如何选择一个表中存在ID的行，而另一表中没有ID？

Question

I have 2 tables:

1) An Order table (which has OrderNumber as the Id)

2) An Items table which can have multiple rows and also has an OrderNumber column tying each row to the ord er row above.

Most orders have multiple rows in the Items table - but some do not and I need to pull a report of Orders with no associated Items rows.

I could do this on 2 queries in my PHP but there is clearly a smarter way to do it in MySQL. I understand JOINS but usually that's when there IS data in both tables. How do I tackle this if there ISN'T a tie in both?

Answer 1

I would use a LEFT OUTER JOIN or FULL OUTER JOIN, depending on what you need.. It would help if you post the 2 queries, or some sample data...

OUTER handles any data that isn't there

here is a link to help you out: http://www.codinghorror.com/blog/2007/10/a-visual-explanation-of-sql-joins.html

Answer 2

假设您的Items表主键是Items_Id

SELECT * FROM Orders LEFT JOIN Items USING ( `OrderNumber` ) where Items.Items_Id IS NULL

Answer 3

In addition to the ways listed above, I'd say that I'd use this structure:

SELECT * FROM Order where not exists (SELECT NULL FROM Items where OrderNumber = Orders.OrderNumber);

You can also add an index on OrderNumber column in Items for better performance:

CREATE INDEX Idx_Items on Items (OrderNumber);

Answer 4

You could use a the NOT IN phrase for the where clause. This would get Order records who's Id's aren't in the Items table.

SELECT * FROM Order WHERE Id NOT IN (SELECT OrderNumber FROM Items)