Question

我有一张包含类别信息的表。

该表有一个标记父类别和子类别的字段。

我尝试通过查询列出父类别，然后列出其子类别。

我试过像

这样的东西

SELECT 
  CategoryID.categories as ParentID,
  CategoryName.categories AS ParentName, 
  parents.* 
FROM 
  categories AS parents LEFT JOIN categories AS child 
    ON child.Parent = parents.ParentID

但遗憾的是它对我不起作用。它给了我一个奇怪的错误。

[SQL] SELECT CategoryID.categories as ParentID, CategoryName.categories 
AS ParentName, parents.* FROM categories as parents LEFT JOIN categories as child on child.Parent = parents.ParentID

[Err] 1054 - Unknown column 'CategoryID.categories' in 'field list'

我确信所有字段名称都是正确的。

Answer 1

首先，db引擎读取列[table]。[column]，而不是相反。所以它不会理解CategoryID.categories是什么 - 没有名为CategoryID的表。

您已将表格定义为

     FROM categories AS parents 
LEFT JOIN categories AS child

所以你需要使用你给他们的名字来引用它们。即。

SELECT
      parents.CategoryId as ParentId,
      child.CategoryId as ChildId,
      ...

Answer 2

尝试

SELECT
  parents.CategoryId AS ParentId,
  parents.CategoryName AS ParentName,
  parents.* -- < this is probably child.*
FROM
   categories AS child LEFT JOIN categories AS parents
     ON child.parent = parents.CategoryId

SQL的命名约定是[table]。[column]。您首先使用列名称。

Answer 3

在SELECT下，您正在撰写CategoryID.categories和CategoryName.categories ...规则是将其写为 alias name.field name 。

因此，您收到错误Unknown column 'CategoryID.categories' in 'field list'。

我建议你试试这个 -

SELECT parent.CategoryID as ParentCategory, parent.CategoryName as ParentCatName,
     child.CategoryID as ChdCategory, child.CategoryName as ChdCatName
FROM categories AS parents LEFT JOIN categories AS child 
     ON child.Parent = parents.ParentID

您也可以在此处查询：mysql joins with alias

未知的复合列名称

3 个答案: