Odd INNER JOIN syntax and encapsulation

问题

I'm usually pretty well versed with JOINS, but this is new.

Suppose three tables (classic case of two tables and a third, linker table):

Customer  Product  Transaction
--------  -------  -----------
ID        ID       CustomerID
Name      Desc     ProductID
          Cost     Date

(Simplistic on purpose, I can't reproduce the actual structure, it's not my code.)

Normally, to get a table of "who bought what when", I'd do this:

SELECT Customer.Name, Product.Desc, Transaction.Date
FROM Product
INNER JOIN Transaction ON Transaction.ProductID = Product.ID
INNER JOIN Customer ON Transaction.CustomerID = Customer.ID

But I've been presented with this:

SELECT Customer.Name, Product.Desc, Transaction.Date
FROM Product
INNER JOIN ( Transaction
      INNER JOIN Customer ON Transaction.CustomerID = Customer.ID)
ON Transaction.ProductID = Product.ID

What's this? Just another syntax, or a performance trick?

(It's on SQLServer, FYI, but presumably that could be applied to others...)

回答1:

The parentheses do not change the semantics. The position of the ON clause controls the order of the logical processing of joins.

First Query

SELECT Customer.Name,
       Product.Desc,
       Transaction.Date
FROM   Product
       INNER JOIN Transaction
         ON Transaction.ProductID = Product.ID
       INNER JOIN Customer
         ON Transaction.CustomerID = Customer.ID

Second Query

(Redundant parentheses removed)

SELECT Customer.Name,
       Product.Desc,
       Transaction.Date
FROM   Product
       INNER JOIN Transaction
                  INNER JOIN Customer
                    ON Transaction.CustomerID = Customer.ID
         ON Transaction.ProductID = Product.ID

So logically in your first example the join on Transaction, Product happens first then the virtual table resulting from that is joined onto Customer, whereas in your second example the join on Transaction, Customer happens first then the virtual table resulting from that is joined on to Product

This is only logically and as inner joins are both associative and commutative this likely won't make any difference to the execution plan (unless you add OPTION (FORCE ORDER) to the query) but it can do for outer joins.

This is covered by Itzik Ben Gan here but the article has a number of inaccuracies, see the follow up letter by Lubor Kollar as well.

来源：https://stackoverflow.com/questions/8068301/odd-inner-join-syntax-and-encapsulation

标签

sql

sql-server

inner-join