尝试在 mysql 中加入多个表 - 得到错误 1054，未知列

Question

I want to join multiple tables using JOINs, and mysql/mariadb is refusing to find one column. The column exists, of course, and I can't figure out what the cause might be.

Table Layout

CREATE TABLE `shops` (
  `id` int(11) NOT NULL,
  `name` varchar(32) NOT NULL,
);

CREATE TABLE `shops2categories` (
  `shopid` int(11) NOT NULL,
  `categoryid` int(11) NOT NULL,
);

CREATE TABLE `categories` (
  `id` int(11) NOT NULL,
  `name` varchar(64) NOT NULL,
);

CREATE TABLE `categories2items` (
  `itemid` int(11) NOT NULL,
  `categoryid` int(11) NOT NULL
);

CREATE TABLE `items` (
  `id` int(11) NOT NULL,
  `name` varchar(32) NOT NULL,
);

Query: In order to avoid confusion with aliases, I now ran the query with the original table names.

SELECT
shops.name,
categories.name,
items.name

FROM shops

LEFT JOIN shops2categories ON shops2categories.shopid = shops.id
LEFT JOIN categories2items ON categories2items.categoryid = categories.id

LEFT JOIN categories ON categories.id = shops2categories.categoryid
LEFT JOIN items ON items.id = categories2items.itemid

Error Message:

#1054 - Unknown column 'categories.id' in 'on clause'

No matter how I restructured my query (foreign key first, primary key first, items table first, categories table first, ..., using different JOIN types), I can't seem to get this to work. Also I read through a whole lot of SO questions to this topic, but I feel that the order is correct, I am not inserting nor updating, and it's not about quoting. Something is fundamentally broken in my concept, and I'm very keen on learning what this could be.

Answer 1

Look at your from clause. You reference c.id before you have defined c .

A table cannot be referenced until it is defined in the FROM clause.

You would seem to want:

FROM shops s LEFT JOIN
     shops2categories s2c
     ON s2c.shopid = s.id LEFT JOIN
     categories c
     ON c.id = s2c.categoryid LEFT JOIN
     categories2items c2i
     ON c2i.categoryid = c.id LEFT JOIN
     items i
     ON i.id = c2i.itemid