Getting single records back from joined tables that may produce multiple records
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03-07-2019 - |
Question
I've got a student table and an enrollment table; a student could have multiple enrollment records that can be active or inactive.
I want to get a select that has a single student record and an indicator as to whether that student has active enrollments.
I thought about doing this in an inline UDF that uses the student ID in a join to the enrollment table, but I wonder if there's a better way to do it in a single select statement.
The UDF call might look something like:
Select Student_Name,Student_Email,isEnrolled(Student_ID) from Student
What might the alternative - with one SQL statement - look like?
Solution
select Student_Name,
Student_Email,
(select count(*)
from Enrollment e
where e.student_id = s.student_id
) Number_Of_Enrollments
from Student e
will get the number of enrollments, which should help.
OTHER TIPS
Why not join to a secondary select? Unlike other solutions this isn't firing a subquery for every row returned, but gathers the enrollment data for everyone all at once. The syntax may not be quite correct, but you should get the idea.
SELECT
s.student_name,
s.student_email,
IsNull( e.enrollment_count, 0 )
FROM
Students s
LEFT OUTER JOIN (
SELECT
student_id,
count(*) as enrollment_count
FROM
enrollments
WHERE
active = 1
GROUP BY
student_id
) e
ON s.student_id = e.student_id
The select from enrollments could also be redone as a function which returns a table for you to join on.
CREATE FUNCTION getAllEnrollmentsGroupedByStudent()
RETURNS @enrollments TABLE
(
student_id int,
enrollment_count int
) AS BEGIN
INSERT INTO
@enrollments
(
student_id,
enrollment_count
) SELECT
student_id,
count(*) as enrollment_count
FROM
enrollments
WHERE
active = 1
GROUP BY
student_id
RETURN
END
SELECT
s.student_name,
s.student_email,
e.enrollment_count
FROM
Students s
JOIN
dbo.getAllEnrollmentsGroupedByStudent() e
ON s.student_id = e.student_id
Edit:
Renze de Waal corrected my bad SQL!
Try someting like this:
SELECT Student_Name, Student_Email, CAST((SELECT TOP 1 1 FROM Enrollments e WHERE e.student_id=s.student_id) as bit) as enrolled FROM Student s
I think you can also use the exists statement in the select but not positive
try to avoid using udfs or subqueries, they are performance killers. banjolity seems to havea good solution otherwise because it uses a derivd table instead of a UDF or subselect.
select students.name,
decode(count(1), 0, "no enrollments", "has enrollments")
from students, enrollments
where
students.id = enrollments.sutdent_id and
enrollments.is_active = 1 group by students.name
Of course, replace the decode with a function your database uses (or, a case statement).