對於 MySQL，我需要一份與平均值偏差超過 1 個標準差的行報告（以百分比表示）

Question

我正在嘗試編寫一個MySQL 查詢來計算與每種產品最常見的行有很大不同的行，並報告百分比差異，其中低於100% 的價格低於該產品的平均價格，超過100%的價格較高。忽略與平均值小於 1 個標準差的價格。

範例資料：

_rowid	_時間戳記	程式碼	fk_product_id	fk_po_id	成本
5952	2021-01-10 10:19:01	00805	1367	543	0.850
9403	2022-05-23 14:54:34	00805	1367	2942	0.850
41595	2022-11-23 11:20:26	00805	1367	3391	1.350
39635	2022-01-18 12:49:32	水1	344	3153	0.140
40134	2022-04-06 22:39:34	水1	344	2747	0.190
41676	2022-12-09 16:28:28	水1	344	3398	0.140
39634	2022-01-18 12:49:31	gr309203 344400	1024	3154	0.770
35634	2021-03-03 15:23:23	gr309203 344400	1024	3203	0.790
41264	2022-11-16 11:41:44	gr309203 344400	1024	3357	0.970

SELECT code, fk_product_id, cost, cost/
  (SELECT avg(cost) FROM po_line aa WHERE aa.code = code) AS percent 
FROM po_line 
WHERE (SELECT STDDEV(cost) FROM po_line ss WHERE ss.code = code)>1;

這不會傳回任何行，但報告中應顯示三行（每種產品各一行）。

預期結果應該是：

程式碼	fk_product_id	成本	百分比
00805	1367	1.350	133
水1	344	0.190	121
gr309203 344400	1024	0.970	115

Answer 1

此查詢顯示如何使用視窗函數來計算每個給定程式碼的標準差數量以及成本相對於平均成本的百分比。

SELECT code, cost, avg, std, 
  ABS(cost-avg)/std AS num_std, 
  cost*100/avg AS pct
FROM (
  SELECT code, cost,
    AVG(cost) OVER (PARTITION BY code) AS avg,
    STDDEV(cost) OVER (PARTITION BY code) AS std
  FROM po_line
) AS p;

結果：

+-----------------+-------+-----------+---------------------+--------------------+-------------+
| code            | cost  | avg       | std                 | num_std            | pct         |
+-----------------+-------+-----------+---------------------+--------------------+-------------+
| 00805           | 0.850 | 1.0166667 | 0.23570226039551592 | 0.7071069226079035 |  83.6065546 |
| 00805           | 0.850 | 1.0166667 | 0.23570226039551592 | 0.7071069226079035 |  83.6065546 |
| 00805           | 1.350 | 1.0166667 | 0.23570226039551592 | 1.4142134209517383 | 132.7868809 |
| gr309203 344400 | 0.770 | 0.8433333 |  0.0899382504215469 | 0.8153738777025533 |  91.3043514 |
| gr309203 344400 | 0.790 | 0.8433333 |  0.0899382504215469 | 0.5929990827042229 |  93.6758930 |
| gr309203 344400 | 0.970 | 0.8433333 |  0.0899382504215469 | 1.4083740722807512 | 115.0197674 |
| Water1          | 0.140 | 0.1566667 | 0.02357022603955158 | 0.7071081954001099 |  89.3616831 |
| Water1          | 0.190 | 0.1566667 | 0.02357022603955158 | 1.4142121481595331 | 121.2765699 |
| Water1          | 0.140 | 0.1566667 | 0.02357022603955158 | 0.7071081954001099 |  89.3616831 |
+-----------------+-------+-----------+---------------------+--------------------+-------------+

（請注意，視窗函數需要 MySQL 8.0）。

該查詢僅向您顯示計算是如何完成的。要獲得您想要的結果：

SELECT code, fk_product_id, cost, ROUND(cost*100/avg) AS pct
FROM (
  SELECT code, cost, fk_product_id,
    AVG(cost) OVER (PARTITION BY code) AS avg,
    STDDEV(cost) OVER (PARTITION BY code) AS std
  FROM po_line
) AS p
WHERE ABS(cost-avg)/std > 1;

+-----------------+---------------+-------+------+
| code            | fk_product_id | cost  | pct  |
+-----------------+---------------+-------+------+
| 00805           |          1367 | 1.350 |  133 |
| gr309203 344400 |          1024 | 0.970 |  115 |
| Water1          |           344 | 0.190 |  121 |
+-----------------+---------------+-------+------+