Mysql查询涉及多个表和一个OR语句

Question

Sorry in advance for an ambiguous title, but I can't think of a good one.

I have 2 tables:

bookings(booking_id, customer_id)
charges(customer_id, booking_id, amount)

where in charges table, either booking_id or customer_id must be entered. Not both.

I'm trying to get amount that are associated with a customer_id

Because the customer_id's are null sometimes in charges table, I have to use booking_id to acquire customer_id through bookings table.

So, I had a query like this:

SELECT c.amount
FROM charges as c, bookings as b
WHERE (c.customer_id = 1234) OR 
(c.customer_id = null AND c.booking_id = b.booking_id AND b.customer_id = 1234)

However, it seems to create an infinite loop.

Any suggestions?

Thanks,

Answer 1

The problem is that you are doing a cross join (cartesian product), based on the structure of your where clause.

A better approach is to use left outer join (and proper join syntax). So, join to the bookings table, if it exists. Then use or in the where clause to look for a match on the customer_id in either place:

SELECT c.amount
FROM charges as c left outer join
     bookings as b
     on c.booking_id = b.booking_id 
WHERE (c.customer_id = 1234) OR (b.customer_id = 1234)

Answer 2

Why you don't use a JOIN ? like

SELECT c.amout 
FROM charges AS c
LEFT JOIN bookings AS b ON c.bookin_id = b.booking_id
WHERE c.customer_id = 1234 OR b.customer_id = 1234

Answer 3

SELECT amount, customer_id from charges where customer_id = 1234
UNION
select amount, b.customer_id from charges c, bookings b where b.booking_id = c.booking_id and c.customer_id is null and b.customer_id = 1234