I have this cross-database query...
SELECT
`DM_Server`.`Jobs`.*,
`DM_Server`.servers.Description AS server,
digital_inventory.params,
products.products_id,
products.products_pdfupload,
customers.customers_firstname,
customers.customers_lastname
FROM `DM_Server`.`Jobs`
INNER JOIN `DM_Server`.servers ON servers.ServerID = Jobs.Jobs_ServerID
JOIN `cpod_live`.`digital_inventory` ON digital_inventory.jobname = Jobs.Jobs_Name
JOIN `cpod_live`.`products` ON products.products_pdfupload = CONCAT(digital_inventory.jobname, ".pdf")
JOIN `cpod_live`.`customers` ON customers.customers_id = products.cID
ORDER BY `DM_Server`.`Jobs`.Jobs_StartTime DESC LIMIT 50
it runs fine until I make them LEFT JOINs. I thought that by not specifying a type of join it was assumed to be a LEFT JOIN. Is this not the case?
I thought that by not specifying a type of join it was assumed to be a LEFT JOIN. Is this not the case?
No, the default join is an INNER JOIN.
Here is a visual explanation of SQL joins.
Inner join
Left join
No. When a type isn't specified, an INNER JOIN is used. To read up on differences; wikipedia
I believe the default is INNER JOIN if you just specify JOIN.
If you just mentioned JOIN in query by default it will be considered
as a INNER JOIN.
Left join:Left join will take all the elements from Left table and only matching records from the Right table as Follows.
example:
SELECT column_name(s)
FROM table_name1 #(Left table)
LEFT JOIN table_name2 #(Right table)
ON table_name1.column_name=table_name2.column_name
Hope this helps.
Related
When I use a left join on different databases, it works but not when I use inner join. Why ?
SELECT tkblue_tklabel_dev_data.EmailContent.*, tkblue_tklabel_dev_archdata.EmailTracking.*
FROM tkblue_tklabel_dev_data.EmailContent
FULL JOIN tkblue_tklabel_dev_archdata.EmailTracking ON tkblue_tklabel_dev_archdata.EmailTracking.idEmailTracking = tkblue_tklabel_dev_data.EmailContent.idEmailTracking
Unknown table 'tkblue_tklabel_dev_data.EmailContent'
But when using
SELECT tkblue_tklabel_dev_data.EmailContent.*, tkblue_tklabel_dev_archdata.EmailTracking.*
FROM tkblue_tklabel_dev_data.EmailContent
LEFT JOIN tkblue_tklabel_dev_archdata.EmailTracking ON tkblue_tklabel_dev_archdata.EmailTracking.idEmailTracking =
tkblue_tklabel_dev_data.EmailContent.idEmailTracking
I have not this error message, but I can only have the results of the left table. Or, I wish all the results like an full join.
Full join don't exists in mysql but you can produce the same result using both left join and right join in UNION
SELECT tkblue_tklabel_dev_data.EmailContent.*
, tkblue_tklabel_dev_archdata.EmailTracking.*
FROM tkblue_tklabel_dev_data.EmailContent
LEFT JOIN tkblue_tklabel_dev_archdata.EmailTracking
ON tkblue_tklabel_dev_archdata.EmailTracking.idEmailTracking = tkblue_tklabel_dev_data.EmailContent.idEmailTracking
UNION
SELECT tkblue_tklabel_dev_data.EmailContent.*
, tkblue_tklabel_dev_archdata.EmailTracking.*
FROM tkblue_tklabel_dev_data.EmailContent
RIGHT JOIN tkblue_tklabel_dev_archdata.EmailTracking
ON tkblue_tklabel_dev_archdata.EmailTracking.idEmailTracking = tkblue_tklabel_dev_data.EmailContent.idEmailTracking
I'm trying to study SQL.
I have a problem with JOIN
I want to display ref_id, pro_name, class_name but I couldn't.
I find EFFICIENT solution.
MY QUERY (DOESN'T WORK)
SELECT
ref_id, pro_name, class_name
FROM
RC, RP, PP, LP
WHERE
RC.ref_id = RP.ref_id
Avoid using commas be CROSS JOIN
You could use JOIN to instead of commas
like this.
SELECT
RP.ref_id, PP.pro_name, LP.class_name
FROM
RP
LEFT JOIN RC ON RC.ref_id = RP.ref_id
LEFT JOIN PP ON PP.pro_id = RP.pro_id
LEFT JOIN LP ON LP.lec_id = RP.lec_id
Never use commas in the FROM clause. Always use proper, explicit, standard JOIN syntax.
You would seem to want:
select rp.pro_id, pp.pro_name, lp.class_name
from rp left join
pp
on rp.pro_id = pp.pro_id left join
lp
on rp.lec_id = lp.lec_id;
Note the use of left join. This ensure that all rows are in the result set, even when one or the other joins doesn't find a matching record.
From what I can see, the table rc is not needed to answer this specific question.
I have a query where I am using a full outer join. But in some instance,
it gives me a syntax error.
What could be the reason for this? I don't see any miscode in my query.
MySQL does not support full outer join, but you can simulate it as a union between a left and right join query:
SELECT * FROM pbsdev3.item t1
LEFT JOIN pbsdev3.item_ledger_entry t2 ON t1.No_ = t2.Item_No_
UNION ALL
SELECT * FROM pbsdev3.item t1
RIGHT JOIN pbsdev3.item_ledger_entry t2 ON t1.No_ = t2.Item_No_
WHERE t1.No_ IS NULL
Note that in general if you find yourself doing full outer joins often, it could imply that your keys and data model are not well defined. One reason why MySQL does not support full joins could be that you should not have to use it.
Full outer join is quite a pain in MySQL. The first thing I would note is that it should not be needed. The items should match in the two tables, so an inner join or left join should be sufficient:
SELECT i.*, ile.*
FROM pbsdev3.item i LEFT JOIN
pbsdev3.item_ledger_entry ile
ON i.No_ = ile.Item_No_;
If you really need full outer join, then gather together all the items and use left join:
select it.*, ile.*
from (select i.No_ from item i union
select ile.Item_No_ from item_ledger_entry ile
) i left join
item it
on it.No_ = i.No_ left join
item_ledger_entry ile
on ile.No = i.No;
I am running a query:
select course.course,iars.id,
students.rollno,
students.name as name,
teachers.name as tname,
students.studentid,
attndata.studentid ,sum(attndata.obt) as obt
sum(attndata.benefits) as ben , (sum(attndata.max)) as abc
from groups, students
left join iars
on iars.id
left join str
on str.studentid=students.studentid
left join course
on course.c_id=students.course
left join teachers
on teachers.id=iars.teacherid
join sgm
on sgm.studentid=students.studentid
left join attndata
on attndata.studentid=students.studentid and iars.id=attndata.iarsid
left join sps
on sps.studentid=students.studentid and iars.paperid=sps.paperid
left join semdef
on semdef.semesterid=str.semesterid
where students.course='1'
and students.status='regular'
and sps.paperid='5'
and iars.courseid=students.course
and iars.semester=str.semesterid
and semdef.month=9
and iars.paperid='5'
and str.semesterid='1'
and str.sessionid='12'
and groups.id=sgm.groupid
group by sps.studentid,
teachers.id,
semdef.month
order by
students.name
In this query whenever I am having left join on semdef.id=attndata.mon, I am getting zero result when the value of semdef.id=null but I want all the results, irrespective of semdef, but I want to use it. As in it should fetch result, if the values are null. Can you please help it out.
It's probably because your where clause is saying
and semdef.month=9
and you probably want
and (semdef.month=9 OR semdef.id IS NULL)
or something similar.
It's because your where clause has statements relating to the semdef table. Add these to the join clause as putting these in the where is implying an inner join.
Eg:
Left join semdef on xxx and semdef.id = attndata.min
I have this query:
SELECT hit.timestamp,hit.id,config.Name,hit.meter_id,levels.LevelName, pos.sm_pos , hit.hit_value
FROM pos,hit,controllers,levels,config
WHERE hit.id=config.id
AND hit.meter_id=levels.id
AND pos.id=hit.id
AND pos.controller_id=controllers.id;
How to make an inner join query from this? With aliases or something? I was looking and I can't find anything for multiple table query.
The way you are joining tables is outdated now. It was used eartlier, now we use keyword like INNER/NATURAL/LEFT OUTER/RIGHT OUTER/CROSS etc. to join tables on basis of requirement.
Refer Join in Mysql
SELECT hit.timestamp,
hit.id,
config.Name,
hit.meter_id,
levels.LevelName,
pos.sm_pos,
hit.hit_value
FROM hit
INNER JOIN config
ON hit.id = config.id
INNER JOIN levels
ON hit.meter_id = levels.id
INNER JOIN POS
ON pos.id = hit.id
INNER JOIN controllers
ON pos.controller_id = controllers.id;
Note : The query posted by you is according the SQL-89 standard and the second posted by me is according to SQL-92.
The SQL-92 standard introduced INNER JOIN .. ON and OUTER JOIN .. ON in order to replace the more complex(?) syntax of SQL-89.
If you want to reformat your query, you can do it like this:
SELECT
H.timestamp,
H.id,
F.Name,
H.meter_id,
L.LevelName,
P.sm_pos,
H.hit_value
FROM pos AS P
INNER JOIN controllers AS C ON P.controller_id = C.id
INNER JOIN hit AS H ON P.id = H.id
INNER JOIN levels AS L ON H.meter_id = L.id
INNER JOIN config AS F ON H.id =F.id;
Notice that I've taken the liberty to add aliases on your table names, this can simplify your queries alot.
To understand how joins work in MySQL, read about it here in the manual. A good tutorial about joins was written by Jeff Atwood, you can find it here.