取决于两列的SQL TOP 10查询

Question

I am new to SQL (beginner programmer), so I apologize if this might seem like a simple question. I am trying to create a table on my website that displays the lowest 10 grades along with some information about the student. I am pretty comfortable from this aspect, but I am having trouble coding the write SQL query. I am using SQL Server 2008.

I have a table in my database with 10 columns and 500 rows. Two of these columns contain grades (grade1 and grade2). My goal is to display in my website table the TOP 10 lowest GRADE1, but if GRADE1 is NULL I want it to take GRADE2 into consideration and display that instead. So in context, if a student named Billy has no GRADE1 (its NULL) but his GRADE2 is the lowest of all (GRADE1's AND GRADE2's combined), he should be first in the list.

I would really appreciate help making a query capable of accomplishing this task, I have been researching for a solution but it has only confused me more.

Answer 1

In SQL Server you want to use isNull()

SELECT TOP 10 isNull(grade1,grade2) AS `Grade`
FROM mytable
ORDER BY Grade DESC

Answer 2

You can use case in the order by clause

select top 10 * 
from students 
order by case when grade1 is null then grade2 else grade1 end desc

EDIT

Following BillyCode comment on including only those students that apears 3 or more times in the table I suggest this

select top 10 s.*  
from students s
inner join (select StudentId, Count(*) as total from students) c on s.StudentId = c.StudentId  
where c.total >= 3
order by case when grade1 is null then grade2 else grade1 end desc

But I'm not sure if you can join to a subquery.

Answer 3

The COALESCE function does what you want. COALESCE(GRADE1, GRADE2) will display GRADE1 if not null, or GRADE2 if GRADE1 is null.

So instead of ORDER BY GRADE1 , you can do ORDER BY COALESCE(GRADE1, GRADE2)

See here for more details about COALESCE .

Answer 4

Try this:

select Top 10 
student , (case when grade1 is null then grade2 else grade1 end ) as g1 , grade2 as g2 from table order by g1 desc

Answer 5

Another method is to first find the 10 students with the lowest grade1. Rename grade1 into grade. And then find the 10 students with the lowest grade2. Rename grade2 into grade. After that union these two results. From there, find the 10 students with with lowest grade. The SQL is something like:

SELECT id, grade
FROM   (SELECT   id, grade1 AS grade
        FROM     students
        ORDER BY grade1 DESC
        LIMIT 10
        UNION
        SELECT   id, grade2 AS grade
        FROM     students
        ORDER BY grade2 DESC
        LIMIT 10)
ORDER BY grade DESC
LIMIT 10