如何使用外键从三个表中选择所有数据

Question

Hi i am trying to select all data from three tables with a foreign key. i have tried something like that

$getId = $_GET['id'];

$getAll="SELECT * FROM employee_info ,over_view, employee_attendance
WHERE employee_info.id=$getId AND id=over_view.id AND id=employee_info.id";
$info_data = $dbcon->query($getAll);
$store = $info_data->fetch(PDO::FETCH_ASSOC);
echo $store['first_name'];

but it shows that error ans as a newbie i am finding it hard to solve. now how can i select all the data with a single query from three table over_view and employee_attendance tables id's are foreign key of employee_info.

Answer 1

I think you need to qualify your column names in the WHERE clause as below

SELECT * 
  FROM employee_info
      ,over_view
      ,employee_attendance
 WHERE employee_info.id = $getId
   AND employee_info.id = over_view.id
   AND employee_attendance.id = employee_info.id

Answer 2

Ok. First of all: you are using the implicit join structure for your query while you should use the explicit one. Second, the error you get is because you have more than one column id in one of the tables and MySql needs to know what to take into consideration. This is easier to see using the explicit join syntax:

SELECT * 
    FROM employee_info
    JOIN over_view ON employee_info.id=over_view.id
    JOIN employee_attendance ON employee_info.id=employee_attendance.id
    WHERE employee_info.id = $getId

With explicit joins you can use all the functionalities of all the kinds of joins you can create between tables: left, right, inner... an easy to read guide to join is here