Question

I have three tables

    Employee    
    SSN Name
    1   a
    2   b
    3   c

    Projects    
    Pno Name
    1   x
    2   y
    3   z

    Works_On    
    ESSN    Pno
    1       1
    1       2
    2       1
    3       1
    3       2
    3       3

Okay so the question is, how do I know (by query) which employee works on ALL projects. I cant seem to compare multi with multi.

Thank you and sorry for a silly question.

Answer 1

One way to do it is to count all projects, count number of projects for employee and then select all employees that work on the same number of projects:

DECLARE @ProjectCount int
SELECT @ProjectCount = Count(*) FROM Projects

SELECT e.* 
FROM Employee e
WHERE (SELECT COUNT(1) FROM Works_On w WHERE w.ESSN = e.SSN) = @ProjectCount

ProjectCount variable is there to skip counting projects for every employee.

UPDATE: How it works:

For each employee count how many records are in Works_On table with field ESSN equal to current employee’s SSN and then compare that number to total number of projects.
Query:

SELECT COUNT(1) FROM Works_On

will give you total number of records in Works_On table (6 in your case). We want to count how many records are there for each employee and that’s why we embedded that query in our main query with condition to select only records from Works_On table that have current employee’s SSN. If you had to write this in some programming language, this would be logically equivalent to foreach loop on employees collection where in body of the loop you select all records from Works_On table for current employee and then compare it with total number of projects. If current employee has number of projects equal to the total number of projects, count it.

I hope this helps.