1);

1.range 限定范围

range是对order by 的值进行判断范围计算的，根据实际的值和当前的值进行判断取定数据范围的。

preceding 在…之前

following 在…之后

select dt,video_id,if_follow, sum(if_follow) over(partition by video_id order by if_follow range between 1 preceding and 1 following) as num from test ;

current row 当前行

select video_id,dt,if_follow, sum(if_follow) over(partition by video_id order by if_follow range BETWEEN  CURRENT ROW and  3 following) as num from test ;

unbounded preceding 区间的第一行

select video_id,dt,if_follow, sum(if_follow) over(partition by video_id order by if_follow range BETWEEN  unbounded preceding and  3 following) as num from test ;

unbounded following 区间的最后一行

select video_id,dt,if_follow, sum(if_follow) over(partition by video_id order by if_follow range BETWEEN  current row and  unbounded following) as num from test ;

unbounded preceding and unbounded following 最后一行的值和第一行的值

select video_id,dt,if_follow, sum(if_follow) over(partition by video_id order by if_follow range BETWEEN  unbounded preceding and  unbounded following) as num from test ;

2.ROW 限定行

row是对order by 的值进行判断范围计算的，根据实际的值和当前的值进行判断取定数据范围的

preceding 在…之前

following 在…之后

select dt,video_id,if_follow, sum(if_follow) over(partition by video_id order by if_follow rows between 1 preceding and 1 following) as num from test ;

current row 当前行

select video_id,dt,if_follow, sum(if_follow) over(partition by video_id order by if_follow rows BETWEEN  CURRENT ROW and  3 following) as num from test ;

unbounded preceding 区间的第一行

select video_id,dt,if_follow, sum(if_follow) over(partition by video_id order by if_follow rows BETWEEN  unbounded preceding and  3 following) as num from test ;

unbounded following 区间的最后一行

select video_id,dt,if_follow, sum(if_follow) over(partition by video_id order by if_follow rows BETWEEN  current row and  unbounded following) as num from test ;

unbounded preceding and unbounded following 最后一行的值和第一行的值

select video_id,dt,if_follow, sum(if_follow) over(partition by video_id order by if_follow rows BETWEEN  unbounded preceding and  unbounded following) as num from test ;

lag上一个数(lag取值的列，向上几个取值(默认是1))

SELECT video_id,dt,if_follow,LAG(if_follow,2) over(partition by video_id ORDER BY if_follow) FROM flink_data ;

lead下一个数(lead取值的列，向下几个取值(默认是1))

SELECT video_id,dt,if_follow,LEAD(if_follow) over(partition by video_id ORDER BY if_follow) FROM flink_data ;