如何使用mysql进行联接

Question

I'm trying to create a page where there is content on top and at the bottom there is a section that if you click on the heading it will slide down to reveal some more content. I'm having a slight problem on the bottom section. When I try to do a join statement nothing shows up.

I have 3 tables. For the content table the id is the primary key. For the menu table the id is the primary key. For the content_menu both the id_menu and id_content is the primary key.

content:
            id | title    | content
            1  | content1 | This is content 1
            2  | content2 | This is content 2

menu: 
            id | title   |active
            1  | menu 1  | 1
            2  | menu 2  | 1

content_menu:
                id_menu | id_content
                    1   |   1                       

This is the code that I'm using

$exQuery = "SELECT * FROM menu M, content C LEFT JOIN content_menu CxM ON M.`id`=CxM.`id_menu` C.`id`=CxM.`id_content` WHERE M.`active`='1'"; 
$query = mysql_query($exQuery);
while( $result = mysql_fetch_object($query) ){

    $str.='<div class="additional-navigation-wrapper">';
    $str.=      '<div class="additional-navigation">';
    $str.=          '<a class="border-bottom-white padding-level-one inactive additional-nav-info" href="javascript:void(0);">';
    $str.=              $result->title;
    $str.=              '<img class="nav-arrow no-action floatright" src="images/nav-arrow-white.png" />';
    $str.=              '<span class="clearboth"></span>';
    $str.=          '</a>';
    $str.=          '<div class="additional-nav-info-wrapper">';
    $str.=              '<div class="additional-nav-info-inner" style="display:none;">';
    $str.=                  '<h2>'.$result->title.'</h2>';
    $str.=                  '<p>'.$page->content.'</p>';
    $str.=              '</div>';
    $str.=          '</div>';
    $str.=      '</div>';
    $str.='</div>';
    $i++;   
}

return $str;

If you need any more information or if its unclear please let me know

Answer 1

SELECT * FROM menu M, content C LEFT JOIN content_menu CxM ON M.`id`=CxM.`id_menu`
 C.`id`=CxM.`id_content` WHERE M.`active`='1'

In addition to the missing AND between the two join terms that @user3591614 mentioned, you also have a problem when you mix comma-style join syntax with JOIN syntax.

The problem is that JOIN has higher precedence than comma. So at the time the query is evaluating the term M.id = CxM.id_menu , it only knows about tables C and CxM . Therefore the table M cannot be referenced by the join condition.

The solution is to stop using comma-style joins, and use modern syntax for joins consistently. Here's how I would write that query:

SELECT * FROM menu M 
LEFT JOIN content CxM ON M.id = CxM.id_menu
LEFT JOIN content C ON C.id = CxM.id_content
WHERE M.active=1