Not unique table/alias? - sql

Question

I get this error: Not unique table/alias: 'postcode'
"SELECT sub.city AS city, sub.postalcode AS postalcode FROM
(SELECT postcode.city AS city, customers.postalcode AS postalcode, COUNT(customers.postalcode) AS postcode_numbers
FROM orders, postcode
INNER JOIN customers ON orders.userID = customers.ID
INNER JOIN postcode ON customers.postalcode = postcode.city
GROUP BY customers.postalcode) sub
WHERE sub.postcode_numbers > 3";

Answer 1

Remove postcode table in the from statement
SELECT sub.city AS city, sub.postalcode AS postalcode FROM
(SELECT postcode.city AS city, customers.postalcode AS postalcode, COUNT(customers.postalcode) AS postcode_numbers
FROM orders
INNER JOIN customers ON orders.userID = customers.ID
INNER JOIN postcode ON customers.postalcode = postcode.city
GROUP BY customers.postalcode) sub
WHERE sub.postcode_numbers > 3

Answer 2

You have postcode in both your from clause and your second join clause. Remove it from the from clause and you should be OK:
SELECT sub.city AS city, sub.postalcode AS postalcode FROM
(SELECT postcode.city AS city, customers.postalcode AS postalcode, COUNT(customers.postalcode) AS postcode_numbers
FROM orders -- Removed postcode from here
INNER JOIN customers ON orders.userID = customers.ID
INNER JOIN postcode ON customers.postalcode = postcode.city
GROUP BY customers.postalcode) sub
WHERE sub.postcode_numbers > 3

Answer 3

You can write this as:
SELECT p.city, c.postalcode , COUNT(*) AS num_orders
FROM orders o JOIN
customers c
ON o.userID = c.ID JOIN
postcode p
ON c.postalcode = p.city
GROUP BY p.city, c.postalcode
HAVING COUNT(*) > 3;
It is unclear whether you want one row per city or per city/postalcode. This assumes the latter. If you want one row per city, remove postalcode from both the SELECT and the GROUP BY.
Notes:
Never use commas in the FROM clause. Always use proper, explicit, standard JOIN syntax. That is your specific problem.
Table aliases make the query easier to write and to read.
The unaggregated columns in the SELECT should match the GROUP BY keys.
A subquery is not necessary.

Answer 4

You are using postcode table in both FROM clause and and INNER JOIN clause, neither of which have an alias, when you do an INNER JOIN you don't need to have that table in the FROM clause, and other way around of course.
If you want to join the tables in FROM clause you would do something like this:
SELECT
sub.city AS city, sub.postalcode AS postalcode
FROM
(SELECT
postcode.city AS city, customers.postalcode AS postalcode,
COUNT(customers.postalcode) AS postcode_numbers
FROM
orders, postcode
INNER JOIN
customers ON orders.userID = customers.ID
--INNER JOIN postcode ON customers.postalcode = postcode.city
WHERE
postcode.city = customers.postalcode
GROUP BY
customers.postalcode) sub
WHERE
sub.postcode_numbers > 3;
So basically you would lose the INNER JOIN postcode... line
The other way to do this is to use only the INNER JOIN clause, which is my preferred way. To do this you only need to lose the postcode in FROM clause, like so:
SELECT
sub.city AS city, sub.postalcode AS postalcode
FROM
(SELECT
postcode.city AS city, customers.postalcode AS postalcode,
COUNT(customers.postalcode) AS postcode_numbers
FROM
orders
INNER JOIN
customers ON orders.userID = customers.ID
INNER JOIN
postcode ON customers.postalcode = postcode.city
GROUP BY
customers.postalcode) sub
WHERE
sub.postcode_numbers > 3;
Hope this helps!