mysql -=- DDL

↘锁芯ラ 提交于 2020-01-16 09:02:06
net start mysql
mysql -uroot -p
show databases
use 数据库名
how tables

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select goods_id ,goods_name ,shop_price+1,market_price,market_price-shop_price from goods;
select * from goods where goods_id = 32;
select goods_id,goods_name,cat_id from goods where cat_id <> 32;//不等于
select goods_id,goods_name,cat_id from goods where cat_id > 32;//不等于
select goods_id,goods_name,cat_id from goods where cat_id <= 32 and cat_id >= 2;//不等于
select goods_id,cat_id from goods where cat_id = 4 or cat_id =11;//或者
select goods_id,cat_id from goods where cat_id  in (4,11);//4,11选一个
select goods_id,cat_id from goods where cat_id  between  4 and 11;//4--11之间
select goods_id,cat_id from goods where cat_id  not in (4,11);//不是4,11
select click_count ,cat_id,shop_price from goods where cat_id = 3 and (shop_price < 1000 or shop_price > 3000) and click_count>5;  //and优先级高于or
select goods_name from goods where goods_name like '诺基亚%';//%是任意字符
select goods_name from goods where goods_name like '诺基亚___';//___3个下划线是3个字符
select cat_id,sum(goods_number*shop_price) as k from goods group by cat_id having k>20000
(5个聚合函数,sum,avg,max,min,count与group综合运用) select cat_id,max(shop_price) from ecs_goods  group by cat_id;

select goods_id,cat_id,goods_name,shop_price  from ecs_goods where 
cat_id=3 and shop_price>1000 and shop_price <3000 and click_count>5 and goods_name like '诺基亚%';

select goods_id,cat_id,goods_name,shop_price  from ecs_goods where 
shop_price between 1000 and 3000 and cat_id=3  and click_count>5 and goods_name like '诺基亚%';

select goods_id,cat_id,goods_name,shop_price  from ecs_goods where goods_name like '诺基亚N__';
select goods_id,cat_id,goods_name,shop_price from ecs_goos where goods_name not like '诺基亚%';

select goods_id,cat_id,goods_name,shop_price,click_count from ecs_goods where cat_id=3 and (shop_price <1000 or shop_price>3000) and click_count>5;

select goods_id,cat_id,goods_name,shop_price  from ecs_goods     where cat_id not in (3,11);

取出100<=价格<=500的商品(不许用and)
select goods_id,cat_id,goods_name,shop_price  from ecs_goods where shop_price between 100 and 500;



1.15 一道面试题,有如下表和数组,把num值处于[20,29]之间,改为20,num值处于[30,39]之间的,改为30
select floor(num/10)*10 from mian;
updata mian set num=floor(num/10)*10;
update mian set num=floor(num/10)*10 where num between 20 and 29;

1.16 练习题:
把good表中商品名为'诺基亚xxxx'的商品,改为'HTCxxxx',
update goods set goods_name = concat('htc',substring(goods_name,4))  where goods_name like '诺基亚%';

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NULL : 
insert into user values (1,null,99,'rr');//Column 'name' cannot be null


mysql> create table tmp(
    -> id int,
    -> name varchar(20)
    -> ) charset utf8 engine myisam;
insert into tmp values (1,'ll'),(2,'ffff'),(3,null);

mysql> select * from tmp;
+----+------+
| id | name |
+----+------+
|  1 | ll   |
|  1 | ll   |
|  2 | ffff |
|  3 | NULL |
+----+------+
4 rows in set

mysql> select * from tmp where name = null;
Empty set

mysql> select * from tmp where name != null;
Empty set

mysql> select * from tmp where name is null;
+----+------+
| id | name |
+----+------+
|  3 | NULL |
+----+------+
1 row in set

mysql> select * from tmp where name is not null;
+----+------+
| id | name |
+----+------+
|  1 | ll   |
|  1 | ll   |
|  2 | ffff |
+----+------+
3 rows in set

mysql> desc tmp;//查看表的结构: 
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| id    | int(11)     | YES  |     | NULL    |       |
| name  | varchar(20) | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
2 rows in set


mysql> show create table  tmp;//查看表的创建过程: 
+-------+---------------+
| Table | Create Table                                                                                                          |
+-------+--------------+
| tmp   | CREATE TABLE `tmp` (
  `id` int(11) DEFAULT NULL,
  `name` varchar(20) DEFAULT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8 |
+-------+------------+
1 row in set

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mysql> select avg(shop_price) from goods;
mysql> select count(*) from goods;
mysql> select sum(goods_number*shop_price) from goods;//求2列相乘后的和

mysql> select avg(shop_price) ,cat_id from goods group by cat_id;//先根据cat_id分组,再求每组的平均值。
+-----------------+--------+
| avg(shop_price) | cat_id |
+-----------------+--------+
| 823.330000      |      2 |
| 1746.066667     |      3 |
| 2297.000000     |      4 |
| 3700.000000     |      5 |
| 75.333333       |      8 |
| 31.000000       |     11 |
| 33.500000       |     13 |
| 54.000000       |     14 |
| 70.000000       |     15 |
+-----------------+--------+
9 rows in set

mysql>  select count(*) ,cat_id from goods group by cat_id;//根据cat_id分组,再查每组的数量。
+----------+--------+
| count(*) | cat_id |
+----------+--------+
|        1 |      2 |
|       15 |      3 |
|        3 |      4 |
|        1 |      5 |
|        3 |      8 |
|        2 |     11 |
|        2 |     13 |
|        2 |     14 |
|        2 |     15 |
+----------+--------+
9 rows in set

mysql>  select max(shop_price) ,cat_id from goods group by cat_id;
+-----------------+--------+
| max(shop_price) | cat_id |
+-----------------+--------+
| 823.33          |      2 |
| 5999            |      3 |
| 2878            |      4 |
| 3700            |      5 |
| 100             |      8 |
| 42              |     11 |
| 48              |     13 |
| 90              |     14 |
| 95              |     15 |
+-----------------+--------+
9 rows in set

mysql> select goods_id, goods_name, market_price - shop_price from goods;
+----------+---------------------------+---------------------------+
| goods_id | goods_name                | market_price - shop_price |
+----------+---------------------------+---------------------------+
|        1 | kd876                     | 277.6                     |
|        4 | htcn85原装充电器          | 11.6                      |
+----------+---------------------------+---------------------------+
31 rows in set

-------------------------------------------------------------------------------------------------------

having对结果集操作: where---group by---having---order by ---limit

数据库数据保存在goods.MYD文件里面,select xxx from goods where cat_id=3 查询的临时结果集result放在内存中,where是针对表起作用,select (market_price - shop_price) as less中的less在内存结果集中不在表中,所以对表中where less > 700不起作用,想好针对结果集发挥作用只能用having了,
mysql> select goods_id, goods_name ,(market_price - shop_price) as less from goods [where 1] having less > 700;
+----------+----------------+--------+
| goods_id | goods_name     | less   |
+----------+----------------+--------+
|       22 | 多普达touch hd | 1199.8 |
|       23 | htcn96         | 740    |
+----------+----------------+--------+
2 rows in set

所以where不能写在having后面,因为只有where从表中查询出结果集后,然后用having对结果集操作。


有如下表及数据
+------+---------+-------+
| name | subject | score |
+------+---------+-------+
| 张三 | 数学    |    90 |
| 张三 | 语文    |    50 |
| 张三 | 地理    |    40 |
| 李四 | 语文    |    55 |
| 李四 | 政治    |    45 |
| 王五 | 政治    |    30 |
+------+---------+-------+   要求:查询出2门及2门以上不及格者的平均成绩

drop table tmp;

mysql> select count(score<=60) as num from temp;
+-----+
| num |
+-----+
|   6 |
+-----+
1 row in set

mysql> select count(score<=60) as num from temp group by name;
+-----+
| num |
+-----+
|   3 |
|   1 |
|   2 |
+-----+
3 rows in set

mysql>  select name, count(score<=60) as num, avg(score) from temp where 1 group by name having num>=2;
+------+-----+------------+
| name | num | avg(score) |
+------+-----+------------+
| 三   |   3 | 60.0000    |
| 四   |   2 | 30.0000    |
+------+-----+------------+

-------------------------------------------------------------------------------------------------------

order by:在内存中排序
mysql> select * from goods order by shop_price , cat_id;

取出点击量前三名到前5名的商品
select goods_id,goods_name,click_count from ecs_goods order by click_count desc limit 2,5;//跳过2个从第三个开始,取出5个,

where 子查询:
查询出最新的商品(以商品编号goods_id最大为最新,用子查询实现)
select goods_id,goods_name from goods where goods_id =(select max(goods_id) from ecs_goods);
查询出每一个cat_id下最新的商品(先查询每个cat_id下goods_id的最大值)
mysql> select goods_id,cat_id from goods where goods_id in (select max(goods_id) from goods group by cat_id) order by cat_id asc;

from子查询:
mysql> select goods_id from (select goods_id,goods_name from goods order by goods_id asc, goods_name desc) as temp where goods_id between 10 and 13;
+----------+
| goods_id |
+----------+
|       10 |
|       11 |
|       12 |
|       13 |
+----------+

mysql> select goods_id as gid from (select goods_id,goods_name from goods  order by goods_id asc, goods_name desc) as temp where goods_id between 10 and 13 order by gid desc limit 1,2;//有的地方要加as不加as不行, 空格作为分隔符逗号作为连接符。
+-----+
| gid |
+-----+
|  12 |
|  11 |
+-----+

exists型子查询:
查出所有有商品的栏目cart_id
select * from category where exists (select * from goods where goods.cat_id=category.cat_id);

mysql> select goods.cat_id,category.cat_id from goods,category where goods.cat_id = category.cat_id limit 1,5;//2个表查询
+--------+--------+
| cat_id | cat_id |
+--------+--------+
|      8 |      8 |
|      8 |      8 |
|     11 |     11 |
|     11 |     11 |
|      8 |      8 |
+--------+--------+

-----------------------------------------------------------------------------------------------------------------
1+N模式查询:
------------------------------------------------------------------------------------------------------------------
连接查询:
左连接,left join on,table A left join table B on tableA.col1 = tableB.col2 ; 
连接完了之后就成了一个大表格(from boy left join girl on boy.hid=girl.hid就是一张大表,后面的写法跟一张表一样写法,)。on是表连接的条件。

mysql> select * from girl;   mysql> select * from boy;
+-----+--------+        +-----+--------+
| hid | gname  |        | hid | bname  |
+-----+--------+        +-----+--------+
| B   | 小龙女 |        | A   | 屌丝   |
| C   | 张柏芝 |        | B   | 杨过   |
| D   | mv     |        | C   | 陈冠希 |
| E   | no     |        | D   | yw     |
+-----+--------+        +-----+--------+

mysql> select boy.hid,boy.bname,girl.hid,girl.gname from boy left join girl on boy.hid=girl.hid;//左边的表全部列出来,进行右边表的匹配,匹配不上就没有,
+-----+--------+------+--------+
| hid | bname  | hid  | gname  |
+-----+--------+------+--------+
| B   | 杨过   | B    | 小龙女 |
| C   | 陈冠希 | C    | 张柏芝 |
| D   | yw     | D    | mv     |
| A   | 屌丝   | NULL | NULL   |
+-----+--------+------+--------+

mysql> select boy.hid,boy.bname,girl.hid,girl.gname from boy right join girl on boy.hid=girl.hid;//右边的表全部列出来,进行左边表的匹配,匹配不上就没有,
+------+--------+-----+--------+
| hid  | bname  | hid | gname  |
+------+--------+-----+--------+
| B    | 杨过   | B   | 小龙女 |
| C    | 陈冠希 | C   | 张柏芝 |
| D    | yw     | D   | mv     |
| NULL | NULL   | E   | no     |
+------+--------+-----+--------+

mysql> select boy.hid,boy.bname,girl.hid,girl.gname from boy inner join girl on boy.hid=girl.hid;//剔除没有匹配上的,inner join是求交集,求并集是外连接,mysql没有外连接,oracle和sqlsercver有外连接。
+-----+--------+-----+--------+
| hid | bname  | hid | gname  |
+-----+--------+-----+--------+
| B   | 杨过   | B   | 小龙女 |
| C   | 陈冠希 | C   | 张柏芝 |
| D   | yw     | D   | mv     |
+-----+--------+-----+--------+

select goods_name,cat_name,shop_price from 
goods left join category on goods.cat_id=category.cat_id //大表,
where goods.cat_id = 4;



mysql> select * from boygirl;
+--------+--------+
| boygid | byname |
+--------+--------+
| A      | aaa    |
| B      | bbb    |
| C      | ccc    |
| D      | ddd    |
+--------+--------+
mysql> select * from boy left join girl on gid=bid;
+-----+--------+------+--------+
| bid | bname  | gid  | gname  |
+-----+--------+------+--------+
| B   | 杨过   | B    | 小龙女 |
| C   | 陈冠希 | C    | 张柏芝 |
| D   | yw     | D    | mv     |
| A   | 屌丝   | NULL | NULL   |
+-----+--------+------+--------+
mysql> select * from boy left join girl on gid=bid     //left join成一张大表,这张大表再去右连接,
right join boygirl on boygid=gid;
+------+--------+------+--------+--------+--------+
| bid  | bname  | gid  | gname  | boygid | byname |
+------+--------+------+--------+--------+--------+
| B    | 杨过   | B    | 小龙女 | B      | bbb    |
| C    | 陈冠希 | C    | 张柏芝 | C      | ccc    |
| D    | yw     | D    | mv     | D      | ddd    |
| NULL | NULL   | NULL | NULL   | A      | aaa    |
+------+--------+------+--------+--------+--------+
mysql> select * from boy left join girl as tmp on gid=bid  right join boygirl on boygirl.boygid=tmp.gid;//取boy left join girl大表的别名为tmp,
+------+--------+------+--------+--------+--------+
| bid  | bname  | gid  | gname  | boygid | byname |
+------+--------+------+--------+--------+--------+
| B    | 杨过   | B    | 小龙女 | B      | bbb    |
| C    | 陈冠希 | C    | 张柏芝 | C      | ccc    |
| D    | yw     | D    | mv     | D      | ddd    |
| NULL | NULL   | NULL | NULL   | A      | aaa    |
+------+--------+------+--------+--------+--------+
mysql> select * from boy left join girl as t1 on boy.bid = t1.gid;//t1为girl的别名。
+-----+--------+------+--------+
| bid | bname  | gid  | gname  |
+-----+--------+------+--------+
| B   | 杨过   | B    | 小龙女 |
| C   | 陈冠希 | C    | 张柏芝 |
| D   | yw     | D    | mv     |
| A   | 屌丝   | NULL | NULL   |
+-----+--------+------+--------+


select gc.goods_name,gc.cat_name,gb.brand_name from 
goods 
left join category as gc
on goods.cat_id=category.cat_id
left join brand as gb
on goods.brand_id=brand.brand_id
where goods.cat_id = 4;
----------------------------------------------------------------------------------------------------

union查询:把多个sql的查询结果合并成一个结果集。必须满足表的列数相同。字段不同都可以,以前面表格的字段为新字段。完全相同的行将会被合并,union all就不会合并完全相同的行。
mysql> select * from study_new union all select * from m;
+----+----------+-------+-----+------------+
| id | username | class | sex | addtime    |
+----+----------+-------+-----+------------+
|  1 | 小王     |     1 | 男  | 0          |
|  2 | 小四     |     2 | 女  | 0          |
|  1 | 1        |     2 | 2:0 | 2006-05-21 |
|  2 | 2        |     3 | 1:2 | 2006-06-21 |
|  3 | 3        |     1 | 2:5 | 2006-06-25 |
|  4 | 2        |     1 | 3:2 | 2006-07-21 |
+----+----------+-------+-----+------------+
一、数学函数
abs(x)   返回x的绝对值
bin(x)   返回x的二进制(oct返回八进制,hex返回十六进制)
ceiling(x)   返回大于x的最小整数值
exp(x)   返回值e(自然对数的底)的x次方
floor(x)   返回小于x的最大整数值
greatest(x1,x2,...,xn)返回集合中最大的值
least(x1,x2,...,xn)      返回集合中最小的值
ln(x)                    返回x的自然对数
log(x,y)返回x的以y为底的对数
mod(x,y)                 返回x/y的模(余数)
pi()返回pi的值(圆周率)
rand()返回0到1内的随机值,可以通过提供一个参数(种子)使rand()随机数生成器生成一个指定的值。
round(x,y)返回参数x的四舍五入的有y位小数的值
sign(x) 返回代表数字x的符号的值
sqrt(x) 返回一个数的平方根
truncate(x,y)            返回数字x截短为y位小数的结果

二、聚合函数(常用于group by从句的select查询中)
avg(col)返回指定列的平均值
count(col)返回指定列中非null值的个数
min(col)返回指定列的最小值
max(col)返回指定列的最大值
sum(col)返回指定列的所有值之和
group_concat(col) 返回由属于一组的列值连接组合而成的结果

三、字符串函数
ascii(char)返回字符的ascii码值
bit_length(str)返回字符串的比特长度
concat(s1,s2...,sn)将s1,s2...,sn连接成字符串
concat_ws(sep,s1,s2...,sn)将s1,s2...,sn连接成字符串,并用sep字符间隔
insert(str,x,y,instr) 将字符串str从第x位置开始,y个字符长的子串替换为字符串instr,返回结果
find_in_set(str,list)分析逗号分隔的list列表,如果发现str,返回str在list中的位置
lcase(str)或lower(str) 返回将字符串str中所有字符改变为小写后的结果
left(str,x)返回字符串str中最左边的x个字符
length(s)返回字符串str中的字符数
ltrim(str) 从字符串str中切掉开头的空格
position(substr,str) 返回子串substr在字符串str中第一次出现的位置
quote(str) 用反斜杠转义str中的单引号
repeat(str,srchstr,rplcstr)返回字符串str重复x次的结果
reverse(str) 返回颠倒字符串str的结果
right(str,x) 返回字符串str中最右边的x个字符
rtrim(str) 返回字符串str尾部的空格
strcmp(s1,s2)比较字符串s1和s2
trim(str)去除字符串首部和尾部的所有空格
ucase(str)或upper(str) 返回将字符串str中所有字符转变为大写后的结果

四、日期和时间函数
curdate()或current_date() 返回当前的日期
curtime()或current_time() 返回当前的时间
date_add(date,interval int keyword)返回日期date加上间隔时间int的结果(int必须按照关键字进行格式化),如:selectdate_add(current_date,interval 6 month);
date_format(date,fmt)  依照指定的fmt格式格式化日期date值
date_sub(date,interval int keyword)返回日期date加上间隔时间int的结果(int必须按照关键字进行格式化),如:selectdate_sub(current_date,interval 6 month);
dayofweek(date)   返回date所代表的一星期中的第几天(1~7)
dayofmonth(date)  返回date是一个月的第几天(1~31)
dayofyear(date)   返回date是一年的第几天(1~366)
dayname(date)   返回date的星期名,如:select dayname(current_date);
from_unixtime(ts,fmt)  根据指定的fmt格式,格式化unix时间戳ts
hour(time)   返回time的小时值(0~23)
minute(time)   返回time的分钟值(0~59)
month(date)   返回date的月份值(1~12)
monthname(date)   返回date的月份名,如:select monthname(current_date);
now()    返回当前的日期和时间
quarter(date)   返回date在一年中的季度(1~4),如select quarter(current_date);
week(date)   返回日期date为一年中第几周(0~53)
year(date)   返回日期date的年份(1000~9999)
一些示例:
获取当前系统时间:select from_unixtime(unix_timestamp());
select extract(year_month from current_date);
select extract(day_second from current_date);
select extract(hour_minute from current_date);
返回两个日期值之间的差值(月数):select period_diff(200302,199802);
在mysql中计算年龄:
select date_format(from_days(to_days(now())-to_days(birthday)),'%y')+0 as age from employee;
这样,如果brithday是未来的年月日的话,计算结果为0。
下面的sql语句计算员工的绝对年龄,即当birthday是未来的日期时,将得到负值。
select date_format(now(), '%y') - date_format(birthday, '%y') -(date_format(now(), '00-%m-%d') <date_format(birthday, '00-%m-%d')) as age from employee

五、加密函数
aes_encrypt(str,key)  返回用密钥key对字符串str利用高级加密标准算法加密后的结果,调用aes_encrypt的结果是一个二进制字符串,以blob类型存储
aes_decrypt(str,key)  返回用密钥key对字符串str利用高级加密标准算法解密后的结果
decode(str,key)   使用key作为密钥解密加密字符串str
encrypt(str,salt)   使用unixcrypt()函数,用关键词salt(一个可以惟一确定口令的字符串,就像钥匙一样)加密字符串str
encode(str,key)   使用key作为密钥加密字符串str,调用encode()的结果是一个二进制字符串,它以blob类型存储
md5()    计算字符串str的md5校验和
password(str)   返回字符串str的加密版本,这个加密过程是不可逆转的,和unix密码加密过程使用不同的算法。
sha()    计算字符串str的安全散列算法(sha)校验和
示例:
select encrypt('root','salt');
select encode('xufeng','key');
select decode(encode('xufeng','key'),'key');#加解密放在一起
select aes_encrypt('root','key');
select aes_decrypt(aes_encrypt('root','key'),'key');
select md5('123456');
select sha('123456');

六、控制流函数
mysql有4个函数是用来进行条件操作的,这些函数可以实现sql的条件逻辑,允许开发者将一些应用程序业务逻辑转换到数据库后台。
mysql控制流函数:
case when[test1] then [result1]...else [default] end如果testn是真,则返回resultn,否则返回default
case [test] when[val1] then [result]...else [default]end  如果test和valn相等,则返回resultn,否则返回default
if(test,t,f)   如果test是真,返回t;否则返回f
ifnull(arg1,arg2) 如果arg1不是空,返回arg1,否则返回arg2
nullif(arg1,arg2) 如果arg1=arg2返回null;否则返回arg1
这些函数的第一个是ifnull(),它有两个参数,并且对第一个参数进行判断。如果第一个参数不是null,函数就会向调用者返回第一个参数;如果是null,将返回第二个参数。
如:select ifnull(1,2), ifnull(null,10),ifnull(4*null,'false');
nullif()函数将会检验提供的两个参数是否相等,如果相等,则返回null,如果不相等,就返回第一个参数。
如:select nullif(1,1),nullif('a','b'),nullif(2+3,4+1);
和许多脚本语言提供的if()函数一样,mysql的if()函数也可以建立一个简单的条件测试,这个函数有三个参数,第一个是要被判断的表达式,如果表达式为真,if()将会返回第二个参数,如果为假,if()将会返回第三个参数。
如:selectif(1<10,2,3),if(56>100,'true','false');
if()函数在只有两种可能结果时才适合使用。然而,在现实世界中,我们可能发现在条件测试中会需要多个分支。在这种情况下,mysql提供了case函数,它和php及perl语言的switch-case条件例程一样。
case函数的格式有些复杂,通常如下所示:
case [expression to be evaluated]
when [val 1] then [result 1]
when [val 2] then [result 2]
when [val 3] then [result 3]
......
when [val n] then [result n]
else [default result]
end
这里,第一个参数是要被判断的值或表达式,接下来的是一系列的when-then块,每一块的第一个参数指定要比较的值,如果为真,就返回结果。所有的when-then块将以else块结束,当end结束了所有外部的case块时,如果前面的每一个块都不匹配就会返回else块指定的默认结果。如果没有指定else块,而且所有的when-then比较都不是真,mysql将会返回null。
case函数还有另外一种句法,有时使用起来非常方便,如下:
case
when [conditional test 1] then [result 1]
when [conditional test 2] then [result 2]
else [default result]
end
这种条件下,返回的结果取决于相应的条件测试是否为真。
示例:
mysql>select case 'green'
     when 'red' then 'stop'
     when 'green' then 'go' end;
select case 9 when 1 then 'a' when 2 then 'b' else 'n/a' end;
select case when (2+2)=4 then 'ok' when(2+2)<>4 then 'not ok' end asstatus;
select name,if((isactive = 1),'已激活','未激活') as result fromuserlogininfo;
select fname,lname,(math+sci+lit) as total,
case when (math+sci+lit) < 50 then 'd'
when (math+sci+lit) between 50 and 150 then 'c'
when (math+sci+lit) between 151 and 250 then 'b'
else 'a' end
as grade from marks;
select if(encrypt('sue','ts')=upass,'allow','deny') as loginresultfrom users where uname = 'sue';#一个登陆验证

七、格式化函数
date_format(date,fmt)  依照字符串fmt格式化日期date值
format(x,y)   把x格式化为以逗号隔开的数字序列,y是结果的小数位数
inet_aton(ip)   返回ip地址的数字表示
inet_ntoa(num)   返回数字所代表的ip地址
time_format(time,fmt)  依照字符串fmt格式化时间time值
其中最简单的是format()函数,它可以把大的数值格式化为以逗号间隔的易读的序列。
示例:
select format(34234.34323432,3);
select date_format(now(),'%w,%d %m %y %r');
select date_format(now(),'%y-%m-%d');
select date_format(19990330,'%y-%m-%d');
select date_format(now(),'%h:%i %p');
select inet_aton('10.122.89.47');
select inet_ntoa(175790383);

八、类型转化函数
为了进行数据类型转化,mysql提供了cast()函数,它可以把一个值转化为指定的数据类型。类型有:binary,char,date,time,datetime,signed,unsigned
示例:
select cast(now() as signed integer),curdate()+0;
select 'f'=binary 'f','f'=cast('f' as binary);

九、系统信息函数
database()   返回当前数据库名
benchmark(count,expr)  将表达式expr重复运行count次
connection_id()   返回当前客户的连接id
found_rows()   返回最后一个select查询进行检索的总行数
user()或system_user()  返回当前登陆用户名
version()   返回mysql服务器的版本
示例:
select database(),version(),user();
selectbenchmark(9999999,log(rand()*pi()));#该例中,mysql计算log(rand()*pi())表达式9999999次。

 

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