MYSQL: Insert column if not exists

Question

I am trying to add a column by checking if it exists. If not it should give message that it already exists with the code below.

$prefix = 'vm_';
$col_name = 'checking';

$col = "SELECT ".$col_name." FROM ".$prefix."users";

if (!$col){
    $insert_col = "ALTER TABLE ".$table." ADD ".$col_name." DATETIME NOT NULL";

    mysql_query($insert_col);

    echo $col_name.' has been added to the database';
} else {
    echo $col_name.' is already exists';
}

But it doesn't add any column and directly displays message that the column already exists.

Answer 1

You never execute your query, your condition is instead your query string:

if (!$col) { //always false, since a nonempty string is always truthy

Answer 2

Here is the final code. There was stupid mistake; I didn't use mysql_query for $col

$prefix = 'vm_';
$col_name = 'checking';

$col = mysql_query("SELECT ".$col_name." FROM ".$prefix."users");


if (!$col){
    //$insert_col = "ALTER TABLE ".$table." ADD ".$col_name." DATETIME NOT NULL";

    mysql_query("ALTER TABLE ".$prefix."users ADD ".$col_name." DATETIME NOT NULL");

    echo $col_name.' has been added to the database';
} else {
    echo $col_name.' is already exists';
}