MySql Combine two queries (subselect or join or union) - mysql

Question

I have two working queries that I would like to combine to provide one lot of output for a wall display. I am NOT a DB person by any means but have managed to create these queries from scratch, albeit with a lot of info from here!
I have four tables with relevant columns:
Hosts[host, hostid]
Items[hostid, itemid, itemname]
History_unit[itemid, value]
History_log[itemid, value]
hostid and itemid are our identifiers
The history_xxx tables are just that, and entry for every record of that data.
After trying to combine these for too many hours I just don't understand enough to make it work.
Query 1
SELECT hosts.host,
max((case when items.name='RP_Dayend_OK' then history_uint.value end) *1000) as 'Day End',
max((case when items.name='RP_Sync_OK' then history_uint.value end) *1000) as 'Sync',
max((case when items.name='RP_Monthend_OK' then history_uint.value end) *1000) as 'Month End',
max(case when items.name='RP_Version' then history_uint.value end) as 'Version'
from hosts, items, history_uint
where hosts.hostid=items.hostid and items.itemid=history_uint.itemid and items.name like '%RP\_%'
group by hosts.host
Output:
Host | Day End | Sync | Month End | Version
Host 1 | date | date | date | 7xx
Host 2 | date | date | date | 7xx
Query 2
SELECT hosts.host,
max(case when items.name='RP_Cron' then history_log.value end) as 'cron'
from hosts, items, history_log
where hosts.hostid=items.hostid and items.itemid=history_log.itemid and items.name like '%RP\_%'
group by hosts.host
Output:
Host | Cron
Host 1 | string
Host 2 | string
What I would like is:
Host | Day End | Sync | Month End | Version | Cron
Host 1 | date | date | date | 7xx | string
Host 2 | date | date | date | 7xx | string
I did manage a sub select but I ended up with a different row for each host for each item, and no data for 'cron'. I also tried joins to no avail. It is simply my lack of understanding.
Thanks for any help!

Answer 1

You should abandon the use of implicit (comma separated) joins in favour of explicit joins. In your case LEFT (outer) joins are appropriate.
DROP TABLE IF EXISTS Hosts;
DROP TABLE IF EXISTS Items;
DROP TABLE IF EXISTS History_unit;
DROP TABLE IF EXISTS History_uint;
DROP TABLE IF EXISTS History_log;
CREATE TABLE Hosts(host VARCHAR(20), hostid INT);
CREATE TABLE Items(hostid INT, itemid INT, name VARCHAR(20));
CREATE TABLE History_uint(itemid INT, value INT);
CREATE TABLE History_log(itemid INT, value INT);
INSERT INTO HOSTS VALUES ('HOST1',1),('HOST2',2);
INSERT INTO ITEMS VALUES
(1,1,'RP_Dayend_OK'),
(1,2,'RP_Sync_OK'),
(1,3,'RP_Monthend_OK'),
(1,4,'RP_Version' ),
(2,1,'RP_Dayend_OK'),
(2,2,'RP_Sync_OK'),
(2,2,'RP_cron')
;
INSERT INTO HISTORY_uint VALUES
(1,10),(2,10),(3,10),(4,10),
(1,50),(3,60);
INSERT INTO HISTORY_log VALUES
(1,10),(2,10),(3,10),(4,10)
;
SELECT hosts.host,
max((case when items.name='RP_Dayend_OK' then history_uint.value end) *1000) as 'Day End',
max((case when items.name='RP_Sync_OK' then history_uint.value end) *1000) as 'Sync',
max((case when items.name='RP_Monthend_OK' then history_uint.value end) *1000) as 'Month End',
max(case when items.name='RP_Version' then history_uint.value end) as 'Version',
max(case when items.name='RP_Cron' then history_log.value end) as 'cron'
from hosts
left join items on items.hostid = hosts.hostid
left join history_uint on history_uint.itemid = items.itemid
left join history_log on history_log.itemid = items.itemid
where items.name like '%RP\_%'
group by hosts.host;
+-------+---------+-------+-----------+---------+------+
| host | Day End | Sync | Month End | Version | cron |
+-------+---------+-------+-----------+---------+------+
| HOST1 | 50000 | 10000 | 60000 | 10 | NULL |
| HOST2 | 50000 | 10000 | NULL | NULL | 10 |
+-------+---------+-------+-----------+---------+------+
2 rows in set (0.00 sec)
Note it's usually best if the OP provides the data.