我有如下表:
+------------+-------+
| SchoolName | Marks |
+------------+-------+
| A | 71 |
| A | 71 |
| A | 71 |
| B | 254 |
| B | 135 |
| B | 453 |
| B | 153 |
| C | 453 |
| C | 344 |
| C | 223 |
| B | 453 |
| D | 300 |
| D | 167 |
+------------+-------+
这里是按学校名称分组的分数平均值:
+------------+------------+
| SchoolName | avg(Marks) |
+------------+------------+
| A | 71.0000 |
| B | 289.6000 |
| C | 340.0000 |
| D | 233.5000 |
+------------+------------+
https://www.db-fiddle.com/f/5t7N3Vx8FSQmwUJgKLqjfK/9
然而,我想计算按学校名称分组的成绩的中位数。
我正在使用:
SELECT AVG(dd.Marks) as median_val
FROM (
SELECT d.Marks, @rownum:=@rownum+1 as `row_number`, @total_rows:=@rownum
FROM tablename d, (SELECT @rownum:=0) r
WHERE d.Marks is NOT NULL
ORDER BY d.Marks
) as dd
WHERE dd.row_number IN ( FLOOR((@total_rows+1)/2), FLOOR((@total_rows+2)/2) );
我想要计算整个成绩列的平均值,但不知道如何为每个学校单独计算。