Question

Confession: a mysql newb requires simple example to locate duplicate rows in somewhat large table. I have searched for and read many other threads with similar titles, but the examples are so complex that I cannot apply them to my basic situation.

A MySQL table has only 5 fields, but there are hundreds of rows. I wish to locate duplicate rows — I know there is one for sure and wonder if there are others.

Example Row: (rel_id is auto-incrementing, primary key field)

'rel_id' => 1
'host' => 17
'host_type' => 'client'
'rep' => 7
'rep_type => 'cli_mgr'

My approach was to:
1. Read entire table into mysql query
2. row-by-row compare the 4 data fields to those of previous (“done”) rows
3. after comparing a “new” row, append it to array of “done” rows

Here is what I have tried. I am sure that there must be a much simpler solution. You will see that I am bogged down in trying to append the “new” row to the array of “done” rows:

$rRels = mysql_query("SELECT * FROM `rels`");
$a = array();
$e = array();
$c1 = 0;
$c2 = 0;
While ($r = mysql_fetch_assoc($rRels)) {
    $i = $r['rel_id'];
    $h = $r['host'];
    $ht = $r['host_type'];
    $r = $r['rep'];
    $rt = $r['rep_type'];

    foreach($a as $row) {
        $xh = $row['host'];
        $xht = $row['host_type'];
        $xr = $row['rel'];
        $xrt = $row['rel_type'];

        if (($h==$xh) && ($ht==$xht) && ($r==$xr) && ($rt==$xrt)) {
            echo 'Found one<br>';
            $e[] = $r;
        }
        $c2++;
    }
    $a = array_merge(array('rel_id'=>$i, 'host'=>$h, 'host_type'=>$ht, 'rep'=>$r, 'rep_type'=>$rt), $a);
    $c1++;
}

echo '<h3>Duplicate Rows:</h3>';
foreach ($e as $row) {
    print_r($row);
    echo '<br>';
}
echo '<br><br>';
echo 'Counter 1: ' . $c1 . '<br>';
echo 'Counter 2: ' . $c2 . '<br>';

Answer 1

This should do the trick:

SELECT COUNT(*) as cnt, GROUP_CONCAT(rel_id) AS ids
FROM rels
GROUP BY host, host_type, rep, rep_type
HAVING cnt > 1

any “duplicate” records will have a cnt > 1, and the group_concat will give you the ids of the duped records.