Question

I have three tables:

tbl_customers
tbl_users
tbl_reviews

I have an application where, customer submit reviews for users. What I want to be able to do is not have multiple reviews of the same user from the same customer. This is what I want to do.

SELECT
  *
FROM
  tbl_customers
WHERE
  tbl_customer.CustomerID != tbl_review.CustomerID
  AND tbl_user.UserID != tbl_reviews.UserID

• CustomerID is p_id for tbl_customer
• UserID is p_id for tbl_users.
• Foreign key relationship for both of them in tbl_review.

Thanks in advance.

If more information is needed please let me know.

Answer 1

Put a unique constraint on your tbl_reviews table which uses both columns. This will disallow multiple entries altogether which will allow you to do a simple join. You will just have to add validation for when the entry is added so it doesn’t error out when a user tries to enter more than one review for the same person.

alter table tbl_reviews add unique index(reviewid, userid);

select *
from tbl_users as u
join tbl_reviews as r on r.userid = u.userid