My SQL query is returning results but they are repeated ~50 times. I don't understand why

Question

The query I'm using calls on a few tables in the database and works fine. However, when I add line 10 to the mix it returns 50 or more repeated results. I'm still somewhat new to SQL and Sequel Pro so I'm sure the solution isn't too complicated but I am truly stumped right now.

Here is the code:

SELECT c.first_name, c.last_name, ca.company, ca.city, ca.state, ct.certificate_number,   ct.certificate_date
FROM customer c, customer_type ctype, cust_address ca, certification ct, cust_prof_cert cp
WHERE ca.id_customer = c.id_customer LIKE cp.prof_cert_id_prof_cert
AND c.customer_type_id_customer_type = ctype.id_customer_type
AND ct.customer_id_customer = c.id_customer
AND ca.id_customer = c.id_customer
AND ctype.customer_type IN('CIRA','CIRA, CDBV')
AND ct.course_type_id_course_type = 1
AND ct.certificate_number IS NOT NULL
AND cp.prof_cert_id_prof_cert = "1"
ORDER BY ct.certificate_number ASC, c.last_name ASC;

Thank you for your time.

Answer 1

By Doing your SQL like that you are not relating the data, just selecting it. I would recommend changing your SQL to use JOINS.

SELECT Orders.OrderID, Customers.CustomerName, Orders.OrderDate
FROM Orders
INNER JOIN Customers
ON Orders.CustomerID=Customers.CustomerID; 

Here is an article that might be able to help you a bit: w3schools, Joins

Answer 2

Here's your query using the SQL92 syntax for joins. You should use this syntax instead of the SQL89 "comma-style" joins.

SELECT c.first_name, c.last_name, ca.company, ca.city, ca.state, 
  ct.certificate_number, ct.certificate_date
FROM customer AS c
INNER JOIN customer_type AS ctype ON c.customer_type_id_customer_type = ctype.id_customer_type
INNER JOIN cust_address AS ca ON ca.id_customer = c.id_customer
INNER JOIN certification AS ct ON ct.customer_id_customer = c.id_customer
INNER JOIN cust_prof_cert AS cp    -- what's this join condition?
WHERE ca.id_customer = c.id_customer LIKE cp.prof_cert_id_prof_cert
  AND ctype.customer_type IN('CIRA','CIRA, CDBV')
  AND ct.course_type_id_course_type = 1
  AND ct.certificate_number IS NOT NULL
  AND cp.prof_cert_id_prof_cert = '1'
ORDER BY ct.certificate_number ASC, c.last_name ASC;

A few weird things I notice in this query:

• The first term in the WHERE clause is strange. You should know that LIKE has higher precedence than = so this might not be doing what you think it's doing. It's as if you wrote

   WHERE ca.id_customer = (c.id_customer LIKE cp.prof_cert_id_prof_cert) 

  Which means evaluate the LIKE and produce a 0 or a 1 to represent the boolean condition. Then look for a ca.id_customer matching that 0 or 1.

• Given that strange term, I can find no other join condition for the cp table. The default join if you give no restriction for it is that every row matches every row in the joined tables. So if you have 50 rows where cp.prof_cert_id_prof_cert = 1 , then it will effectively multiply the results from the rest of the joined tables by 50.

  This is called a Cartesian product , or in MySQL parlance it's counted in SHOW STATUS as a Full join .

• ctype.customer_type IN('CIRA','CIRA, CDBV') You have quoted the second and third strings together. Basically, this means you are trying to match the column against two strings, one of which happens to contain a comma.

  You probably meant to write ctype.customer_type IN('CIRA','CIRA','CDBV') so the column may match any of these three values.

Answer 3

I would suggest not querying multiple tables in your FROM clause, I believe this is the cause of your duplicate rows. If you separate out the tables into separate inner or left joins, (whichever you need) you should be able to match which ever keys in each table manually, instead of having SQL attempt to automatically do this.