MySQL：如何检查是否列出了用户

Question

我想知道如何检查用户是否已经在数据库中。

在 PHP 中，我有一个包含一些用户 ID 的数组。 即用户ID [0] = 1234；用户 IDs[1] = 2345;

现在我想构建一个查询，如果可能的话只进行一次 sql 调用以获得以下结果：

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

是否有 sql 解决方案或者我必须通过单独的调用检查每个 ID？ 感谢您的帮助！

Answer 1

根据您可以执行的列表检查现有的ids

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

但是要检查ids 的列表是否在数据库中，您可以这样做

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:

你在 PHP 中建立你的查询，它是字符串。

$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);
//...

差不多就是这样。因为查询是一个字符串，你可以像你想要的那样构建它:)