欢迎您访问程序员文章站本站旨在为大家提供分享程序员计算机编程知识!
您现在的位置是: 首页  >  数据库

mysql流程函数存储引擎InnoDB简单特性_MySQL

程序员文章站 2022-06-08 12:56:15
...
建表及插入数据语句:

mysql> create table salary(userid int,salary decimal(9,2));
Query OK, 0 rows affected (0.11 sec)
mysql> insert into salary values(1,1000),(2,2000),(3,3000),(4,4000),(5,5000),(1,
null);
Query OK, 6 rows affected (0.13 sec)
Records: 6 Duplicates: 0 Warnings: 0

1.if(value,t,f)函数:这里认为高于2000就是'high',其他都是low

mysql> select if(salary>2000,'high','low') from salary;
+------------------------------+
| if(salary>2000,'high','low') |
+------------------------------+
| low |
| low |
| high |
| high |
| high |
| low |
+------------------------------+
6 rows in set (0.00 sec)

2.ifnull(value1,value2)函数:这个函数可以用来将NULL值换成0

mysql> select ifnull(salary,0) from salary;
+------------------+
| ifnull(salary,0) |
+------------------+
| 1000.00 |
| 2000.00 |
| 3000.00 |
| 4000.00 |
| 5000.00 |
| 0.00 |
+------------------+
6 rows in set (0.00 sec)

3.case when [value1] then [result]...else [default] end函数:

mysql> select case when salary select case salary when 1000 then 'low' when 2000 then 'mid' else 'high'e
nd from salary;
+----------------------------------------------------------------------+
| case salary when 1000 then 'low' when 2000 then 'mid' else 'high'end |
+----------------------------------------------------------------------+
| low |
| mid |
| high |
| high |
| high |
| high |
+----------------------------------------------------------------------+
6 rows in set (0.02 sec)

5.关于mysql存储引擎的一些东西:

存储引擎是mysql不同于其他数据库的一个重要特性,用户可以根据实际需要利用这个特性定制自己的存储引擎.

mysql的引擎有:

mysql> show engines \G;
*************************** 1. row ***************************
Engine: MyISAM
Support: YES
Comment: Default engine as of MySQL 3.23 with great performance
*************************** 2. row ***************************
Engine: MEMORY
Support: YES
Comment: Hash based, stored in memory, useful for temporary tables
*************************** 3. row ***************************
Engine: InnoDB
Support: DEFAULT
Comment: Supports transactions, row-level locking, and foreign keys
*************************** 4. row ***************************
Engine: BerkeleyDB
Support: NO
Comment: Supports transactions and page-level locking
*************************** 5. row ***************************
Engine: BLACKHOLE
Support: NO
Comment: /dev/null storage engine (anything you write to it disappears)
*************************** 6. row ***************************
Engine: EXAMPLE
Support: NO
Comment: Example storage engine
*************************** 7. row ***************************
Engine: ARCHIVE
Support: YES
Comment: Archive storage engine
*************************** 8. row ***************************
Engine: CSV
Support: NO
Comment: CSV storage engine
*************************** 9. row ***************************
Engine: ndbcluster
Support: NO
Comment: Clustered, fault-tolerant, memory-based tables
*************************** 10. row ***************************
Engine: FEDERATED
Support: NO
Comment: Federated MySQL storage engine
*************************** 11. row ***************************
Engine: MRG_MYISAM
Support: YES
Comment: Collection of identical MyISAM tables
*************************** 12. row ***************************
Engine: ISAM
Support: NO
Comment: Obsolete storage engine
12 rows in set (0.00 sec)

或者使用这个cmd:

mysql> show variables like 'have%';
+-----------------------+----------+
| Variable_name | Value |
+-----------------------+----------+
| have_archive | YES |
| have_bdb | NO |
| have_blackhole_engine | NO |
| have_compress | YES |
| have_crypt | NO |
| have_csv | NO |
| have_example_engine | NO |
| have_federated_engine | NO |
| have_geometry | YES |
| have_innodb | YES |
| have_isam | NO |
| have_ndbcluster | NO |
| have_openssl | DISABLED |
| have_query_cache | YES |
| have_raid | NO |
| have_rtree_keys | YES |
| have_symlink | YES |
+-----------------------+----------+
17 rows in set (0.00 sec)

disabled说明mysql支持该engine,但是启动的时候被禁用.

创建表的时候,可以使用engine关键字指定该表使用哪个engine:

mysql> create table ai(i bigint(20) NOT NULL AUTO_INCREMENT,PRIMARY KEY(I)) ENGI
NE=MyISAM DEFAULT CHARSET=GBK;
Query OK, 0 rows affected (0.03 sec)

也可以修改表的引擎:

mysql> alter table ai engine=innodb;
Query OK, 0 rows affected (0.13 sec)
Records: 0 Duplicates: 0 Warnings: 0
mysql> show create table ai \G;
*************************** 1. row ***************************
Table: ai
Create Table: CREATE TABLE `ai` (
`i` bigint(20) NOT NULL auto_increment,
PRIMARY KEY (`i`)
) ENGINE=InnoDB DEFAULT CHARSET=gbk
1 row in set (0.00 sec)

常见的存储引擎有:

MyISAM,InnoDB,MEMORY,MERGE,NDB

上述引擎中只有InnoDB支持外键。

mysql的默认存储引擎是MyISAM

每个MyISAM在磁盘上存储成3个文件:

.frm(存储表定义)

.MYD(MYData,存储数据)

.MYI(MYIndex,存储索引)

关于InnoDB的一些特性:

a.自动增长字段:

mysql> create table autoincre(i smallint not null auto_increment,name varchar(20
),primary key(i))engine=innodb;
Query OK, 0 rows affected (0.05 sec)

mysql> insert into autoincre values(1,'1'),(2,'2'),(null,'3');
Query OK, 3 rows affected (0.03 sec)
Records: 3 Duplicates: 0 Warnings: 0
mysql> select * from autoincre;
+---+------+
| i | name |
+---+------+
| 1 | 1 |
| 2 | 2 |
| 3 | 3 |
+---+------+
3 rows in set (0.00 sec)

向自增长字段插入记录后,不影响该字段自己增加值.

对于InnoDB表,自动增长字段必须是索引,如果是组合索引也必须是组合索引的第一个列.

但是对于MyISAM表,自增长字段可以不是组合索引的第一个列,可以作为第二个列出现:

mysql> create table autoincre_demo(d1 smallint not null auto_increment,d2 smalli
nt not null,name varchar(10),index(d2,d1))engine=myisam;
Query OK, 0 rows affected (0.05 sec)
mysql> insert into autoincre_demo(d2,name)values(2,'2'),(3,'3'),(4,'4'),(2,'2'),
(3,'3'),(4,'4');
Query OK, 6 rows affected (0.00 sec)
Records: 6 Duplicates: 0 Warnings: 0
mysql> select * from autoincre_demo;
+----+----+------+
| d1 | d2 | name |
+----+----+------+
| 1 | 2 | 2 |
| 1 | 3 | 3 |
| 1 | 4 | 4 |
| 2 | 2 | 2 |
| 2 | 3 | 3 |
| 2 | 4 | 4 |
+----+----+------+
6 rows in set (0.00 sec)

自增长字段d1作为组合索引在第二列中出现,自增长字段的记录按照组合索引d2进行排序后递增.

b.mysql的存储引擎中只有InnoDB支持fk:

建表语句:

mysql> create table country(country_id smallint unsigned not null auto_increment
,country varchar(50) not null,primary key(country_id))
-> engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.14 sec)

mysql> create table city(
-> city_id smallint unsigned not null auto_increment,
-> country_id smallint unsigned not null,
-> primary key(city_id),
-> foreign key(country_id) references country(country_id)
-> on delete restrict on update cascade
-> engine=innodb default charset=utf8;
Query OK, 0 rows affected (0.05 sec)

插入记录:

mysql> insert into country(country) values('china');
Query OK, 1 row affected (0.03 sec)
mysql> select * from country;
+------------+---------+
| country_id | country |
+------------+---------+
| 1 | china |
+------------+---------+
1 row in set (0.00 sec)
mysql> insert into city(country_id) values(1);
Query OK, 1 row affected (0.11 sec)

mysql> select * from city;
+---------+------------+
| city_id | country_id |
+---------+------------+
| 1 | 1 |
+---------+------------+
1 row in set (0.00 sec)

city表依赖country表的country_id字段,删除会出错:

mysql> delete from country where country_id=1;
ERROR 1451 (23000): Cannot delete or update a parent row: a foreign key constrai
nt fails (`tom1/city`, CONSTRAINT `city_ibfk_1` FOREIGN KEY (`country_id`) REFER
ENCES `country` (`country_id`) ON UPDATE CASCADE)

现在更新country表中的country_id字段,city表的country_id字段也会被同步更新,这是因为在创建city表的最后加了:on update cascade,即:更新时做级联操作

mysql> update country set country_id=1000 where country_id=1;
Query OK, 1 row affected (0.03 sec)
Rows matched: 1 Changed: 1 Warnings: 0
mysql> select * from city;
+---------+------------+
| city_id | country_id |
+---------+------------+
| 1 | 1000 |
+---------+------------+
1 row in set (0.00 sec)