Group by group and get the first data

MYSQL GROUP BY 

mysql  I use sorting to filter , Guarantee grouping 、 After sorting, my first data is the data I want

SELECT  a.code , 
	a.type AS  Class type  ,MAX(a.num)
FROM
	(
		SELECT
			*
		FROM
			cent_ylb_numclass
		GROUP BY
			CODE,
			type
		ORDER BY
			CODE,
			num DESC
	) AS a
GROUP BY
	`code`

ORACLE GROUP BY 

oracle Use the windowing function . Access data according to serial number

 grammar ：
ROW_NUMBER() OVER(PARTITION BY COLUMN1 ORDER BY COLUMN2 [desc])
 according to column1 grouping , according to column2 Sort 

This time we need to code according to the teacher code Grouping , Teacher code （）code） And the number of teacher course groups （count（ Course grouping ）） null

We need to determine which course a teacher teaches the most

SELECT
	localtable. Teacher code  as  Teacher code ,localtable. Course   as  grouping 
FROM
(SELECT
			tablebiao. Teacher code  as  Teacher code ,
			tablebiao. Teacher's name  as   Teacher's name   ,
			tablebiao. Course grouping  as  Course ,
			COUNT(tablebiao. Course grouping ) AS num,
			row_number () over (
				PARTITION BY tablebiao. Teacher code ,
				tablebiao. Teacher's name 
			ORDER BY
				tablebiao. Teacher code ,
				COUNT(tablebiao. Course grouping ) DESC
			) AS rn
		FROM
			tablebiao
		GROUP BY
			tablebiao. Teacher code ,
			tablebiao. Teacher's name ,
			tablebiao. Course grouping 
		ORDER BY
			tablebiao. Teacher code ,
			COUNT(tablebiao. Course grouping ) DESC
)  localtable
WHERE
	localtable.rn = 1

 

If there is a better implementation method , Or for guidance, please contact wechat c243126035