Question

Now I have to create a query for search page depends on database that have 30 tables:

I will describe my requirement with an example:

there is a relation between table A and table B -> one to many relation

If we consider that table A is for Students info and table B to save the courses for that student each year . it is only allowed to take 3 subjects each year so the structure will be as the following:

Table A: ID ,Student-Name ,Tel#,Birthdate ……

Table B: :ID,StudentID,Date,Subject-one,Subject-two,Subject-three

so the student can take any 3 subjects per year
I want to make a query for subject called X to get all students who take it at any year but only once

example for valid student:

  StudentID  Sub1  Sub2  Sub3  Year  
  60         Z     (X)    Y      1
  60         L      W     V      2
  60         M      P     Y      3

example for invalid student:

  StudentID  Sub1  Sub2  Sub3  Year  
  10         Z     (X)    O      1
  10         L      W     V      2
  10         O      P    (x)     3

i hope that to be clear enough for my problem

Answer 1

I agree with comments that you would be better with one row per subject in table 2 (a normalised structure is virtually always better and more SQL friendly).

That said, you can solve it in your structure.

SELECT
  studentID
FROM
  table2
WHERE
     sub1 = 'x'
  OR sub2 = 'x'
  OR sub3 = 'x'
GROUP BY
  studentID
HAVING
  COUNT(*) = 1

(This assumes the student can’t take the same subject twice in the same year.)

Some varieties of SQL may allow the WHERE clause to be shortened…

WHERE
  'x' IN (sub1, sub2, sub3)

You can then pick up all the student details by putting this in a sub-query…

SELECT
  *
FROM
  table1
INNER JOIN
(
  <put the above query here>
)
  AS lookup
    ON lookup.studentID = table1.studentID

EDIT

I don’t understand your comment. But an alternative (and potentially more generalisable) HAVING clause could be..

HAVING
  SUM(CASE WHEN sub1 = 'x' THEN 1 ELSE 0 END +
      CASE WHEN sub2 = 'x' THEN 1 ELSE 0 END +
      CASE WHEN sub3 = 'x' THEN 1 ELSE 0 END)
  = 1