I'm trying to make left join in one query, but it seems that I'm wrong somewhere.
table machines
--------------
machineID
FaNo
Barcode
RoutingCode
Name
table log
-------------
logID
lineBarcode
machineBarcode
In the log table there are records on the machines and the lines. On one line there can be many different machines and machines from the same type.
The machine type is routingCode
, so I'm interested in selecting all the machines in the line and group them. Only machines with different routingCode
should display separately, and I want to get the count of the machines of every type.
This is done this way.
SELECT routingcode, name, count(1)
FROM machines
JOIN log ON log.machinebarcode = machines.barcode
WHERE log.linebarcode = 100000000001
GROUP BY routingcode, name
Okay everything runs smoothly, but this way I get only machines which are related in log
table and have record according to linebarcode
.
I thinked that if I LEFT JOIN
the log table I will get all the machines from the machines
table and display them and of course only machines which are found in log
table will have proper count
, but no.
Where am I mistaking and how to find a proper workaround?
You need to put the condition on log
into the on
clause not the where
. Non matching rows preserved by the left outer join will be null extended for all columns in log
.
Rows with NULL
for log.linebarcode
will be removed again if the condition is in the where
.
Also instead of COUNT(1)
you need to count a column from log
that won't be NULL
SELECT routingcode,
name,
count(log.linebarcode)
FROM machines
LEFT JOIN log
ON log.machinebarcode = machines.barcode
AND log.linebarcode = 100000000001
GROUP BY routingcode,
name
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