mySQL - 使用select返回多行更新多列

Question

我有一個郵政編碼表，我想用它的3個最近的鄰居更新每個郵政編碼。即填補空白此表：

postcode nearestPostcode1 nearestPostcode2 nearestPostcode3 
_______________________________________________________________ 

KY6 1DA  -    -     - 
KY6 1DG  -    -     - 
KY6 2DT  -    -     - 
KY6 1RG  -    -     - 
.... 

我已經想通了一個SELECT查詢找到最近的郵政編碼，這裏是一個笨拙的方式，第一行可能被更新：

update table1 set 
nearestPostcode1 = (select query for returning the first nearest postcode), 
nearestPostcode2 = (select query for returning the second nearest postcode), 
nearestPostcode3 = (select query for returning the third nearest postcode) 
where postcode = 'KY6 1DA'; 

但是，這將導致爲每個行更新運行3個選擇查詢。

update table1 set 
(nearestPostcode1, nearestPostcode2, nearestPostcode3) = 
(select query to return the 3 nearest postcodes) 
where postcode = 'KY6 1DA'; 

在上面看起來「選擇查詢」類似這樣的：如果有一些方法做的是通過這個僞代碼表示這將是更有效的

select postcode from postcodeTable 
order by <equation to calculate distance> ASC 
limit 3 

反正對從select中返回的行將被放入一個可用於更新多個字段的表單中？ 謝謝。

Answer 1

Update Table1 
    Cross Join (
       Select Min(Case When Z1.Num = 1 Then Z1.postcode End) As PostCode1 
        , Min(Case When Z1.Num = 2 Then Z1.postcode End) As PostCode2 
        , Min(Case When Z1.Num = 3 Then Z1.postcode End) As PostCode3 
       From (
         Select postcode 
          , @num := @num + 1 As Num 
         From postcodeTable 
         Where postcode = 'KY6 IDA' 
         Order By <equation to calculate distance> ASC 
         Limit 3 
         ) As Z1 
       ) As Z 
Set nearestPostCode1 = Z.PostCode1 
    , nearestPostCode2 = Z.PostCode2 
    , nearestPostCode3 = Z.PostCode3 
Where Table1.postcode = 'KY6 IDA' 

Answer 2

我想你可以用僞代碼做到這一點：

REPLACE INTO table1 (postcode, nearestPostcode1, nearestPostcode2, nearestPostcode3) 
    SELECT "KY6 1DA", col1, col2, col3 FROM myTable WHERE ...; 

它會更容易指定它看到了真正的SQL。

注意第一列被指定爲引號中的常量。爲此，postcode必須是UNIQUE或PRIMARY索引。

Answer 3

你可以做一些類似這樣的東西：

UPDATE table1 
SET 
nearestPostcode1 = pc1, 
nearestPostcode2 = pc2, 
nearestPostcode3 = pc3 
FROM 
(SELECT pc1, pc2, pc3 FROM ....) t 
WHERE 
postcode = 'KY6 1DA'; 

我發現此相關的問題＃2如何列轉換爲行：

MySQL - Rows to Columns

你的情況，你可以做點像

SELECT 
IF(@rownum=1,postcode,'')) AS pc1, 
IF(@rownum=2,postcode,'')) AS pc2, 
IF(@rownum=3,postcode,'')) AS pc2, 
FROM 
(SELECT postcode 
FROM postcodeTable 
ORDER BY <equation to calculate distance> ASC 
LIMIT 3) 

這裏是模擬在MySQL [1]的ROW_NUMBER（）功能的黑客：

SELECT @rownum:=@rownum+1 rownum, t.* 
FROM (SELECT @rownum:=0) r, mytable t; 

Answer 4

每當我看到一個表，有他們的名字後1專櫃專欄中，我得到關注。

一般而言，存儲可以從已經存儲的數據中計算出來的數據是一個壞主意（TM）。如果您的應用突然需要最接近的郵政編碼，會發生什麼情況？如果郵政編碼邊界改變怎麼辦？

假設距離計算不是很複雜，從長遠來看，最好不要顯式存儲這些數據。