How to use a where clause with a join - sql

Question

Table Student
Id
Name
1
Name
2
Name
Table Classes
Id
Name
Number
1
class1
2
2
class1
3
and a join table
StudentId
ClassId
Income
1
1
5
2
2
6
and the query
select sum(scr.Income)
from Student s
left join StudentClassRelation scr on scr.classId = s.Id
left join Class c on c.Id = scr.classId and c.Number > 4
I want to move c.Number > 4 on the second line in order to receive only the income from classes with number greater than 4. I cannot change the query significantly because it is a part of a bigger one. I need to filter StudentClassRelation somehow
CREATE TABLE Student
(
Id INT PRIMARY KEY,
Name VARCHAR(100),
);
CREATE TABLE Class
(
Id INT PRIMARY KEY,
Name VARCHAR(100),
Number int
);
CREATE TABLE StudentClassRelation
(
Income int,
StudentID INT NOT NULL,
ClassID INT NOT NULL,
FOREIGN KEY (StudentID) REFERENCES Student(Id),
FOREIGN KEY (ClassID) REFERENCES Class(Id),
UNIQUE (StudentID, ClassID)
);
INSERT INTO Class (Id, Name, Number)
VALUES (1, '1', 1), (2, '5', 5)
INSERT INTO Student (Id, Name)
VALUES (1, '1'), (2, '5')
INSERT INTO StudentClassRelation (StudentID, ClassID, Income)
VALUES (1, 1, 10), (2, 1, 20), (2, 2, 5)

Answer 1

To keep the join to StudentClassRelation as a LEFT JOIN and apply a filter based on a column in the class table, you could use LEFT JOIN to a subquery that uses an INNER JOIN on Class, e.g.
SELECT SUM(scr.Income)
FROM Student AS s
LEFT JOIN
( SELECT scr.StudentId, scr.Income
FROM StudentClassRelation AS scr
INNER JOIN Class AS c
ON c.Id = scr.ClassId
WHERE c.Number > 4
) AS scr
ON scr.StudentId = s.Id;
You can however rewrite this in a less verbose way as follows:
SELECT SUM(scr.Income)
FROM Student AS s
LEFT JOIN (StudentClassRelation AS scr
INNER JOIN Class AS c
ON c.Id = scr.ClassId
AND c.Number > 4)
ON scr.StudentId = s.Id;
The execution plans of the two are exactly the same, which is "better" would be entirely personal preference as to which you find more readable. You spend more time reading code than writing it, so the least verbose method does not equate to the best method.
Also worth noting, that if this was the entire query, there is no difference at all from simply using INNER JOIN throughout
SELECT SUM(scr.Income)
FROM Student AS s
INNER JOIN StudentClassRelation AS scr
ON scr.classId = s.Id
INNER JOIN Class AS c
ON c.Id = scr.classId
AND c.Number > 4;
But since you have mentioned that this is part of a larger query, I will assume that there is more to it than just the posted sample, and there is in fact a need for the LEFT JOIN to StudentClassRelation, e.g. If you were to do something like:
SELECT s.Id, Income = SUM(scr.Income)
FROM Student AS s
LEFT JOIN (StudentClassRelation AS scr
INNER JOIN Class AS c
ON c.Id = scr.ClassId
AND c.Number > 4)
ON scr.StudentId = s.Id
GROUP BY s.Id;
This would yield different results to the version with an INNER JOIN to both tables
SELECT s.Id, Income = SUM(scr.Income)
FROM Student AS s
INNER JOIN StudentClassRelation AS scr
ON scr.classId = s.Id
INNER JOIN Class AS c
ON c.Id = scr.classId
AND c.Number > 4
GROUP BY s.Id;

Answer 2

Here a workaround (2options) if u dont want to change the rest of your query
select sum(scr.Income) from Student s
left join StudentClassRelation scr on scr.classId = s.Id
and EXISTS(select top 1 1 from Class where Class.Id = scr.classId and Class.Number > 4)
--> 5
select sum(scr.Income) from Student s
left join StudentClassRelation scr on scr.classId = s.Id
and (select top 1 Class.Number from Class where Class.Id = scr.classId) > 4
--> 5
Is it working as expected?
EDIT: my bad, the solution provided by #GarethD is way better (using inner join instead of left join)