mySQL - 使用返回多行的选择更新多列

Question

我有一个邮政编码表，我想用它最近的 3 个邻居更新每个邮政编码。即填表中的空白：

postcode  nearestPostcode1  nearestPostcode2  nearestPostcode3
_______________________________________________________________

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

我想出了一个 SELECT 查询来查找最近的邮政编码，这是一种可以更新第一行的笨拙方法：

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，这将导致为每次行更新运行 3 个选择查询。如果有一些方法可以做这个伪代码所表达的事情会更有效：

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，从选择返回的行是否可以放入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

你可以做类似这样的事情：

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

我在 Stackoverflow 上找到了有关如何将列转换为行的相关问题：

MySQL - Rows to Columns

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)

这是在 MySQL [1] 中模拟 ROW_NUMBER() 功能的 hack：

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

Answer 3

我认为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 ...;

查看真正的 SQL 会更容易指定它。

请注意，第一列在引号中指定为常量。为此，postcode 必须是 UNIQUE 或 PRIMARY 索引。

Answer 4

每当我看到一个表，其名称后有一个加一计数器的列，我都会担心。

一般来说，存储可以从已存储的数据中计算出来的数据是一个坏主意 (TM)。如果您的应用程序突然需要 4 个最接近的邮政编码会怎样？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，最好不要明确存储这些数据。