NATURAL JOIN无法正常运作

Question

I have three tables describes like that:

+----------+    +-----------------+    +----------+ 
| products |    | products_stores |    | stores   | 
+----------+    +-----------------+    +----------+ 
| barecode |    | #barecode       |    | storeID  | 
| name     |----| #storeID        |----| location | 
+----------+    | price           |    +----------+ 
                +-----------------+    

Created them like this:

CREATE TABLE IF NOT EXISTS `products` (
  `barecode` varchar(100) NOT NULL UNIQUE,
  `name` varchar(25) NOT NULL,
  PRIMARY KEY (`barecode`)
);
CREATE TABLE IF NOT EXISTS `stores` (
  `idStore` int(10) NOT NULL AUTO_INCREMENT,
  `name` varchar(25) NOT NULL,
  `location` varchar(100) NOT NULL,
  PRIMARY KEY (`idStore`)
);
CREATE TABLE IF NOT EXISTS `products_stores` (
  `idStore` int(10) NOT NULL,
  `barecode` VARCHAR(100) NOT NULL,
  `price` double NOT NULL,
  FOREIGN KEY (`barecode`) REFERENCES `products`(`barecode`),
  FOREIGN KEY (`idStore`) REFERENCES `stores`(`idStore`)
);

I wanted to get all the product and its price with the right store so I tried NATURAL JOIN but it returned nothing (code below 1-). So I tried with only one NATURAL JOIN just to check and it works (code below 2- and 3-).

1- SELECT * FROM products NATURAL JOIN products_stores NATURAL JOIN stores;
2- SELECT * FROM products JOIN products_stores NATURAL JOIN stores;
3- SELECT * FROM products NATURAL JOIN products_stores JOIN stores;

I do not understand why the "double" NATURAL JOIN does not work. Does anyone can help me through this? Thanks.

Answer 1

It depends on your data,here is what the documentation has to say about it

The evaluation of multi-way natural joins differs in a very important way that affects the result of NATURAL or USING joins and that can require query rewriting. Suppose that you have three tables t1(a,b), t2(c,b), and t3(a,c) that each have one row: t1(1,2), t2(10,2), and t3(7,10). Suppose also that you have this NATURAL JOIN on the three tables:

SELECT ... FROM t1 NATURAL JOIN t2 NATURAL JOIN t3; Previously, the left operand of the second join was considered to be t2, whereas it should be the nested join (t1 NATURAL JOIN t2). As a result, the columns of t3 are checked for common columns only in t2, and, if t3 has common columns with t1, these columns are not used as equi-join columns. Thus, previously, the preceding query was transformed to the following equi-join:

SELECT ... FROM t1, t2, t3 WHERE t1.b = t2.b AND t2.c = t3.c; That join is missing one more equi-join predicate (t1.a = t3.a). As a result, it produces one row, not the empty result that it should. The correct equivalent query is this:

SELECT ... FROM t1, t2, t3 WHERE t1.b = t2.b AND t2.c = t3.c AND t1.a = t3.a;

In short the second JOIN only checks for the previous table`s column not the WHOLE result of the first JOIN.

One of the reasons that NATURAL JOINs are not recommended,not much control and little to gain.