SQL三个排序函数 ROW_NUMBER()、RANK()、DENSE_RANK()
ROW_NUMBER()不并列 连续的
RANK()分组不连续排序(跳跃排序)
DENSE_RANK()并列连续
创建实例表:
DROP table IF EXISTS con;
Create Table IF NOT EXISTS con(id int,Num int);
INSERT INTO con VALUES(1,1);
INSERT INTO con VALUES(2,1);
INSERT INTO con VALUES(3,1);
INSERT INTO con VALUES(4,4);
INSERT INTO con VALUES(5,2);
INSERT INTO con VALUES(6,2);
INSERT INTO con VALUES(7,3);
INSERT INTO con VALUES(8,3);
实例表:
三个函数 ROW_NUMBER()、 RANK()、 DENSE_RANK() 的区别如下图:
SELECT *,
ROW_NUMBER() over(ORDER BY Num DESC) AS 'ROW_NUMBER()',
RANK() over(ORDER BY Num DESC) AS 'RANK()',
DENSE_RANK() over(ORDER BY Num DESC) AS 'DENSE_RANK()'
FROM con;
函数区别结果:
1. TOP N 问题
返回第二高的数字
-- 方法一:DENSE_RANK() 窗口函数
SELECT DISTINCT Num
FROM (
SELECT *,
DENSE_RANK() over(ORDER BY Num) AS dense
FROM con
) AS a
WHERE dense = 2;
-- 方法二: 相关子查询
SELECT DISTINCT Num
FROM con AS c1
WHERE 2 = (
SELECT COUNT(DISTINCT Num)
FROM con AS c2
WHERE c1.Num >= c2.Num
)
结果如下:
2. 返回连续出现 N 次的数
返回连续出现三次的数字
情况一:要求 id 连续,而不是表格中的原顺序 的情况下Num连续
-- 方法一: join on
SELECT DISTINCT c1.Num
FROM con AS c1
JOIN con AS c2 ON c1.Num = c2.Num AND c2.id = c1.id + 1
JOIN con AS c3 ON c3.Num = c2.Num AND c3.id = c2.id + 1
-- 方法二:in
SELECT DISTINCT Num
SELECT *
FROM con
WHERE (id + 1, Num) IN (SELECT * FROM con)
AND (id + 2, Num) IN (SELECT * FROM con)
结果如下:
情况二:按 表格中的行顺序 Num连续
-- 方法一:排序函数
SELECT DISTINCT Num
FROM (
SELECT *,
-- id 索引值
ROW_NUMBER() over(ORDER BY id) AS number,
-- 分组排序
DENSE_RANK() over(PARTITION BY Num ORDER BY id) AS dense,
-- 差值
ROW_NUMBER() over(ORDER BY id) - DENSE_RANK() over(PARTITION BY Num ORDER BY id) AS different
FROM con
) AS a
GROUP BY Num, different
HAVING count(*) >= 3
-- 方法二: 偏移函数
-- lead()向上偏移
SELECT DISTINCT Num
FROM (
SELECT *,
-- 按照表格中的原顺序, num连续出现相同值
LEAD(Num, 1) over(ORDER BY id) AS l1,
LEAD(Num,2) over(ORDER BY id) AS l2
FROM con
) AS a
WHERE Num = l1 AND l1 = l2;
-- lag() 向下偏移
SELECT DISTINCT Num
FROM (
SELECT *,
-- 按照表格中的原顺序, num连续出现相同值
LAG(Num, 1) over(ORDER BY id) AS l1,
LAG(Num, 2) over(ORDER BY id) AS l2
FROM con
) AS a
WHERE Num = l1 AND l1 = l2;
结果如下:
总结:
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