PHP PDO fetchColumn没有返回结果

Here is my code:

$result = $link->query($query);
//Get query results 

$x = 0; 
$tableA = [];
$tableB = [];


while($tables = $result->fetch(PDO::FETCH_ASSOC))
{
    $test[] = $tables; 
    $assoc = $test[$x]; 

    $tableName = $assoc['TABLE_NAME']; 
    $columnName = $assoc['COLUMN_NAME']; 
    $refTableName = $assoc['REFERENCED_TABLE_NAME'];
    $refColumnName = $assoc['REFERENCED_COLUMN_NAME']; 

    $fkQueryA = "SELECT DISTINCT " . $columnName . " FROM " . $dbName . "." . $tableName . " ORDER BY ".$columnName." ;"; 
    $fkQueryB = "SELECT DISTINCT " . $refColumnName . " FROM " . $dbName . "." . $refTableName . " ORDER BY ".$refColumnName." ;"; 

    echo $fkQueryA."
"; 
    echo $fkQueryB."
"; 

    $resultA = $link->query($fkQueryA);
    $resultB = $link->query($fkQueryB);

    $tableA[] = $assoc['COLUMN_NAME'];
    while($var = $resultA->fetchColumn())
    {
        $tableA[] = $var;
    }

    $tableB[] = $assoc['REFERENCED_COLUMN_NAME'];
    while($vari = $resultB->fetchColumn())
    {
        $tableB[] = $vari; 
    }

    $x++; 
}

var_dump($tableA); 
//var_dump($tableB);

The code is returning this result when run:

SELECT DISTINCT salesRepEmployeeNumber FROM classicmodels.Customers ORDER BY     salesRepEmployeeNumber ;
SELECT DISTINCT employeeNumber FROM classicmodels.Employees ORDER BY     employeeNumber ;
SELECT DISTINCT officeCode FROM classicmodels.Employees ORDER BY officeCode ;
SELECT DISTINCT officeCode FROM classicmodels.Offices ORDER BY officeCode ;
SELECT DISTINCT orderNumber FROM classicmodels.OrderDetails ORDER BY orderNumber ;
SELECT DISTINCT orderNumber FROM classicmodels.Orders ORDER BY orderNumber ;
SELECT DISTINCT productCode FROM classicmodels.OrderDetails ORDER BY productCode ;
SELECT DISTINCT productCode FROM classicmodels.Products ORDER BY productCode ;
SELECT DISTINCT customerNumber FROM classicmodels.Payments ORDER BY customerNumber ;
SELECT DISTINCT customerNumber FROM classicmodels.Customers ORDER BY customerNumber ;
array(545) {
  [0]=>
  string(22) "salesRepEmployeeNumber"
  [1]=>
  string(10) "officeCode"
  [2]=>
  string(1) "1"
  [3]=>
  string(1) "2"
  [4]=>
  string(1) "3"
  [5]=>
  .....and so on. 

fetchColumn() is returning the proper results for all the other columns of my query results, just not that column's worth of information. Any suggestions as to how to fix this?

Data in salesRepEmployeeNumber column:

mysql> SELECT DISTINCT salesRepEmployeeNumber FROM classicmodels.Customers;                                         +------------------------+
| salesRepEmployeeNumber |
+------------------------+
|                   NULL |
|                   1165 |
|                   1166 |
|                   1188 |
|                   1216 | 
|                   1286 |
|                   1323 |
|                   1337 |
|                   1370 |
|                   1401 |
|                   1501 |
|                   1504 |
|                   1611 |
|                   1612 |
|                   1621 |
|                   1702 |
+------------------------+