mysql problem in a php book

["Ralph Kessel" <kesselr1@xxxxxxxxx>]

I've been wrestling with a mysql script as part of a PHP script found in a text book:

Message d'erreur : Unknown column 'a.type_id' in 'on clause' 
SELECT a.name, a.type_id, b.title,
                b.description, b.content_date,
                b.create_date, b.created_by,
                b.last_upd_date, b.last_upd_by,
                c.name as dept_name, content_id
              FROM Content_Type a, Department c
              LEFT OUTER JOIN Content b on
                a.type_id = b.content_type
                and a.type_id = b.content_type
                and b.dept_id = 3
                and b.content_type = 4
              WHERE c.dept_id = 3
              ORDER BY content_date DESC


table structure 
DROP TABLE IF EXISTS Department;

CREATE TABLE Department ( 
  dept_id       SERIAL,
  name          VARCHAR(255)  NOT NULL,
  description   VARCHAR(255) NOT NULL,
  
  PRIMARY KEY (dept_id)
);

DROP TABLE IF EXISTS Content_Type;

CREATE TABLE Content_Type (
  type_id       SERIAL,
  name          VARCHAR(255) NOT NULL,
  
  PRIMARY KEY (type_id)
);

DROP TABLE IF EXISTS Content;

CREATE TABLE Content (
  content_id    SERIAL,
  dept_id       INT           NOT NULL,
  content_type  INT           NOT NULL,
  title         VARCHAR(255)  NOT NULL,
  description   TEXT          NOT NULL,
  content_date  DATE,
  creation_date TIMESTAMP    
                DEFAULT now() NOT NULL,
  created_by    VARCHAR(255)  NOT NULL,
  last_upd_date TIMESTAMP     NOT NULL,
  last_upd_by   VARCHAR(255)  NOT NULL,
  
  PRIMARY KEY(content_id)
);

DROP TABLE IF EXISTS Dept_User;

CREATE TABLE Dept_User ( 
  user_name     VARCHAR(255) 
                       UNIQUE NOT NULL,
  dept_id       INT           NOT NULL,
  first_name    VARCHAR(255)  NOT NULL,
  last_name     VARCHAR(255)  NOT NULL,
  password      VARCHAR(255)  NOT NULL,
  creation_date TIMESTAMP    
                DEFAULT now() NOT NULL,
  email         VARCHAR(255),
  
  PRIMARY KEY(user_name)
);

DROP TABLE IF EXISTS Content_Download;

CREATE TABLE Content_Download (
  download_id   SERIAL,
  content_id    INT(6)       NOT NULL,
  file_name     VARCHAR(255) NOT NULL
);

INSERT Content_Type (name) values ("News");
INSERT Content_Type (name) values ("Events");
INSERT Content_Type (name) values ("FAQ");
INSERT Content_Type (name) values ("Meeting Materials");

INSERT Department (name, description)
  values ("Human Resources", "
    Bringing the right people together to get the job done.");

INSERT Department (name, description)
  values ("Sales", 
    "Our experienced sales team markets our great products to the public.");

INSERT Department (name, description)
  values ("Distribution", "We get the goods to the customers.");

INSERT Department (name, description)
  values ("Information Techology", 
    "Building the applications that give us the edge.");
I would appreciate assistance

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