SQL function calculation and union

• truncated mean

  SELECT
  	b.tag,
  	b.difficulty,
  	-- 去掉最大值与最小值取平均，保留一位小数
  	ROUND(( SUM( a.score )- MAX( a.score )- MIN( a.score ))/( COUNT( score )- 2 ), 1 ) 
  FROM
  	examination_info b
  	LEFT JOIN exam_record a ON b.exam_id = a.exam_id 
  WHERE
  	b.tag = "SQL" 
  	AND b.difficulty = "hard"
  

• Minimum value greater than average query
  Subquery divides values ​​greater than average. Then find the minimum value that meets the conditions. Remember: Be sure to connect to the query again. Otherwise, all values ​​greater than the SQL mean are queried instead of only the SQL values ​​greater than the SQL mean.

  SELECT
  	MIN( score ) 
  FROM
  	exam_record er
  	JOIN examination_info ei ON er.exam_id = ei.exam_id 
  WHERE
  	ei.tag = 'SQL' 
  	AND score >= (
  	SELECT
  		AVG( score ) 
  	FROM
  		examination_info a
  		LEFT JOIN exam_record b ON a.exam_id = b.exam_id 
  	WHERE
  	a.tag = "SQL" 
  	)
  

• Calculate the number of days in the month

  SELECT
  	LAST_DAY(submit_time),  -- 月度最后一天
  	DATE(DATE_ADD( submit_time, INTERVAL - DAY ( submit_time )+ 1 DAY )), -- 月度第一天
  	DATEDIFF(LAST_DAY(submit_time),DATE(DATE_ADD( submit_time, INTERVAL - DAY ( submit_time ) DAY ))),  -- 该月一共有几天（法1）
  	DAY(LAST_DAY(submit_time))  -- 该月一共有几天(法2)
  from  practice_record
  

• UNION

  • When using the keyword union, the multiple query results to be unioned cannot be ordered by themselves first, but they must be united first, and then order by according to a certain field.
  • The SELECT statements inside the UNION must have the same number of columns. Columns must also have similar data types. Also, the order of the columns in each SELECT statement must be the same.

  SELECT
  	DATE_FORMAT( submit_time, '%Y%m' ) as submit_month ,
  	COUNT( question_id ) `month_q_cnt`,
  	ROUND( COUNT( question_id )/ DAY ( LAST_DAY( max( submit_time ) )), 3 ) `avg_day_q_cnt` 
  FROM
  	practice_record 
  WHERE
  	YEAR ( submit_time )= '2021' 
  	AND score IS NOT NULL 
  GROUP BY
  	DATE_FORMAT( submit_time, '%Y%m' ) 
  UNION all
  SELECT
  	CONCAT( YEAR ( MIN( submit_time )), '汇总' ) `submit_month `,
  	COUNT( 1 ) `month_q_cnt`,
  	ROUND( COUNT( question_id )/ 31, 3 ) `avg_day_q_cnt` 
  FROM
  	practice_record 
  WHERE
  	YEAR ( submit_time )= '2021' 
  	AND score IS NOT NULL
  ORDER BY submit_month   -- 只能在最后排序
  

• The moving average needs to use the window function, for details, please refer to the details of the window function