MySQL: How can I check whether Users are listed or not

Question

I was wondering how I can check whether users are already in the database or not.

In PHP I have an array with some UserIDs. ie userIDs[0] = 1234; userIDs[1] = 2345;

Now I wanted to build a query to make just one sql call if possible to get following result:

############################
#    UserID    #   Exists  #
############################
#     1234     #     0     #
#     2345     #     1     #
############################

Is there a sql solution or do I have to check each ID with a seperate call? Thank you for your help!

Answer 1

To check existing id s against a list you can do

select userId, userId in (2345,5678) as `exists`
from your_table
order by userId

But to check if a list of id s is in the DB you can do

select tmp.userId, tab.userId is not null as `exists`
from
(
  select 1234 as userId
  union all
  select 2345
  union all
  select 3456
) tmp
left join db_table tab on tab.userId = tmp.userId

Answer 2

x:

you build up your query in PHP which is string.

$idsFromFacebook = array(1234,2345,3456,4567);

//start query
$strQuery = "select tmp.userId, tab.userId is not null as `exists` from (";
$bfirstRun = true;

//Go through each element of the array and connect them with the query string
foreach( $idsFromFacebook as $userID )
{
     //There is no union all before the first element and after the last
     if ( !$bFirstRun )
          $strQuery .= " union all ";

     $strQuery .= "select ".$userID;

     if ( $bFirstRun )
     {
          //"as userId" is only needed the first time
          $strQuery .= " as userId";
          $bFirstRun = False
     }
}

//Finishing the query by joining the generated table with database
$strQuery .=  ") tmp left join db_table tab on tab.userId = tmp.userId;";

//Call Query
$result = $dbms->query($strQuery);
//...

That is pretty much it. Because the query is a string, you can build it up just like you want to have it :)