根據另一列值獲取一列的值

Question

所以我有這張桌子

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

現在我想根據 Col1 從 Col2 獲取值，假設我想要 A 的所有值，所以結果將是 [30, 20, 10] 。

我嘗試了以下查詢，但似乎效果不佳：

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

我在 php 中的 foreach 循環中使用它，所以這是帶有 php 代碼的查詢：

$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));
        }
    }
}

這里$text是包含以逗號分隔的結果的字符串。

我在這里遇到的問題是，除了第一次迭代，我得到了重復的結果，它給了我 A 30, 20, 40, 10 ，但它給了我 B 30, 20, 40, 10。

Answer 1

正如@Barmar 建議的那樣，使用單個查詢：

$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);
}

輸出：

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

檢查我的演示查詢。

如果你不知道GROUP_CONCAT()他是一個文檔