SQL foundation tutorial (2nd edition) Chapter 7 set operations: 7-2 tie (in units of columns of table join)

7-2 coupled (in units of columns in a table join)

■ cross-linking - the CROSS the JOIN
■ coupling a specific syntax grammar and outdated

● join ( JOIN ) is to add additional columns in the table over a set operation "add column" of. UNION is a row (vertical) is operated as a unit, and is coupled to the column (lateral) performed in units.

● Please be sure to use standard SQL syntax to write linking operation, or for those outdated wording in particular SQL, you can find out, it is not recommended.

■ What is the link

■ inner coupling - the INNER JOIN

First, we learn that the inner coupling ( the INNER JOIN ) , which is the most widely used coupling operation.
In this case we will continue to use the Product table and 6 chapters created ShopProduct table. Here let us look at the contents of these two tables.

试着从 Product 表中取出商品名称（product_name）和销售单价（sale_price），并与ShopProduct 表中的内容进行结合，所得到的结果如下所示。

由于表名太长会影响 SQL 语句的可读性，因此还是希望大家能够习惯使用别名。

像这样使用联结运算将满足相同规则的表联结起来时， WHERE、GROUP BY、 HAVING、 ORDER BY 等工具都可以正常使用。我们可以将联结之后的结果想象为新创建出来的一张表（表 7-4）
如果希望继续使用这张“表”，还是将它创建成视图吧。

■外联结——OUTER JOIN

相反，对于外联结来说，只要数据存在于某一张表当中，就能够读取
出来。在实际的业务中，例如想要生成固定行数的单据时，就需要使用外
联结。如果使用内联结的话，根据 SELECT 语句执行时商店库存状况的
不同，结果的行数也会发生改变，生成的单据的版式也会受到影响，而使
用外联结能够得到固定行数的结果。

●外联结要点——每张表都是主表吗？

大家可能会犹豫外联结到底应该使用 LEFT 还是 RIGHT，其实它们的功能没有任何区别，使用哪一个都可以。通常使用 LEFT 的情况会多一些。

■ 3张以上的表的联结

通常联结只涉及 2 张表，但有时也会出现必须同时联结 3 张以上的表的情况。

--MySQL
-- DDL：创建表
CREATE TABLE InventoryProduct
( inventory_id          CHAR(4)      NOT NULL,
  product_id          CHAR(4)      NOT NULL,
  inventory_quantity. INTEGER      NOT NULL,
  PRIMARY KEY (inventory_id, product_id));

-- DML：插入数据
START TRANSACTION;

INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S001',    '0001',    0);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity)VALUES ('S001',    '0002',    120);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S001',    '0003',    200);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S001',    '0004',    3);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S001',    '0005',    0);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S001',    '0006',    99);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S001',    '0007',    999);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S001',    '0008',    200);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity)VALUES ('S002',    '0001',    10);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S002',    '0002',    25);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S002',    '0003',    34);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S002',    '0004',    19);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S002',    '0005',    99);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S002',    '0006',    0);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S002',    '0007',    0);
INSERT INTO InventoryProduct (inventory_id, product_id, inventory_quantity) VALUES ('S002',    '0008',    18);

COMMIT;

View Code --MySQL

SQL foundation tutorial (2nd edition) Chapter 7 set operations: 7-2 tie (in units of columns of table join)

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