mysql select from where different - php

Question

I need to select a list of entries, but need to skip those that have matching fields in 2 different tables.
Here's my DB structure
orders:
| orders_id | customers_id |
| 100 | 01 |
| 101 | 20 |
| 102 | 32 |
| 103 | 48 |
| 104 | 99 |
customers (for reference only):
| firstname | lastname | customers_id |
| John | Doe | 20 |
| Fred | Flinty | 22 |
| Mark | Smith | 32 |
testimonials:
| customers_id | testimonial |
| 20 | aaa |
| 32 | bbb |
| 38 | ccc |
| 49 | ddd |
| 55 | eee |
So, I need to select all customers who are in my Orders table, but need to skip them if they are in my Testimonials table. In the example shown above, I would need to select only customers 01, 48 and 99 because they don't exist in Testimonials table.
This is what I tried, but am obviously missing something:
SELECT c.firstname, c.lastname, c.customers_id, o.orders_id,
o.customers_id, s.date_added as status_date
FROM (orders o, testimonials t )
JOIN customers c
ON c.customers_id = o.customers_id
JOIN status_history s
ON s.orders_id = o.orders_id
and s.orders_status_id = o.orders_status
and o.customers_id != t.customer_id
order by o.orders_id ASC;
Can somebody please tell me what I'm doing wrong and how to skip customers that are found in both tables (orders and testimonials)?
I feel I'm on the right track because, if I change the and o.customers_id != t.customer_id to and o.customers_id = t.customer_id I get only the customers that are in both tables (in this case, 20 and 32).

Answer 1

You can LEFT JOIN on this.
The reason for using LEFT JOIN is because it will show all records from the table defined on lefthand side whether it has a matching record or not on the table define on the righthand side. When table Orders is joined with table testimonials, all the records that have no match will have a value of null for the columns in the testimonials table and that's the one you are looking for. To filter out, we only need to get records with NULL value by checking the column with IS NULL.
SELECT a.*, b.*
FROM orders a
LEFT JOIN testimonials c
ON a.customers_ID = c.customers_ID
LEFT JOIN customers b
ON a.customers_ID = b.customers_ID
WHERE c.customers_ID IS NULL
SQLFiddle Demo
SQLFiddle Demo (added some info on the mismatched customer)
To further gain more knowledge about joins, kindly visit the link below:
Visual Representation of SQL Joins
Adding up INDEX.
If on the real database the Orders table as well as Testimonials are always dependent on Customers table, then a FOREIGN KEY constraint should be enforce to preserve referential integrity.
Here's how:
ALTER TABLE Orders ADD CONSTRAINT tb_fk1
FOREIGN KEY (Customers_ID) REFERENCES Customers(Customers_ID);
ALTER TABLE Testimonials ADD CONSTRAINT tb_fk2
FOREIGN KEY (Customers_ID) REFERENCES Customers(Customers_ID);

Answer 2

This is easy way.
select c.* from order as o
join customers as c on o.customers_id = c.customers_id
where o.customers_id not in(select customers_id from testimonials)