|
一、窗口函数的基本用法
从MySQL8之后才开始支持窗口函数
OVER ([PARTITION BY ] ORDER BY )
二、LAG()和LEAD()函数介绍
lag和lead分别是向前向后的意思参数有三个。expression：列名；offset：偏移量；default_value：超出记录窗口的默认值(默认为null，可以设置为0)
三、数据准备(建表sql在最后)
1、LAG()函数：统计与前一天相比温度更高的日期Id
我们先按照日期进行排序，然后找到当天比前一天温度高的id；使用lag()函数，将温度向后推一天。
SELECT
id,
date,
temperature,
LAG( temperature, 1, 0 ) OVER ( ORDER BY date ) AS temp
FROM
weather;
查询结果：  然后将temperature大于temp 并且temp不等于0的数据挑选出来
SELECT
id
FROM
(SELECT id, date, temperature, LAG( temperature, 1, 0) OVER ( ORDER BY date ) AS temp FROM weather) tmp
WHERE
temperature > temp
AND temp != 0;
结果如下： 
2、LEAD()函数：统计与后一天相比温度更高的日期Id
我们还是先按照日期进行排序，然后找到当天比后一天温度高的id；使用lead()函数，将温度向后推一天。
SELECT
id,
date,
temperature,
LEAD( temperature, 1, 0 ) OVER ( ORDER BY date ) AS temp
FROM
weather;
查询结果：  然后将temperature大于temp 并且temp不等于0的数据挑选出来
SELECT
id
FROM
( SELECT id, date, temperature, LEAD( temperature, 1, 0 ) OVER ( ORDER BY date ) AS temp FROM weather ) tmp
WHERE
temperature > temp
AND temp != 0;
查询结果： 
四、建表数据sql
DROP TABLE IF EXISTS `weather`;
CREATE TABLE `weather` (
`id` int(11) NOT NULL,
`date` date NULL DEFAULT NULL,
`temperature` int(11) NULL DEFAULT NULL,
PRIMARY KEY (`id`) USING BTREE
) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;
-- ----------------------------
-- Records of weather
-- ----------------------------
INSERT INTO `weather` VALUES (1, '2022-08-01', 20);
INSERT INTO `weather` VALUES (2, '2022-08-02', 25);
INSERT INTO `weather` VALUES (3, '2022-08-03', 22);
INSERT INTO `weather` VALUES (4, '2022-08-04', 22);
INSERT INTO `weather` VALUES (5, '2022-08-05', 26);
INSERT INTO `weather` VALUES (6, '2022-08-06', 28);
INSERT INTO `weather` VALUES (7, '2022-08-07', 20);
SET FOREIGN_KEY_CHECKS = 1;
|