SQL语法基础之DROP语句

　　　　　　　　　　　　　　　SQL语法基础之DROP语句

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　作者：尹正杰

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一.查看DROP帮助信息

mysql> ? DROP
Many help items for your request exist.
To make a more specific request, please type 'help <item>',
where <item> is one of the following
topics:
   ALTER TABLE
   DEALLOCATE PREPARE
   DROP DATABASE
   DROP EVENT
   DROP FUNCTION
   DROP FUNCTION UDF
   DROP INDEX
   DROP PROCEDURE
   DROP RESOURCE GROUP
   DROP ROLE
   DROP SERVER
   DROP SPATIAL REFERENCE SYSTEM
   DROP TABLE
   DROP TABLESPACE
   DROP TRIGGER
   DROP USER
   DROP VIEW

mysql>
mysql> 

二.DROP  DATABASE

　　DROP DATABASE语句用来删除数据库操作，即删除了数据库也删除数据库里的所有表。

1>.查看DROP DATABASE的帮助信息

mysql> ? DROP DATABASE
Name: 'DROP DATABASE'
Description:
Syntax:
DROP {DATABASE | SCHEMA} [IF EXISTS] db_name

DROP DATABASE drops all tables in the database and deletes the
database. Be very careful with this statement! To use DROP DATABASE,
you need the DROP privilege on the database. DROP SCHEMA is a synonym
for DROP DATABASE.

*Important*:

When a database is dropped, privileges granted specifically for the
database are not automatically dropped. They must be dropped manually.
See [HELP GRANT].

IF EXISTS is used to prevent an error from occurring if the database
does not exist.

URL: http://dev.mysql.com/doc/refman/8.0/en/drop-database.html

mysql> 

mysql> ? DROP DATABASE

2>.删除数据库的操作将删除改数据库所在文件夹乳腺癌格式的文件，以及db.opt文件

mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| A1                 |
| A2                 |
| A3                 |
| course             |
| day03              |
| devops             |
| information_schema |
| mysql              |
| performance_schema |
| sys                |
| yinzhengjie        |
+--------------------+
11 rows in set (0.00 sec)

mysql>
mysql> DROP DATABASE A1;　　　　　　　　　　　　　　#删除一个已经存在的库，由于我知道该库肯定存在，因此这里我就没有使用判断语句！
Query OK, 1 row affected (0.00 sec)

mysql>
mysql> DROP DATABASE IF EXISTS A2;　　　　　　　　#删除一个库如果你不确定该库是否存在时，你可以使用判断语句，既然改库不存在你也不怕报错啦～
Query OK, 1 row affected (0.01 sec)

mysql>
mysql> SHOW DATABASES;
+--------------------+
| Database           |
+--------------------+
| A3                 |
| course             |
| day03              |
| devops             |
| information_schema |
| mysql              |
| performance_schema |
| sys                |
| yinzhengjie        |
+--------------------+
9 rows in set (0.00 sec)

mysql> 

三.DROP INDEX语句

　　DROP INDEX 语句是用来删除索引操作的

1>.查看DROP INDEX语句的帮助信息

mysql> ? DROP INDEX
Name: 'DROP INDEX'
Description:
Syntax:
DROP INDEX index_name ON tbl_name
    [algorithm_option | lock_option] ...

algorithm_option:
    ALGORITHM [=] {DEFAULT|INPLACE|COPY}

lock_option:
    LOCK [=] {DEFAULT|NONE|SHARED|EXCLUSIVE}

DROP INDEX drops the index named index_name from the table tbl_name.
This statement is mapped to an ALTER TABLE statement to drop the index.
See [HELP ALTER TABLE].

To drop a primary key, the index name is always PRIMARY, which must be
specified as a quoted identifier because PRIMARY is a reserved word:

DROP INDEX `PRIMARY` ON t;

URL: http://dev.mysql.com/doc/refman/8.0/en/drop-index.html

mysql> 

mysql> ? DROP INDEX

2>.案例展示

mysql> SHOW CREATE TABLE teacher;
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                  |
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| teacher | CREATE TABLE `teacher` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(30) DEFAULT NULL,
  `course_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `index_teacher_id_unique` (`id`),
  UNIQUE KEY `index_teacher_id_comment` (`id`) COMMENT '这就是创建一个唯一索引而已!',
  KEY `teacher_course` (`course_id`),
  KEY `index_teacher_id` (`id`),
  KEY `index_teacher_id_and_name` (`id`,`name`),
  CONSTRAINT `teacher_course` FOREIGN KEY (`course_id`) REFERENCES `course` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=16 DEFAULT CHARSET=utf8              |
+---------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql>
mysql> DROP INDEX index_teacher_id_and_name ON teacher;
Query OK, 0 rows affected (0.01 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql>
mysql> SHOW CREATE TABLE teacher;
+---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table   | Create Table                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                 |
+---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| teacher | CREATE TABLE `teacher` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(30) DEFAULT NULL,
  `course_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `index_teacher_id_unique` (`id`),
  UNIQUE KEY `index_teacher_id_comment` (`id`) COMMENT '这就是创建一个唯一索引而已!',
  KEY `teacher_course` (`course_id`),
  KEY `index_teacher_id` (`id`),
  CONSTRAINT `teacher_course` FOREIGN KEY (`course_id`) REFERENCES `course` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=16 DEFAULT CHARSET=utf8              |
+---------+------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

mysql> 

四.DROP TABLE语句

　　DROP TABLE 语句用来删除一个或多个表操作，当然也可以删除临时表。

1>.查看帮助信息

mysql> ?  DROP TABLE
Name: 'DROP TABLE'
Description:
Syntax:
DROP [TEMPORARY] TABLE [IF EXISTS]
    tbl_name [, tbl_name] ...
    [RESTRICT | CASCADE]

DROP TABLE removes one or more tables. You must have the DROP privilege
for each table.

Be careful with this statement! It removes the table definition and all
table data. For a partitioned table, it permanently removes the table
definition, all its partitions, and all data stored in those
partitions. It also removes partition definitions associated with the
dropped table.

DROP TABLE causes an implicit commit, except when used with the
TEMPORARY keyword. See
http://dev.mysql.com/doc/refman/8.0/en/implicit-commit.html.

*Important*:

When a table is dropped, privileges granted specifically for the table
are not automatically dropped. They must be dropped manually. See [HELP
GRANT].

If any tables named in the argument list do not exist, the statement
fails with an error indicating by name which nonexisting tables it was
unable to drop, and no changes are made.

Use IF EXISTS to prevent an error from occurring for tables that do not
exist. Instead of an error, a NOTE is generated for each nonexistent
table; these notes can be displayed with SHOW WARNINGS. See [HELP SHOW
WARNINGS].

IF EXISTS can also be useful for dropping tables in unusual
circumstances under which there is an entry in the data dictionary but
no table managed by the storage engine. (For example, if an abnormal
server exit occurs after removal of the table from the storage engine
but before removal of the data dictionary entry.)

The TEMPORARY keyword has the following effects:

o The statement drops only TEMPORARY tables.

o The statement does not cause an implicit commit.

o No access rights are checked. A TEMPORARY table is visible only with
  the session that created it, so no check is necessary.

Using TEMPORARY is a good way to ensure that you do not accidentally
drop a non-TEMPORARY table.

The RESTRICT and CASCADE keywords do nothing. They are permitted to
make porting easier from other database systems.

DROP TABLE is not supported with all innodb_force_recovery settings.
See
http://dev.mysql.com/doc/refman/8.0/en/forcing-innodb-recovery.html.

URL: http://dev.mysql.com/doc/refman/8.0/en/drop-table.html

mysql> 

mysql> ? DROP TABLE

2>.RESTRICT/CASCADE两个关键词在5.7版本中没用

mysql> SHOW TABLES;
+--------------+
| Tables_in_A3 |
+--------------+
| B3           |
| v_B3         |
+--------------+
2 rows in set (0.00 sec)

mysql>
mysql> DROP TABLE B3;　　　　　　　　　　　　　　　　　　　　　　#删除已经存在的表
Query OK, 0 rows affected (0.01 sec)

mysql>
mysql> DROP TABLE IF EXISTS B3;　　　　　　　　　　　　　　　　#删除一张表，我这里加了判断条件，意思是当这张表存在就删除，若不存在就不执行删除操作，这样可以避免报错～
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql>
mysql> SHOW TABLES;
+--------------+
| Tables_in_A3 |
+--------------+
| v_B3         |
+--------------+
1 row in set (0.00 sec)

mysql> 

五.DROP VIEW语句

　　DROP VIEW语句用来删除一个或多个视图。

1>.查看DROP VIEW的帮助信息

mysql> ? DROP VIEW
Name: 'DROP VIEW'
Description:
Syntax:
DROP VIEW [IF EXISTS]
    view_name [, view_name] ...
    [RESTRICT | CASCADE]

DROP VIEW removes one or more views. You must have the DROP privilege
for each view.

If any views named in the argument list do not exist, the statement
fails with an error indicating by name which nonexisting views it was
unable to drop, and no changes are made.

*Note*:

In MySQL 5.7 and earlier, DROP VIEW returns an error if any views named
in the argument list do not exist, but also drops all views in the list
that do exist. Due to the change in behavior in MySQL 8.0, a partially
completed DROP VIEW operation on a MySQL 5.7 master fails when
replicated on a MySQL 8.0 slave. To avoid this failure scenario, use IF
EXISTS syntax in DROP VIEW statements to prevent an error from
occurring for views that do not exist. For more information, see
http://dev.mysql.com/doc/refman/8.0/en/atomic-ddl.html.

The IF EXISTS clause prevents an error from occurring for views that
don't exist. When this clause is given, a NOTE is generated for each
nonexistent view. See [HELP SHOW WARNINGS].

RESTRICT and CASCADE, if given, are parsed and ignored.

URL: http://dev.mysql.com/doc/refman/8.0/en/drop-view.html

mysql>
mysql> 

mysql> ? DROP VIEW

2>.同样，RESTRICT/CASCADE两个关键词在MySQL5.7依旧也是无效

mysql> SHOW TABLES;
+--------------+
| Tables_in_A3 |
+--------------+
| v_B3         |
+--------------+
1 row in set (0.00 sec)

mysql>
mysql> DROP VIEW v_B3;
Query OK, 0 rows affected (0.00 sec)

mysql>
mysql> SHOW TABLES;
Empty set (0.00 sec)

mysql>
mysql> DROP VIEW IF EXISTS v_B3;
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql>
mysql> SHOW TABLES;
Empty set (0.00 sec)

mysql>
mysql>