```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<=2000 then 'low' else 'high' end from salary;
+---------------------------------------------------+
| case when salary<=2000 then 'low' else 'high' end |
+---------------------------------------------------+
| low |
| low |
| high |
| high |
| high |
| high |
+---------------------------------------------------+
6 rows in set (0.00 sec)

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

mysql> 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:

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)

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 |
+-----------------------+----------+
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.

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)

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)

MyISAM,InnoDB,MEMORY,MERGE,NDB

mysql的默认存储引擎是MyISAM

.frm(存储表定义)

.MYD(MYData,存储数据)

.MYI(MYIndex,存储索引)

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)

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)

b.mysql的存储引擎中只有InnoDB支持fk:

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)

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)

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)```