SQL基础语法—其他语句

1 alter database语句

alter database语句用来修改数据库的属性

Syntax:
ALTER {DATABASE | SCHEMA} [db_name]
    alter_specification ...

alter_specification:
    [DEFAULT] CHARACTER SET [=] charset_name
  | [DEFAULT] COLLATE [=] collation_name

db_name可以不指定,如果不指定说明是修改当前数据库的属性;

character set代表修改数据库的默认字符集;

collate代表修改数据库的默认排序规则;

如果修改了数据库的默认字符集或排序规则,那数据库中的所有存储过程和函数都需要重新创建一遍。

2 alter view语句

alter view语句用来修改视图的定义,本身的语法结构和create view相同,语句所起到的作用和create or replace view语句相同。

Syntax:
ALTER
    [ALGORITHM = {UNDEFINED | MERGE | TEMPTABLE}]
    [DEFINER = { user | CURRENT_USER }]
    [SQL SECURITY { DEFINER | INVOKER }]
    VIEW view_name [(column_list)]
    AS select_statement
    [WITH [CASCADED | LOCAL] CHECK OPTION]

示例:

mysql> alter view v_students_male as select sid,sname from students where sex=0;
mysql> select * from v_students_male;
+------+-------+
| sid | sname |
+------+-------+
| 1 | aaa |

3 drop database语句

drop database语句用来删除数据库操作,即删除了数据库也删除了库里的所有表:

DROP {DATABASE | SCHEMA} [IF EXISTS] db_name

删除数据库的操作将删除该数据库所在文件夹下的.bak .dat .hsh .mrg .myd .myi .trg .trn .cfg .db .frm .ibd .ndb .par以及db.opt文件。

示例:

mysql> drop database test2;
Query OK, 1 row affected (0.07 sec)
mysql> drop database if exists test4;
Query OK, 0 rows affected (0.00 sec)

4 drop index语句

drop index语句用来删除索引操作:

DROP INDEX index_name ON tbl_name

删除tbl_name表中的名字叫index_name的索引。

示例:

mysql> drop index idx_st_sname on students;
Query OK, 0 rows affected (0.01 sec)
Records: 0 Duplicates: 0 Warnings: 0

5 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 :

Syntax:
DROP [TEMPORARY] TABLE [IF EXISTS]
    tbl_name [, tbl_name] ...
    [RESTRICT | CASCADE]

RESTRICT/CASCADE两个关键词在5.7版本中没用。

示例:

mysql> drop table students2;
Query OK, 0 rows affected (0.01 sec)
mysql> drop table if exists students2;
Query OK, 0 rows affected, 1 warning (0.00 sec)

6 drop view语句

drop view语句用来删除一个或多个视图

Syntax:
DROP VIEW [IF EXISTS]
    view_name [, view_name] ...
    [RESTRICT | CASCADE]

RESTRICT/CASCADE两个关键词在5.7版本中没用。

示例:

mysql> drop view v_students_male;
Query OK, 0 rows affected (0.00 sec)
mysql> drop view if exists v_students_male;
Query OK, 0 rows affected, 1 warning (0.00 sec)

7 rename table语句

rename table语句用来重命名一个或多个表:

Syntax:
RENAME TABLE
    tbl_name TO new_tbl_name
    [, tbl_name2 TO new_tbl_name2] ...

当想让两个表名相互调换时,可以执行语句:

RENAME TABLE old_table TO tmp_table,
	new_table TO old_table,
	tmp_table TO new_table;

rename table能将表中的数据,索引,主键定义都自动转换到新表下,但视图和对原表分配的权限不能自动转换到新表,需要手动执行。

mysql> rename table students to students_test;
Query OK, 0 rows affected (0.03 sec)

mysql> show create table students_test;
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table | Create Table |
+---------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| students_test | CREATE TABLE `students_test` (
 `sid` int(11) DEFAULT NULL,
 `sname` varchar(20) DEFAULT NULL,
 `sex` int(11) DEFAULT NULL,
 UNIQUE KEY `idx_st_sid` (`sid`),
 KEY `idx_st_union` (`sname`,`sex`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1 |

mysql> select * from v_students_female; ##原有视图查询失败
ERROR 1356 (HY000): View 'test.v_students_female' references invalid table(s) or column(s) or function(s) or definer/invoker of view lack rights to use them

8 truncate table语句

truncate table语句用来删除/截断表里的所有数据。

TRUNCATE [TABLE] tbl_name

delete删除所有数据在逻辑上含义相同,但性能更快。类似执行了drop tablecreate table两个语句。

示例:

mysql> truncate table students_test;
Query OK, 0 rows affected (0.01 sec)
mysql> select * from students_test; ##查询结果为空
Empty set (0.00 sec)

9 alter table语句

Alter table … add [column_name]
Alter table … add constraint [name] unique [index/key] [name]
Alter table … add constraint [name] foreign key (column_name) references table_name(column_name)
Alter table … drop column [column_name]
Alter table … drop [index/key] [index_name]
posted @ 2020-07-13 20:16  囫囵国  阅读(9)  评论(0编辑  收藏