SELECT vp.vpn_node, vp.cred_id, vp.vpn_conn_id,
	cpool.cred_busy, vp2.n_disconnected FROM
	vpn_pool vp
	LEFT OUTER JOIN (
		SELECT COUNT(vpn_id) AS n_disconnected, vpn_id, cred_id
		FROM vpn_pool GROUP BY cred_id) vp2
	ON vp.vpn_id = vp2.vpn_id
	INNER JOIN vpn_cred_pool cpool
	ON cpool.cred_id = vp.cred_id
	LEFT OUTER JOIN INFORMATION_SCHEMA.PROCESSLIST conns
	ON conns.ID = vp.vpn_conn_id
	WHERE vp.vpn_node IS NOT NULL AND conns.ID IS NULL;


------------------

    UPDATE
    vpn_pool vp
    LEFT OUTER JOIN (
        SELECT COUNT(vpn_id) AS n_disconnected, vpn_id, cred_id
                FROM vpn_pool GROUP BY cred_id) vp2
        ON vp.vpn_id = vp2.vpn_id
    INNER JOIN vpn_cred_pool cpool
        ON cpool.cred_id = vp.cred_id
    LEFT OUTER JOIN INFORMATION_SCHEMA.PROCESSLIST conns
        ON conns.ID = vp.vpn_conn_id
    SET
            vp.vpn_node = NULL,
            vp.cred_id = NULL,
            vp.vpn_conn_id = -1,
            cpool.cred_busy = cpool.cred_busy - vp2.n_disconnected
    WHERE vp.vpn_node IS NOT NULL AND conns.ID IS NULL;