SQL Queries Issues - Essay Example

SELECT DISTINCT Student.stu_fname, Student.stu_no.
FROM Student, Placement.
WHERE Student.stu_no=Placement.stu_no And placement.pl_tdate>now().

In this query simply student table has been joined with placement and then those records whose placement to date is greater than the current date have been filtered.

SELECT DISTINCT Student.stu_fname, Student.stu_no.
FROM Student, Placement.
WHERE Student.stu_no=Placement.stu_no And Placement.pl_tdate-Placement.pl_fdate>20.

In this query, the student table has been joined with placement and then those records whose period of placement is greater than 20 have been filtered.

SELECT DISTINCT Student.stu_fname, Student.stu_no.
FROM Student.
WHERE Student.stu_no is not in (SELECT Placement.stu_no.
FROM Placement).

I have added a subquery in the WHERE clause to filter those students who have no placement.

SELECT DISTINCT Student.stu_fname, Student.stu_no.
FROM Employment, Student, Placement.
WHERE Student.stu_no=Employment.stu_no And Student.stu_no=Placement.stu_no And Placement.co_id=Employment.co_id.

In this query, I have selected the Employment, Student, and Placement tables and have joined them. In addition to that, I have also patched the Company ID of Placement with the Employment table as well.