I learned MySQL last year without putting it into action; that is why I face trouble in formulating my queries. Were it a test, then you would have passed it, because your queries did help me solve my problem. I'll turn to MySQL doc after getting through this pressing project.
Thanks a lot Roger! Babale -----Ursprüngliche Nachricht----- Von: Roger Baklund [mailto:[EMAIL PROTECTED] Gesendet: Samstag, 16. August 2003 14:32 An: [EMAIL PROTECTED] Cc: B. Fongo Betreff: Re: How many join do I need for a query on 3 table? * B. Fongo > I was able to extra the information from the first 2 tables yesterday > using following query: > > SELECT Customers.Name, Customers.City, Orders.Product, > Order.Price FROM Customers inner join Orders USING (cust_id) WHERE > customers.cust_id = "2" > > ------------------------------------------------------------------------ > -------------------------------------------------------- > SELECT Customers.Name, Customers.City, Orders.Product, > Order.Price FROM Customers inner join Orders ON > Customers.cust_id=Order.cust_id WHERE customers.cust_id = "2" That was two queries, and they both have a typo preventing them from working... are you testing us? ;) > Right now I need credit card details from a third table; and that make > the query more complicated for me. > I' m not sure weather 2 inner joins could be used. I' ll appreciate any > help. I can try. :) > I have 2 tables: Customers and orders. What about the third table you just mentioned...? > The have following structures: > Customers Orders Payment > cust_id Product Order_id > Name Price Credit_Card > City cust_id > Order_id This was not very readable on my screen... When you want to show the structure of a table, use "DESC Customers;" or even better: "SHOW CREATE TABLE Customers;". The last one will also include any index definitions, which is often relevant when you ask questions about query performance on this list. Just a friendly advice. :) I think the above means something like this: Customers: cust_id INT PRIMARY KEY, Name VARCHAR, City VARCHAR Orders: Order_id INT PRIMARY KEY, cust_id INT Product VARCHAR, Price INT, Payment: Order_id INT, Credit_Card VARCHAR (You can only have one product per order, and only full payments are allowed, and you should record the date of order and date of payment, but that's not an issue here, I suppose.) Ok, prepare for a "5 minute MySQL joining crash course". :) We will build on your original query, but I will reformat it a little, just to make it easier to read. The original, two table query with INNER JOIN and ON, reformatted: SELECT Customers.Name, Customers.City, Orders.Product, Orders.Price FROM Customers INNER JOIN Orders ON Customers.cust_id=Orders.cust_id WHERE Customers.cust_id = "2" Then we can expand it with another table: SELECT Customers.Name, Customers.City, Orders.Product, Orders.Price, Payment.Credit_Card // new line FROM Customers INNER JOIN Orders ON Customers.cust_id=Orders.cust_id INNER JOIN Payment ON // new line Payment.Order_id=Orders.Order_id // new line WHERE Customers.cust_id = "2" Now we can simplyfy this by removing some table names we don't need (because the column name is unique), and change the ON to USING: SELECT Name, City, Product, Price, Credit_Card FROM Customers INNER JOIN Orders USING(cust_id) INNER JOIN Payment USING(Order_id) WHERE Customers.cust_id = "2" Note that the order of the tables are important when using USING(): the previous table is joined with this table USING the named column(s). For instance, you could not have joined Payment before Orders in the above statement, because " ... Orders USING(cust_id)" then would have referred to the Payment table, which does not have any cust_id column. We could have used the shortcut alias "," instead of INNER JOIN, but then we can not use ON or USING, and must move the join conditions to the WHERE clause: SELECT Name, City, Product, Price, Credit_Card FROM Customers, Orders ,Payment WHERE Customers.cust_id=Orders.cust_id AND Payment.Order_id=Orders.Order_id Customers.cust_id = "2" We could also have used NATURAL JOIN in this case, which is the same as USING naming all columns with the same name in the two joining tables: SELECT Name, City, Product, Price, Credit_Card FROM Customers NATURAL JOIN Orders NATURAL JOIN Payment WHERE Customers.cust_id = "2" The note about the order of the tables when using USING() also goes for NATURAL JOIN. Regarding the question in the subject: when joining 'n' tables, you need 'n-1' JOINS. The JOIN is always placed between two table names, and the first table name is always preceeded with "FROM". Remember that "," when used between table names in a SELECT is an alias for "INNER JOIN". If you wanted to also include customers which have not payed, you would use LEFT JOIN: SELECT Customers.Name, Customers.City, Orders.Product, Orders.Price, IF(Credit_Card,Credit_Card,'*no pay*') // changed line FROM Customers INNER JOIN Orders ON Customers.cust_id=Orders.cust_id LEFT JOIN Payment ON // changed line Payment.Order_id=Orders.Order_id WHERE Customers.City = "Hamburg" // changed line ...or simply: SELECT Name, City, Product, Price, IF(Credit_Card,Credit_Card,'*no pay*') // changed line FROM Customers NATURAL JOIN Orders NATURAL LEFT JOIN Payment // changed line WHERE Customers.City = "Hamburg" // changed line Read more about joining in the manual: <URL: http://www.mysql.com/doc/en/JOIN.html > HTH, -- Roger -- MySQL General Mailing List For list archives: http://lists.mysql.com/mysql To unsubscribe: http://lists.mysql.com/[EMAIL PROTECTED]
