vtable是关键字表的别名，因此可以自联接。与任何别名没有什么不同，但特定别名的选择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 id 和可能的关键字字段上有索引，它应该是相当有效的。
此外 dot.确实将表和列名（或表别名和列名）分开。
这是具有不同别名的相同查询：
DELETE from keywords
USING keywords, keywords as k2
WHERE (keywords.id > k2.id) && (keywords.keyword=k2.keyword)
下面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
这是一种非常棒的查询方法，因为每当您陷入某种情况时，您想通过比较同一张表的记录中的自身来在表中进行操作。
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>