在另一列中使用别名答案

【问题标题】：Using alias name in another column在另一列中使用别名
【发布时间】：2012-06-22 12:52:56
【问题描述】：

这个问题是我之前问题的扩展部分， Finding number position in string.

我有如下表myTable (myWord ==> varchar(10))

++++++++++++
+ myWord   +    
++++++++++++
+ AB123    +
+ A413     +
+ X5231    +
+ ABE921   +
+ 15231    +
+ 523      +
+ ABC      +
++++++++++++

我想要的如下。

++++++++++++++++++++++++++++++++
+ myWord   + myPos + NewString +
++++++++++++++++++++++++++++++++
+ AB123    + 3     + AB        +
+ A413     + 2     + A         +
+ X5231    + 2     + X         +
+ ABE921   + 4     + ABE       +
+ 15231    + 1     +           +
+ 523      + 1     +           +
+ ABC      + 999   + ABC       +
++++++++++++++++++++++++++++++++

为了得到上面的输出，我使用了下面的查询。

SELECT 
  myWord, 
  LEAST (
    if (Locate('0',myWord) >0,Locate('0',myWord),999),
    if (Locate('1',myWord) >0,Locate('1',myWord),999),
    if (Locate('2',myWord) >0,Locate('2',myWord),999),
    if (Locate('3',myWord) >0,Locate('3',myWord),999),
    if (Locate('4',myWord) >0,Locate('4',myWord),999),
    if (Locate('5',myWord) >0,Locate('5',myWord),999),
    if (Locate('6',myWord) >0,Locate('6',myWord),999),
    if (Locate('7',myWord) >0,Locate('7',myWord),999),
    if (Locate('8',myWord) >0,Locate('8',myWord),999),
    if (Locate('9',myWord) >0,Locate('9',myWord),999)
  ) as myPos,
  if (LEAST (
    if (Locate('0',myWord) >0,Locate('0',myWord),999),
    if (Locate('1',myWord) >0,Locate('1',myWord),999),
    if (Locate('2',myWord) >0,Locate('2',myWord),999),
    if (Locate('3',myWord) >0,Locate('3',myWord),999),
    if (Locate('4',myWord) >0,Locate('4',myWord),999),
    if (Locate('5',myWord) >0,Locate('5',myWord),999),
    if (Locate('6',myWord) >0,Locate('6',myWord),999),
    if (Locate('7',myWord) >0,Locate('7',myWord),999),
    if (Locate('8',myWord) >0,Locate('8',myWord),999),
    if (Locate('9',myWord) >0,Locate('9',myWord),999)
  )=999,myWord,SUBSTR(myWord,1,LEAST (
    if (Locate('0',myWord) >0,Locate('0',myWord),999),
    if (Locate('1',myWord) >0,Locate('1',myWord),999),
    if (Locate('2',myWord) >0,Locate('2',myWord),999),
    if (Locate('3',myWord) >0,Locate('3',myWord),999),
    if (Locate('4',myWord) >0,Locate('4',myWord),999),
    if (Locate('5',myWord) >0,Locate('5',myWord),999),
    if (Locate('6',myWord) >0,Locate('6',myWord),999),
    if (Locate('7',myWord) >0,Locate('7',myWord),999),
    if (Locate('8',myWord) >0,Locate('8',myWord),999),
    if (Locate('9',myWord) >0,Locate('9',myWord),999)
  )-1)) as NewString
FROM myTable;

我的问题是

在为列命名为 MyPos 后，为什么我不能在另一列中使用该名称，如下查询所示？

SELECT 
  myWord, 
  LEAST (
    if (Locate('0',myWord) >0,Locate('0',myWord),999),
    if (Locate('1',myWord) >0,Locate('1',myWord),999),
    if (Locate('2',myWord) >0,Locate('2',myWord),999),
    if (Locate('3',myWord) >0,Locate('3',myWord),999),
    if (Locate('4',myWord) >0,Locate('4',myWord),999),
    if (Locate('5',myWord) >0,Locate('5',myWord),999),
    if (Locate('6',myWord) >0,Locate('6',myWord),999),
    if (Locate('7',myWord) >0,Locate('7',myWord),999),
    if (Locate('8',myWord) >0,Locate('8',myWord),999),
    if (Locate('9',myWord) >0,Locate('9',myWord),999)
  ) as myPos,
  if (myPos=999,myWord,SUBSTR(myWord,1,myPos-1)) as NewString
FROM myTable;

但是，这给了我错误

Unknown column 'myPos' in 'field list':

See here for more details

请建议我如何通过使用别名来缩短此查询。

【问题讨论】：

标签： mysql alias

【解决方案1】：

尝试将最少的部分作为子选择。你可以别名。

类似这样的：

SELECT 
  mytable.myWord, 
  myPos.l,
  if (l=999,mytable.myWord,SUBSTR(mytable.myWord,1,l-1)) as NewString
FROM myTable, 
(select myword, LEAST (
    if (Locate('0',myWord) >0,Locate('0',myWord),999),
    if (Locate('1',myWord) >0,Locate('1',myWord),999),
    if (Locate('2',myWord) >0,Locate('2',myWord),999),
    if (Locate('3',myWord) >0,Locate('3',myWord),999),
    if (Locate('4',myWord) >0,Locate('4',myWord),999),
    if (Locate('5',myWord) >0,Locate('5',myWord),999),
    if (Locate('6',myWord) >0,Locate('6',myWord),999),
    if (Locate('7',myWord) >0,Locate('7',myWord),999),
    if (Locate('8',myWord) >0,Locate('8',myWord),999),
    if (Locate('9',myWord) >0,Locate('9',myWord),999)
  ) as l from mytable)
as myPos
where myPos.myword = mytable.myword

SQLFiddle example

【讨论】：

sql fiddle 向我展示了很多行，但没有给我预期的结果
太好了，非常感谢...我也做了一些 RND。见here
this question上的任何输入

【解决方案2】：

根据@juergend 的回答，我也尝试了以下方法，它也可以正常工作。

SELECT 
  t1.myWord, t2.l as MyPos, if (t2.l=999,t1.myWord,SUBSTR(t1.myWord,1,t2.l-1)) as NewString
FROM myTable t1 JOIN  
(select id, LEAST (
    if (Locate('0',myWord) >0,Locate('0',myWord),999),
    if (Locate('1',myWord) >0,Locate('1',myWord),999),
    if (Locate('2',myWord) >0,Locate('2',myWord),999),
    if (Locate('3',myWord) >0,Locate('3',myWord),999),
    if (Locate('4',myWord) >0,Locate('4',myWord),999),
    if (Locate('5',myWord) >0,Locate('5',myWord),999),
    if (Locate('6',myWord) >0,Locate('6',myWord),999),
    if (Locate('7',myWord) >0,Locate('7',myWord),999),
    if (Locate('8',myWord) >0,Locate('8',myWord),999),
    if (Locate('9',myWord) >0,Locate('9',myWord),999)
  ) as l from mytable) as t2
 ON t1.id=t2.id;

注意：为此，我在表中添加了id 以加入两个表。

Demo

【讨论】：