我有一个邮政编码表，我想用3个最近的邻居更新每个邮政编码。即填写此表中的空白：
postcode nearestPostcode1 nearestPostcode2 nearestPostcode3
_______________________________________________________________
KY6 1DA - - -
KY6 1DG - - -
KY6 2DT - - -
KY6 1RG - - -
....
我已经找到了一个SELECT查询来查找最近的邮政编码，这是第一行可以更新的一种笨拙的方式：
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。如果有一些方法来执行这个伪代码所表达的内容，那将会更有效：
update table1 set
(nearestPostcode1, nearestPostcode2, nearestPostcode3) =
(select query to return the 3 nearest postcodes)
where postcode = 'KY6 1DA';
上面的“选择查询”如下所示：
select postcode from postcodeTable
order by <equation to calculate distance> ASC
limit 3
无论如何，从select返回的行被放入一个可以用来更新多个字段的表单中吗？
谢谢。
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'
你可以做类似的事情：
UPDATE table1
SET
nearestPostcode1 = pc1,
nearestPostcode2 = pc2,
nearestPostcode3 = pc3
FROM
(SELECT pc1, pc2, pc3 FROM ....) t
WHERE
postcode = 'KY6 1DA';
我在Stackoverflow上找到了关于如何将列转换为行的相关问题：
MySQL - 行到列
在你的情况下，你可以做类似的事情
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)
这是一个模拟MySQL中的ROW_NUMBER（）功能的hack [1]：
SELECT @rownum:=@rownum+1 rownum, t.*
FROM (SELECT @rownum:=0) r, mytable t;
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 ...;
看到它真正的SQL就更容易了。
请注意，第一列被指定为引号中的常量。为此工作 postcode 必须是一个 UNIQUE 要么 PRIMARY 指数。
任何时候我看到一个表格，其列名在他们的名字后面有一个计数器，我很担心。
通常，存储可以从已经存储的数据计算的数据是坏想法（TM）。如果您的应用程序突然需要，会发生什么 4 最近的邮政编码？如果邮政编码边界发生变化怎么办？
假设距离计算不是很复杂，从长远来看，你最好不要明确地存储这些数据。