使用另一個GROUP BY過濾GROUP BY結果以進行修剪

Question

我已經有了一個基本查詢來從我的數據庫中選擇所有位置，然後按它們的郵編的前2個字符對它們進行分組。

SELECT LEFT(`locationPostcode`, 2) as `postcode`, count(`locationPostcode`) as `locationCount` 
FROM `locations` 
WHERE `locationPostcode` IS NOT NULL 
AND `locationPostcode` <> '' 
GROUP BY `postcode` 
ORDER BY `locationCount` DESC 

因爲有些郵政編碼僅1個字母字符，而不是2開始，我需要這組再次獲得這些郵政編碼（例如E1，W2），只顯示他們的第一個字母。

我在此嘗試將它們分組再次LEFT('postcode', 2)，但它似乎並沒有產生太大的影響（下查詢）

SELECT postcode, locationCount FROM (

    SELECT LEFT(`locationPostcode`, 2) as `postcode`, count(`locationPostcode`) as `locationCount` 
    FROM `locations` 
    WHERE `locationPostcode` IS NOT NULL 
    AND `locationPostcode` <> '' 
    GROUP BY `postcode` 
    ORDER BY `locationCount` DESC 
    ) as `outer` 
GROUP BY left(`postcode`, 1) 
ORDER BY `locationCount` DESC 

澄清：我想我的實際郵編結果如下：

postcode | count 
---------------- 
E  | 3000 
W  | 2200 
SW  | 1300 

不是：

postcode | count 
---------------- 
SW  | 1300 
E1  | 1000 
E2  | 300 
S1  | 200 
S2  | 100 
S3  | 50 

Answer 1

您的問題的描述表明了group by一個有條件的情況下：

SELECT (case when substr(locationPostcode, 2, 1) between 'A' and 'Z' 
      then left(`locationPostcode`, 2) 
      else left(locationPostcode, 1) 
     end) as postcode_prefix, count(`locationPostcode`) as `locationCount` 
FROM `locations` 
WHERE `locationPostcode` IS NOT NULL AND `locationPostcode` <> '' 
GROUP BY postcode_prefix 
ORDER BY `locationCount` DESC;