Where do I need to use backticks or quotes in my MYSQL query?

Question

I am trying to run the following query, but I am not sure if my 's should be `s or not, $form_id = the record's column , $user_id is the primary key of the record called cf_id .

$querydate is going to be echo'd later on in the script, as it pulls the date from the record that equals to $form_id and $user_id .

$querydate = mysql_query("SELECT '$form_id' FROM email_history WHERE cf_id = '$user_id'") or die(mysql_error());

EDIT >>>>>>

After trying some of the solutions below, it seems to work ok, but instead of getting the date stored under the form name, I am getting this echo'd instead, so im not sure whats happening now : :Resource id #120 :Resource id #121 :Resource id #122 :Resource id #123

The table is setup like the followng:

[USER_ID] [FORM_ID1212212]  [FORM_ID1212112]  
 [1]      [2-1-2012]        [2-1-2012]       
 [2]      [1-1-2012]        [1-1-2012]       

Answer 1

You use backticks (`) for table and column names, single quotes (') for strings.

$querydate = mysql_query("SELECT `$form_id` FROM email_history WHERE cf_id = '$user_id'"); 

Backticks are only needed when your table name or column name is a MySQL reserved word... best practise is to avoid reserved words

But also consider switching to PDO and using prepared statements, or at least to mysqli rather than mysql

Answer 2

Best practice would be:

"SELECT `$form_id` FROM `email_history` WHERE `cf_id` = '$user_id'"

Backticks should be used around field names and table names (and DB names), and quotes should be used around values.

Answer 3

You should:

1. ensure that $form_id is a legal table name, especially if it's generated from user-supplied input.

2. use a bound parameter for $user_id

eg:

$sql = "SELECT `$userid` FROM `email_history` WHERE `cf_id` = ?"
$res = $db->query($sql, array($user_id));
while ($row = $res->fetchRow()) {
   ...
}

Back-ticks are appropriate for all table and column names. Unfortunately you can't use variable column names in a parameterised query, so you do need to construct that part of the query by hand.