Joining two tables using subquery in mysql - mysql

Question

Im trying to write a SELECT statement with a subquery that determines the customer_id and the last_name of the customer
whose credit card expires soonest as shown in my orders table
and for this i want to use a WHERE clause but im stuck since the result only displays one customer...
here is what i have so far. if someone could guide me in the right path..
SELECT customer_id, last_name
FROM customers
WHERE NOT EXISTS
(SELECT card_expires
FROM orders
WHERE customers.customer_id = orders.customer_id
);

Answer 1

While I am not clear on your exact filtering or ordering requirements, I think you may want to start with the following.
SELECT customer_id, last_name, card_expires
FROM customers
left join (
SELECT card_expires
FROM orders
) o on (customers.customer_id = orders.customer_id)
Using this method you can see a full list of all customers and card_expires, and THEN you can start to filter the list down to what you want. You might build up the subquery on orders to select a specific subset of orders. Again I am not clear on what you are trying to do exactly. But by selecting all of the information first you will be able to see all of the information and be able to decide what to filter out.
You may decide you would ALSO like to see more columns from the order table to help you decide which orders to include / exclude.
If you wanted to only see a list of customers that do not have a card_expires entry
SELECT customer_id, last_name, card_expires
FROM customers
left join (
SELECT card_expires
FROM orders
) o on (customers.customer_id = orders.customer_id)
where o.card_expires is null
If you want to see only customers WITHOUT a card that expires in the future.
SELECT customer_id, last_name, card_expires
FROM customers
left join (
SELECT card_expires
FROM orders
where card_expires > now()
) o on (customers.customer_id = orders.customer_id)
where o.card_expires is null
When crafting an SQL statement, something that can make it easier to do is FIRST create an SQL statement which returns ALL of the columns you will use to filter the list by, THEN apply the filter. This will allow you to more easily add and remove criteria until you have it right.
I like to split each criteria on to its own separate line so that I can easily comment lines out with two dashes. EX:
SELECT customer_id, last_name, card_expires
FROM customers
left join (
SELECT card_expires
FROM orders
where card_expires > now()
) o on (customers.customer_id = orders.customer_id)
where 0=0
and o.card_expires is null
-- and last_name = 'Smith'

Answer 2

Use LEFT JOIN/INNER JOIN, i assume you want ORDER BY ASC, since you want the soonest
SELECT customer_id, last_name
FROM customers c
LEFT JOIN orders o ON o.customer_id = c.customer_id
ORDER BY card_expires ASC;