MySQL中的“使用table1，table1作为vtable”是什么意思？答案

【问题标题】：What does "USING table1, table1 as vtable" mean in MySQL?MySQL中的“使用table1，table1作为vtable”是什么意思？
【发布时间】：2013-01-20 23:36:36
【问题描述】：

我是 MySQL 的新手，我很难弄清楚这意味着什么：

DELETE from keywords USING keywords, keywords as vtable 
WHERE (keywords.id > vtable.id) && (keywords.keyword=vtable.keyword)

具体来说，USING keywords, keywords as vtable这部分是什么意思？

有没有更好的方法来编写这个等效的查询？我读过创建虚拟表效率不高。

另外，. 是分隔表格和列的吗？

【问题讨论】：

我会说查询删除了重复的关键字

标签： mysql sql join syntax

【解决方案1】：

vtable 是关键字表的别名，因此可以自联接。与任何别名没有区别，但选择特定别名vtable 可能会让您相信其中涉及一些特殊的虚拟表机制。没有。

从效率的角度来看，这看起来像是自联接的正常使用。如果在 id 和可能的关键字字段上有索引，它应该是相当有效的。

还有点.，确实将表名和列名（或表别名和列名）分开。

下面是同一个查询，但别名不同：

DELETE from keywords USING keywords, keywords as k2 WHERE (keywords.id > k2.id) && (keywords.keyword=k2.keyword)

这是整个查询的完成方式略有不同，但可能不那么令人困惑，使用 JOIN：

DELETE keywords FROM keywords INNER JOIN keywords as k2 ON keywords.keyword = k2.keyword WHERE keywords.id > k2.id

【讨论】：

+1 进行解释。使用显式连接语法（ON 而不是 WHERE）显示相同查询的示例怎么样？
非常感谢您的回答！你能评论一下USING keywords, keywords as vtable 做了什么吗？它是用两个keywords 的副本构造一个vtable 吗？或者USING 是否适用于DELETE FROM keywords 语句？
当与 DELETE 一起使用时，USING 关键字执行以下操作：它命名用于选择的表——但删除只发生在以 FROMbefore命名的表上> 使用。请参阅此处给出的第三种语法：dev.mysql.com/doc/refman/5.0/en/delete.html.
另外，请注意 USING 会产生连接。由于没有指定连接条件，这是一个笛卡尔连接。但是，在您的示例查询中，WHERE 条件实际上就像连接条件一样使用。通常，USING 与指定的列（两个表中的列名相同）一起使用以指定要连接的列。

【解决方案2】：

这是一种非常棒的查询方法，因为每当您遇到某种情况时，您想通过比较同一张表的记录来在表中进行操作。

USING 关键字可以使用出现在两个表中的列的列表，相当于说 C.ID = O.ID AND S.ID = O.ID。

            mysql>
            mysql> CREATE TABLE Employee(
                ->     id            int,
                ->     first_name    VARCHAR(15),
                ->     last_name     VARCHAR(15),
                ->     start_date    DATE,
                ->     end_date      DATE,
                ->     salary        FLOAT(8,2),
                ->     city          VARCHAR(10),
                ->     description   VARCHAR(15)
                -> );
            Query OK, 0 rows affected (0.01 sec)

            mysql>
            mysql> create table job (
                ->   id         int,
                ->   title      VARCHAR(20)
                -> );
            Query OK, 0 rows affected (0.01 sec)

            mysql>
            mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
                ->              values (1,'Jason',    'Martin',  '19960725',  '20060725', 1234.56, 'Toronto',  'Programmer');
            Query OK, 1 row affected (0.02 sec)

            mysql>
            mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
                ->               values(2,'Alison',   'Mathews',  '19760321', '19860221', 6661.78, 'Vancouver','Tester');
            Query OK, 1 row affected (0.00 sec)

            mysql>
            mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
                ->               values(3,'James',    'Smith',    '19781212', '19900315', 6544.78, 'Vancouver','Tester');
            Query OK, 1 row affected (0.00 sec)

            mysql>
            mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
                ->               values(4,'Celia',    'Rice',     '19821024', '19990421', 2344.78, 'Vancouver','Manager');
            Query OK, 1 row affected (0.00 sec)

            mysql>
            mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
                ->               values(5,'Robert',   'Black',    '19840115', '19980808', 2334.78, 'Vancouver','Tester');
            Query OK, 1 row affected (0.00 sec)

            mysql>
            mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
                ->               values(6,'Linda',    'Green',    '19870730', '19960104', 4322.78,'New York',  'Tester');
            Query OK, 1 row affected (0.00 sec)

            mysql>
            mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
                ->               values(7,'David',    'Larry',    '19901231', '19980212', 7897.78,'New York',  'Manager');
            Query OK, 1 row affected (0.02 sec)

            mysql>
            mysql> insert into Employee(id,first_name, last_name, start_date, end_Date,   salary,  City,       Description)
                ->               values(8,'James',    'Cat',     '19960917',  '20020415', 1232.78,'Vancouver', 'Tester');
            Query OK, 1 row affected (0.00 sec)

            mysql>
            mysql> insert into job (id, title) values (1,'Tester');
            Query OK, 1 row affected (0.01 sec)

            mysql> insert into job (id, title) values (2,'Accountant');
            Query OK, 1 row affected (0.00 sec)

            mysql> insert into job (id, title) values (3,'Developer');
            Query OK, 1 row affected (0.00 sec)

            mysql> insert into job (id, title) values (4,'Coder');
            Query OK, 1 row affected (0.00 sec)

            mysql> insert into job (id, title) values (5,'Director');
            Query OK, 1 row affected (0.00 sec)

            mysql> insert into job (id, title) values (6,'Mediator');
            Query OK, 1 row affected (0.00 sec)

            mysql> insert into job (id, title) values (7,'Proffessor');
            Query OK, 1 row affected (0.00 sec)

            mysql> insert into job (id, title) values (8,'Programmer');
            Query OK, 1 row affected (0.01 sec)

            mysql> insert into job (id, title) values (9,'Developer');
            Query OK, 1 row affected (0.00 sec)

            mysql>
            mysql> select * from job;
            +------+------------+
            | id   | title      |
            +------+------------+
            |    1 | Tester     |
            |    2 | Accountant |
            |    3 | Developer  |
            |    4 | Coder      |
            |    5 | Director   |
            |    6 | Mediator   |
            |    7 | Proffessor |
            |    8 | Programmer |
            |    9 | Developer  |
            +------+------------+
            9 rows in set (0.00 sec)

            mysql> select * from Employee;
            +------+------------+-----------+------------+------------+---------+-----------+-------------+
            | id   | first_name | last_name | start_date | end_date   | salary  | city      | description |
            +------+------------+-----------+------------+------------+---------+-----------+-------------+
            |    1 | Jason      | Martin    | 1996-07-25 | 2006-07-25 | 1234.56 | Toronto   | Programmer  |
            |    2 | Alison     | Mathews   | 1976-03-21 | 1986-02-21 | 6661.78 | Vancouver | Tester      |
            |    3 | James      | Smith     | 1978-12-12 | 1990-03-15 | 6544.78 | Vancouver | Tester      |
            |    4 | Celia      | Rice      | 1982-10-24 | 1999-04-21 | 2344.78 | Vancouver | Manager     |
            |    5 | Robert     | Black     | 1984-01-15 | 1998-08-08 | 2334.78 | Vancouver | Tester      |
            |    6 | Linda      | Green     | 1987-07-30 | 1996-01-04 | 4322.78 | New York  | Tester      |
            |    7 | David      | Larry     | 1990-12-31 | 1998-02-12 | 7897.78 | New York  | Manager     |
            |    8 | James      | Cat       | 1996-09-17 | 2002-04-15 | 1232.78 | Vancouver | Tester      |
            +------+------------+-----------+------------+------------+---------+-----------+-------------+
            8 rows in set (0.00 sec)

            mysql>
            mysql>
            mysql>
            The syntax looks like the following:
            mysql>
            mysql> SELECT C.First_Name, C.Last_Name, O.title
                -> FROM Employee AS C
                -> LEFT JOIN job as O USING (ID);
            +------------+-----------+------------+
            | First_Name | Last_Name | title      |
            +------------+-----------+------------+
            | Jason      | Martin    | Tester     |
            | Alison     | Mathews   | Accountant |
            | James      | Smith     | Developer  |
            | Celia      | Rice      | Coder      |
            | Robert     | Black     | Director   |
            | Linda      | Green     | Mediator   |
            | David      | Larry     | Proffessor |
            | James      | Cat       | Programmer |
            +------------+-----------+------------+
            8 rows in set (0.00 sec)

            mysql>
            mysql>
            mysql>
            mysql> drop table job;
            Query OK, 0 rows affected (0.00 sec)

            mysql> drop table Employee;
            Query OK, 0 rows affected (0.01 sec)

            mysql>
            mysql>
            In this example, the USING keyword would be like using the ON keyword with C.ID = O.ID following it.
            This is another shortcut to save time.
            mysql>

【讨论】：