检查用户是否存在mysql php

I have the following table

uId | groupId
==============
1     12
3     66

How can I write a mySQL statement to check if a user exists in a group?

What I've Tried:

SELECT FROM usertable(uId) WHERE usertable.uId = $uId

the above query only checks if the user exists but doesn't check whether the user belongs to a particular group.

Just append that condition to your where clause, for example:

SELECT uId
FROM usertable
WHERE usertable.uId = 1
AND groupId = 12

Simply add the groupId parameter to your query WHERE clause.

$userId = x; // some user id
$groupId = y; // some group id

$con; // your mysqli instance

if (!$stmt = $con->prepare('SELECT 1 FROM usertable WHERE uId = ? AND groupId = ?')) {
    throw new Exception($con->error, $con->errno);
}

$stmt->bind_param('ii', $userId, $groupId);

if (!$stmt->execute()) {
    throw new Exception($stmt->error, $stmt->errno);
}
if ($stmt->fetch()) {
    // user exists and is in group
}

$query = mysql_query("Select from usertable(uId) where usertable.uId = $uId and groupId=$gid");

if(mysql_num_rows($query)!=0){
echo "user exists!";
}else
{
echo "user not exists!";
}