Question

Class

• ClassId (PK)
• ClassName
• ClassCapacity

Batch

• BatchId (PK)
• BatchName
• Size
• Status {NotStarted, Enrolled, Completed}

BatchClass

• BatchClassId (PK)
• BatchId (FK)
• ClassId (FK)
• NoOfStudents

Relationship

• A class has maximum capacity, therefore a batch can have many classes, but at a given time a class can be allocated only for one batch that status is = Enrolled (This is to be validated from the application end)

I want to get all the classes that are not currently allocated to a batch that status is not equal to Enrolled

This is what I tried,

SELECT C.* 
FROM Class C 
LEFT JOIN (
    Batch B 
      INNER JOIN BatchClass BC 
        ON B.BatchId = BC.BatchId
    ) ON C.ClassId = BC.ClassId 
WHERE B.Status <> "Enrolled";

Even though when I try WHERE B.Status = “Enrolled” it gives all the classes that have a enrolled batch. What I want is the Opposite which didn’t work for me from above SQL.

I’m not sure is it something wrong in my design or SQL statement. Please help. Thank you in advance.

Answer 1

This should work fine:

SELECT DISTINCT C.* 
FROM Class AS C 
INNER JOIN BatchClass AS BC ON C.ClassID = BC.ClassID
INNER JOIN Batch      AS B  ON B.BatchId = BC.BatchId 
WHERE B.Status <> 'Enrolled';

SQL Fiddle Demo

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Update 1

Try this instead:

SELECT C.*
FROM Class AS C 
WHERE c.ClassID NOT IN(SELECT bc.ClassID
                       FROM BatchClass bc
                       INNER JOIN Batch b ON B.BatchId = BC.BatchId
                       WHERE b.status = 'Enrolled');

Updated SQL Fiddle Demo