สอบถามเรื่องการนับรวมจำนวนของคอลัมน์ทั้งหมดในตารางฐานข้อมูลทีค่ะ

$sql = 'SELECT COUNT(*) FROM table WHERE f1 IS NOT NULL';
$results = $db->query($sql);
$numNonNullF1Values = $results->fetchColumn();

$sql = 'SELECT COUNT(*) FROM table WHERE f2 IS NOT NULL';
$results = $db->query($sql);
$numNonNullF2Values = $results->fetchColumn();

$sql = 'SELECT COUNT(*) FROM table WHERE f3 IS NOT NULL';
$results = $db->query($sql);
$numNonNullF3Values = $results->fetchColumn();

$totalNonNullValues = $numNonNullF1Values + $numNonNullF2Values + $numNonNullF3Values;

$sql = 'SELECT f1, f2, f3 FROM table';
$results = $db->query($sql);

$numRows = $results->num_rows;

$nonNullValues = array();
for ($i = 0; $i < $numRows; $i++) {
  $row = $results->fetch_assoc();
  foreach ($row as $column => $value) {
    if ($value !== null) {
      $nonNullValues[$column]++;
    }
  }
}

$totalNonNullValues = array_sum($nonNullValues);

$sql = 'SELECT f1, f2, f3 FROM table';
$results = $db->query($sql);

$nonNullValues = [];
foreach ($results as $row) {
  foreach ($row as $column => $value) {
    if ($value !== null) {
      $nonNullValues[$column]++;
    }
  }
}

$totalNonNullValues = array_sum($nonNullValues);