mySQL - 使用select返回多行更新多列

Question

我有一个邮政编码表，我想用它的3个最近的邻居更新每个邮政编码。即填补空白此表：

postcode nearestPostcode1 nearestPostcode2 nearestPostcode3 
_______________________________________________________________ 

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

我已经想通了一个SELECT查询找到最近的邮政编码，这里是一个笨拙的方式，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，这将导致为每个行更新运行3个选择查询。

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

在上面看起来“选择查询”类似这样的：如果有一些方法做的是通过这个伪代码表示这将是更有效的

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

反正对从select中返回的行将被放入一个可用于更新多个字段的表单中？ 谢谢。

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

我想你可以用伪代码做到这一点：

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

它会更容易指定它看到了真正的SQL。

注意第一列被指定为引号中的常量。为此，postcode必须是UNIQUE或PRIMARY索引。

Answer 3

你可以做一些类似这样的东西：

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

我发现此相关的问题＃2如何列转换为行：

MySQL - Rows to Columns

你的情况，你可以做点像

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) 

这里是模拟在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

每当我看到一个表，有他们的名字后1专柜专栏中，我得到关注。

一般而言，存储可以从已经存储的数据中计算出来的数据是一个坏主意（TM）。如果您的应用突然需要最接近的邮政编码，会发生什么情况？如果邮政编码边界改变怎么办？

假设距离计算不是很复杂，从长远来看，最好不要显式存储这些数据。