The difference between SQL Server sum and count in relation to statistics

Sum is the amount of content that is added, and count counts the number of table row objects

When you use a case, such as

Select Subject,
Count (case if score>80 then score else null end) Excellent,
Count (case if score<80 and score>59 then score else null end) Excellent,
Count (case if score<60 then score else null end) Excellent
From Stuscore group by [subject]

SELECT * FROM Stuscore

Select Subject,
SUM (case if score>80 then score else 1 end) Excellent,
SUM (case if score<80 and score>59 then score else 1end) excellent,
SUM (case if score<60 then score else 1end) excellent
From Stuscore group by [subject]

SELECT * FROM Stuscore

This time the sum is returned at 1, and the Count returns a column (null), or a row object that counts the number of

The difference between SQL Server sum and count in relation to statistics