如何从MySQL表中获取多对多关系数据，并使用php进行排序和显示？

Question

I am very new to php and mysql. I am trying to learn as I go and am stuck on this issue.

I have a many-to-many relationship. The tables are: users and state. Many companies in the users table operate in many states and each state has many different companies. Each company on the user table has a unique id that is stored on the state table. The state table then has a column for the state name that the company operates in. A new row is created in the state table for each state the company is in. That all seems to work ok.

I wrote the following php code to pull all the company id's from the state table for any given state ($state) and then display all the company names (pulled from the users table with the company id) that operate in the given state.

My problem is that I would like to order the list alphabetically of companies that operate in any given state. This code will not do that. Can anyone offer a better way to do this that will allow the list to be ordered (or that will just generally be more efficient and better)?

$state=$_GET['state'];
echo $state;
$aid=array();
$result=mysql_query("SELECT * FROM state WHERE state='$state'");
while($row=mysql_fetch_assoc($result)){
$v=$row['company_id'];
array_push($aid,$v);};

foreach($aid as $val){
$result1=mysql_query("SELECT * FROM users WHERE company_id='$val'");
$row1=mysql_fetch_assoc($result1);
$company=$row1['company'];
echo $company.'<br/>';};

The table structures are:

users

company_id
company
(other columns that are not relevant)

state

index
company_id
state
(no additional columns)

Answer 1

You typically would introduce a third table in a relational database to express a many-to-many relationship. The table could be users_states and have only to fields: user_id and state_id. So you tables should probably look like this:

You do the sort in your SQL. So the query might look like:

SELECT u.*, s.*
FROM users AS u
INNER JOIN users_states AS us ON u.user_id = us.user_id
INNER JOIN states AS s ON us.state_id = s.state_id
WHERE s.state_id = ?
ORDER BY u.name ASC

Here I am assuming that a field name is what you want to sort by and that you will filter on some value passed for state_id.

Answer 2

See this URL:-

Many-to-many relationship select and order by

Probably something similar to this:

SELECT
    a.person_id
FROM
    table AS a,
    table AS b
WHERE
    a.person_id = b.person_id AND
    a.favorite_id = 1 AND
    b.favorite_id = 2
ORDER BY
    ( IF( a.is_main_favorite = "y", 1, 0 )
      +
      IF( b.is_main_favorite = "y", 1, 0 ) ) DESC

By the way: You may want to store 1/0 instead of y/n in the database so that you won't need the IF call

Answer 3

Try this SQL statement:

SELECT 
    s.state, u.company 
FROM 
    states s, users u 
WHERE 
    s.company_id = u.company_id 
AND 
    s.state = "$state" 
GROUP BY 
    s.state 
ORDER BY 
    s.state, u.company

This should give you an alphabetic list of companies per state selected. The ORDER BY uses the state first in case you decide to expand your query to get more than one states.