mySQL - 使用select返回多行来更新多个列

Question

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      -                -                  -
....

我已经想出了一个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.如果有某种方法可以执行此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用于更新多个字段的表单中吗？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'