Get values of one column based on another columns value

Question

So I have this table

TableTBL: 
+--------+------+-------+
|    ID  | Col1 | Col2  |
+--------+------+-------+
|      1 |    A |    30 | 
|      2 |    A |    20 | 
|      3 |    B |    40 |  
|      4 |    A |    10 |  
+--------+------+-------+

Now I want to get values from Col2 based off Col1, say I want all the values of A , so the result would be [30, 20, 10] .

I've tried the following query but it doesn't seem to work well:

SELECT DISTINCT Col2 FROM TableTBL WHERE Col1 = 'A' ORDER BY Col2 DESC;

I am using this in php in a foreach loop, so here's the query with the php code:

$sql = "SELECT DISTINCT Col1 FROM TableTBL ORDER BY Col1 DESC;";
$stmt = mysqli_stmt_init($conn);
if (!mysqli_stmt_prepare($stmt, $sql )) 
{
    echo "SQL statement failed";
}
else
{
    mysqli_stmt_execute($stmt);
    $result = mysqli_stmt_get_result($stmt);

    while ($row = mysqli_fetch_assoc($result)) 
    { 
        $rows[] = $row['Col1']; 

    } 
    for($i = 0; $i < count($rows) ;$i++)
    {
        echo $rows[$i].": {\n";

        $sql = "SELECT DISTINCT Col2 FROM TableTBL WHERE Col1 = '$rows[$i]' ORDER BY Col2 DESC;";
        $stmt = mysqli_stmt_init($conn);
        if (!mysqli_stmt_prepare($stmt, $sql)) 
        {
            echo "SQL statement failed";
        }
        else
        {
            mysqli_stmt_execute($stmt);
            $result = mysqli_stmt_get_result($stmt);

            while ($row = mysqli_fetch_assoc($result)) { 
                $rows[] = $row; 
            } 
            foreach($rows as $row) {
                $arrList[] = $row['Col2'];
            }
            $text =  implode("',' ", array_unique($arrList));
        }
    }
}

Here $text is the string containing the result separated by commas.

The problem I am getting here is that I am getting repeated results in all except the first iteration, It is giving me A with 30, 20, 40, 10 , but it then gives me B with 30, 20, 40, 10.

Answer 1

As @Barmar suggested, use a single query:

$sql = "SELECT Col1, 
               GROUP_CONCAT(DISTINCT Col2 ORDER BY Col2 DESC SEPARATOR ', ') AS Col2 
        FROM TableTBL 
        GROUP BY Col1
        ORDER BY Col1 ";
$stmt = mysqli_stmt_init($conn);
if (!mysqli_stmt_prepare($stmt, $sql)) {
    echo "SQL statement failed";
} else {
    mysqli_stmt_execute($stmt);
    $result = mysqli_stmt_get_result($stmt);
    $arrList = array();
    while ($row = mysqli_fetch_assoc($result)) {
        $arrList[] = array(
            'Col1' => $row['Col1'],
            'Col2' => $row['Col2']
        );
    }
    var_export($arrList);
}

OUTPUT :

array(
    0 => array(
        'Col1' => 'A',
        'Col2' => '30, 20, 10',
    ),
    1 => array(
        'Col1' => 'B',
        'Col2' => '40',
    ),
)

Check my Demo query .

If you don't know GROUP_CONCAT() he is a documentation