MySQL 5.7

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CREATE TABLE device 
( deviceID varchar(12) NOT NULL primary key
, snmpSet varchar(30) NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf16;
CREATE TABLE snmpData 
( deviceID varchar(12)  NOT NULL
, ts datetime NOT NULL
, oid varchar(45)  NOT NULL
, value varchar(30)  NOT NULL
,    constraint pk_snmpData primary key (deviceID, ts) -- assumed 
,    constraint fk_snmpData foreign key (deviceID) 
                            references device (deviceID)
) ENGINE=InnoDB DEFAULT CHARSET=utf16;
CREATE TABLE device_summary 
( deviceID varchar(12) NOT NULL
, firstSeen datetime NOT NULL
, lastSeen datetime NOT NULL
,   constraint pk_device_summary primary key (deviceID) 
,   constraint fk_device_summary foreign key (deviceID) 
                                 references device (deviceID)
) ENGINE=InnoDB DEFAULT CHARSET=utf16;


-- DELIMITER $$

CREATE TRIGGER after_snmpData_insert 
AFTER INSERT ON snmpData
FOR EACH ROW 
BEGIN
    -- if device exists, insert will be ignored
    INSERT IGNORE INTO device_summary
        (deviceID, firstSeen, lastSeen)
    VALUES (NEW.deviceID, NEW.ts, NEW.ts);

    -- update lastSeen
    UPDATE device_summary 
        SET lastSeen = NEW.ts
    WHERE deviceID = NEW.deviceId;
END
;
-- $$
-- DELIMITER ;
insert into device (deviceid, snmpset) values ('d1','D1');
insert into snmpData (deviceid, ts, oid, value) values ('d1', now() - 1, '', '0');
select * from device_summary;


deviceID
firstSeen
lastSeen


d1
2019-07-22 07:59:07
2019-07-22 07:59:07


insert into snmpData (deviceid, ts, oid, value) values ('d1', now(), '', '0');
select * from device_summary;


deviceID
firstSeen
lastSeen


d1
2019-07-22 07:59:07
2019-07-22 07:59:08