Forcing row for each group even when no relevant data at table

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I need to count how many rows of each type are exists in a table. But, even if no relevant rows I still need to show each type at the results set and it's counter. Counter will be 0 if no relevant rows.

This is my current SQL:

SELECT student_type, COUNT(*) as 'count'
FROM students 
WHERE student_type IN (10, 12)
AND registration_time BETWEEN '2018-1-1' AND '2018-12-31'
GROUP BY student_type;

The current SQL return result only if there are row(s) with student_type 10 or 12, at the specified date, but if there is no rows, nothing will return.

I need the result will always be in that form:

student_type    count
 10               0
 12               0

Consider conditional aggregation where you move your WHERE condition to a SELECT expression to avoid filtering out the zero conditional records. Below sums the True conditions by your grouping aggregation which equates to counts where zeroes will return.

SELECT student_type, 
       SUM(student_type IN (10, 12) AND 
           registration_time BETWEEN '2018-01-01' AND '2018-12-31') as 'count'
FROM students
GROUP BY student_type;

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That works for me using left-join

SELECT a.student_type, count(b.student_type) 
FROM students_types a
LEFT JOIN students b ON a.student_type = b.student_type
AND b.registration_time BETWEEN '2018-1-1' AND '2018-12-31'
WHERE a.student_type in(10, 12)
GROUP BY 1;

Or with 'on-the-fly' left join:

SELECT t.student_type, count(b.student_type) 
FROM (SELECT 10 student_type UNION SELECT 12) t
LEFT JOIN students b ON t.student_type = b.student_type
AND b.registration_time BETWEEN '2018-1-1' AND '2018-12-31'
GROUP BY 1; 

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--create 2 datasets table a(all the stydent types you wish to display) --and table b(the count for each type) --and select everything from table a, when you then left outer join to b that means if there is no corresponding record in b then you get NULL(which you can convert to 0)

SELECT a.student_type,ISNULL(b.cnt,0) as 'count' from

( SELECT distinct student_type FROM students WHERE student_type IN (10, 12) ) a

left outer join

( SELECT student_type, COUNT(*) as 'cnt' FROM students WHERE student_type IN (10, 12) AND registration_time BETWEEN '2018-1-1' AND '2018-12-31' GROUP BY student_type; ) b on a.student_type=b.student_type;

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Comments
  • Consider handling issues of data display in application code. Failing that, see: Why should I provide an MCVE for what seems to me to be a very simple SQL query?
  • the SQL COUNT function doesn't work in the way you're hoping. Unless you have all possible student types enumerated in a separate "student_types" table? In which case using a left join from that table to students might help you.
  • @Strawberry - I can solve it with application code but a better solution for my case will be with SQL.
  • @ADyson - May there is an option to create such table 'on-the-fly' at the query?
  • @Ron ideally you should have one already so that you have a foreign key to that tale from the students table. That should mean a) you can store a text description for each student type (and don't have to repeat it elsewhere) and b) you can have integrity in your students table, because the foreign key constraint will ensure that only valid student_type IDs (i.e. ones which occur in the student_type table) can be inserted. A properly normalised database would have such a table in this scenario already.
  • Not worked for me. Still get blank results when no relevant data (i need a zero counter in that case)
  • Really? Absolutely no results show? See rextester demo: rextester.com/JTVY80711 (scroll to very bottom of 500 row insert). As example shows, ALL student_type will show regardless and counts differ by group with zeros. In retrospect, we can remove the IN() clause since it is part of GROUP BY column.
  • You right. Indeed it works, guess i didn't check well.. ;/ Moreover, as I see, that query is running much slower on big tables in comparing to the queries with the join or the 'on-the-fly'-join.
  • Inspiration from here mattmazur.com/2017/11/01/…
  • Ah!! I see, there is a separate student_type table. You did not make that clear in opening post.
  • Yes, but you can see the code without the left-join. Indeed at first time I preferred without the left join.
  • Please format your code and provide an explanation. Code-only answers are not recommended on SO.