[LondoN]

public client_connect(id)
{
	new g_szAuthID[32];
	get_user_authid(id, g_szAuthID, charsmax(g_szAuthID));

	new g_szQuery[256];
	formatex(g_szQuery, charsmax(g_szQuery), "SELECT * FROM tablename WHERE player_auth = '%s'", g_szAuthID);

	new g_szData[1];
	g_szData[0] = id;

	SQL_ThreadQuery(g_SQLTuple, "SQL_Check", g_szQuery, g_szData, sizeof g_szData);
}

public SQL_Check(FailState, Handle: Query, szError[], iErrorCode, szData[], iDataSize)
{
	if(!FailState && !iErrorCode)
	{
		new id = szData[0];

		if(!SQL_NumResults(Query))
			server_cmd("kick #%d", get_user_userid(id));
	}

	SQL_FreeHandle(Query);
}

This type of 'doing sql' always worked fine for me, you can try.