私はMySQLが初めてで、これが何を意味するのか理解するのに少し苦労しています:
DELETE from keywords USING keywords, keywords as vtable
WHERE (keywords.id > vtable.id) && (keywords.keyword=vtable.keyword)
具体的には、この部分は何USING keywords, keywords as vtableを意味していますか？
同等のこのクエリを作成するより良い方法はありますか? 仮想テーブルの作成は効率的ではないことを読みました。
また、.テーブルとカラムを分離していますか?
vtable自己結合できるように、キーワード テーブルのエイリアスです。どのエイリアスとも違いはありませんが、特定のエイリアスを選択したvtableことで、何らかの特別な仮想テーブル メカニズムが関与していると考えるようになった可能性があります。ありません。
効率の観点からは、これは自己結合の通常の使用方法のように見えます。id フィールドとおそらくキーワード フィールドにインデックスがあれば、かなり効率的です。
また、ドット.は実際にテーブルと列名 (またはテーブル エイリアスと列名) を分離します。
別のエイリアス名を使用した同じクエリを次に示します。
DELETE from keywords
USING keywords, keywords as k2
WHERE (keywords.id > k2.id) && (keywords.keyword=k2.keyword)
そして、JOIN を使用して、少し異なる方法で行われたクエリ全体を次に示します。
DELETE keywords
FROM keywords
INNER JOIN keywords as k2 ON keywords.keyword = k2.keyword
WHERE keywords.id > k2.id
これは、同じテーブルのレコードから自分自身を比較することによってテーブルを操作したいという状況に陥ったときはいつでも、非常に優れたクエリ方法です。
USING キーワードは、両方のテーブルに表示される列のリストを使用でき、C.ID = O.ID AND S.ID = O.ID と言うのと同じです。
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>