PHP while statement echoes duplicates

问题

I'm new to PHP and I have spent several hours researching and trying to figure out what I'm doing wrong. I'm joining several tables to populate a client profile page with multiple transactions from a transactions table. The rest of the page is working with the query as expected, but when using a while statement, each transaction in the result is repeating equal to the total number of transactions in the result. For instance, if the result has 3 transaction numbers (1, 2, 3), the while statement is echoing "1, 1, 1, 2, 2, 2, 3, 3, 3" instead of just "1, 2, 3".

I know it is something simple, but it has me pulling my hair out. Please help:

    if(!isset($_GET['PeopleID'])){
      header('Location: people.php');
      die();
    } else {
      if(!filter_var($_GET['PeopleID'], FILTER_VALIDATE_INT)){
        header('Location: people.php');
        die();
      } else {
        $PeopleID = filter_var($_GET['PeopleID'], FILTER_VALIDATE_INT);
        $query = "SELECT *
          FROM
            people
          LEFT JOIN notes ON 
            PersonID = PeopleID
          LEFT JOIN filenumbers ON 
            id = PeopleID   
          LEFT JOIN transactions ON 
            transactions.FileNumberID = filenumbers.FileNumberID
          LEFT JOIN filingdetails ON 
            filingdetails.FileID = filenumbers.FileNumberID
          WHERE PeopleID=:PeopleID";
  
         $result = $db_connection->prepare($query);
         $result->execute([
           'PeopleID' => $PeopleID
         ]);
       }
    }

    while($row = $result -> fetch()){
      echo $row['TransactionNumber'] . ", ";

    }

回答1:

Okay... So, this is expected behaviour from your SQL query.

When you LEFT JOIN in the manner that you have you end up getting all of the data multiple times over. This is best explained with an example...

Example

Lets assume you have three tables:

table1 => your primary table

table2 => many-to-one relationship with table1

table3 => many-to-one relationship with table1

table1
id, value
1, a
2, b
3, c

table2
id, fkey, value
1, a, xxxx
2, a, cxcz
3, a, bdtht
4, a, nbtyt
5, b, ngftht
6, b, thhgfj
7, b, gjfhj
8, c, hjghj
9, c, jhgjh

table3
id, fkey, value
10, 2, sdgvgd
11, 2, gjkmhkjk
12, 3, kjhkl
13, 3, kljj

Next we want to SELECT data from table1 whilst incorporating data from table2, using a LEFT JOIN

SELECT
    table1.id, table1.value, table2.id, table2.value
FROM table1
    LEFT JOIN table2 ON 
        table2.fkey = table1.id

The output of the above SQL would look something like the below...

table1.id, table1.value, table2.id, table2.value
1, a, 1, xxxx
1, a, 2, cxcz
1, a, 3, bdtht
1, a, 4, nbtyt
2, b, 5, ngftht
2, b, 6, thhgfj
2, b, 7, gjfhj
3, c, 8, hjghj
3, c, 9, jhgjh

Notice that the above output is effectively every selected row in table1 multiplied by the number of corresponding rows in table2; OR...

every row in table1 * table2 rows with fkey=table1.id

So, that's simple enough. What if we now look at an example with the three tables and two LEFT JOINs

SELECT
    table1.id, table1.value, table2.id, table2.value
FROM table1
    LEFT JOIN table2 ON 
        table2.fkey = table1.id
    LEFT JOIN table3 ON 
        table3.fkey = table1.id


table1.id, table1.value, table2.id, table2.value, table3.id, table3.value
1, a, 1, xxxx, null, null
1, a, 2, cxcz, null, null
1, a, 3, bdtht, null, null
1, a, 4, nbtyt, null, null
2, b, 5, ngftht, 10, 2, sdgvgd
2, b, 5, ngftht, 11, 2, gjkmhkjk
2, b, 6, thhgfj, 10, 2, sdgvgd
2, b, 6, thhgfj, 11, 2, gjkmhkjk
2, b, 7, gjfhj, 10, 2, sdgvgd
2, b, 7, gjfhj, 11, 2, gjkmhkjk
3, c, 8, hjghj, 12, 3, kjhkl
3, c, 8, hjghj, 13, 3, kljj, 
3, c, 9, jhgjh, 12, 3, kjhkl
3, c, 9, jhgjh, 13, 3, kljj

Here you can see that we now have output equivalent to:

every row in table1 * table2 rows with fkey=table1.id * table3 rows with fkey=table1.id

来源：https://stackoverflow.com/questions/64516874/php-while-statement-echoes-duplicates

标签

php

while-loop

duplicates

echo