Question

请帮助我理解我的问题和最好的例子。

我无法理解如何创建对象并使用它们。

谢谢

Answer 1

她的一些非常简单的东西：

准备DB：

CREATE TABLE `table1` (
  `id` int(11) NOT NULL DEFAULT 0,
  `value` text not null DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB;

CREATE TABLE `table2` (
  `id` int(11) NOT NULL DEFAULT 0,
  `value` text not null DEFAULT '',
  PRIMARY KEY (`id`)
) ENGINE=InnoDB;

INSERT INTO table1 VALUES
(1, 'one'), (2, 'two'), (3, 'three'), (4, 'four'), (5, 'five');

INSERT INTO table2 VALUES
(3, 'three'), (4, 'four'), (5, 'five'), (6, 'six'), (7, 'seven');

执行查询：

SELECT t1.*, t2.* FROM table1 t1 JOIN table2 t2 on t1.id = t2.id;
SELECT t1.*, t2.* FROM table1 t1 INNER JOIN table2 t2 on t1.id = t2.id;
+----+-------+----+-------+
| id | value | id | value |
+----+-------+----+-------+
|  3 | three |  3 | three |
|  4 | four  |  4 | four  |
|  5 | five  |  5 | five  |
+----+-------+----+-------+
SELECT t1.*, t2.* FROM table1 t1 LEFT JOIN table2 t2 on t1.id = t2.id;
SELECT t1.*, t2.* FROM table1 t1 LEFT OUTER JOIN table2 t2 on t1.id = t2.id;
+----+-------+------+-------+
| id | value | id   | value |
+----+-------+------+-------+
|  1 | one   | NULL | NULL  |
|  2 | two   | NULL | NULL  |
|  3 | three |    3 | three |
|  4 | four  |    4 | four  |
|  5 | five  |    5 | five  |
+----+-------+------+-------+

Answer 2

假设你有两张桌子：

ID  Name
1   Smith
2   Novak
3   Tarantino

和第二个表

INNER JOIN将连接两个表中匹配的记录，即仅在结果表中ID将包含2和3，因为它们匹配。结果如下：

ID  Name       Cash
2   Novak      300
3   Tarantino  490

另一方面，

OUTER JOIN将匹配所有内容，无论它们是否匹配。如果记录没有匹配，则对应于某一行的值将为NULL，即Smith NULL列中的Cash和Cash列中的500对应记录将在NULL列中显示Name。结果如下：

ID  Name       Cash
1   Smith      NULL
2   Novak      300
3   Tarantino  490
4   NULL       500

我可以为SQL中的内连接和外连接提供什么基本示例？

2 个答案: