PDO :: FETCH_COLUMN返回的行数少于PDO :: FETCH_ASSOC

Question

我有一个简单的查询，从我的数据库中的库存表中返回所有ISBN，总共应该返回大约6650个结果但由于某种原因PDO :: FETCH_COLUMN很短，更改我只返回4851结果，下面是两个我用于使用pdo mysql检索数据的不同函数

php中的函数

/* returns all select results in multi-dimensional arrays */
public function select($query){ 
    try{
        $stmt = $this->dbh->query($query);
        $arr = array();
        while($row = $stmt->fetch(PDO::FETCH_ASSOC)){
            $arr[] = $row;
        }
        return $arr;

    }catch(PDOException $e){  
        echo "Error with select_array function: ".$e->getMessage();  
    }
}

/* returns only one column in one array, used when selecting one column */
public function select_one_column($query){
    try{
        $stmt = $this->dbh->query($query);
        $arr = array();
        while($row = $stmt->fetch(PDO::FETCH_COLUMN)){
            $arr[] = $row;
        }
        return $arr;

    }catch(PDOException $e){  
        echo "Error with select_array function: ".$e->getMessage();  
    }
}

使用示例

$query = "select isbn from inventory";
print_r($this->select($query));
array(
   [0] => Array
      (
         [isbn] => 1843060469
      )
   [1] => Array
      (
         [isbn] => 184306152X
      )
)

print_r($this->select_one_colum($query));
array(
    [0] => 1843060469
    [1] => 184306152X
)

可能导致此问题的原因是什么？