MySQL的使用和学习(一)

1.查看mysql版本, 当前用户和当前的时间(不区分大小写)

mysql> select user(),version(),currenT_date,now(),sysDate(),CURDATE();
+----------------+-------------------------+--------------+---------------------+---------------------+------------+
| user()         | version()               | currenT_date | now()               | sysDate()           | CURDATE()  |
+----------------+-------------------------+--------------+---------------------+---------------------+------------+
| root@localhost | 5.5.38-0ubuntu0.14.04.1 | 2014-10-27   | 2014-10-27 15:22:48 | 2014-10-27 15:22:48 | 2014-10-27 |
+----------------+-------------------------+--------------+---------------------+---------------------+------------+
1 row in set (0.00 sec)

2.mysql可以作为一个简单的计算器(a simple calculator )

mysql> select cos(PI()/4),(4+2)/3 As basicOperation;
+--------------------+----------------+
| cos(PI()/4)        | basicOperation |
+--------------------+----------------+
| 0.7071067811865476 |         2.0000 |
+--------------------+----------------+
1 row in set (0.00 sec)

3.数据库的创建和使用  

mysql> CREATE DATABASE mytest;
Query OK, 1 row affected (0.02 sec)
//使用数据库
mysql> use mytest;
Database changed
//删除数据库
mysql> drop database mytest;
Query OK, 0 rows affected (0.17 sec)

　　每次登陆都必须选择创建的数据库来开启一次mysql会话，可以使用下面的方式直接指定使用的数据库

user@user-xubuntu:~$ mysql -h 127.0.0.1 -u root -p newtest
Enter password: 

　　-h 指定本机IP或者是localhost或者是127.0.0.1； 
　　newtest 表示指定使用数据库的名字；
　　可以将密码直接跟在-p后面，-p和你所输入的密码之间不能有空格，但是不建议这么做，这样做就将你的密码暴露出去。

4.表的创建和使用

mysql> create table person(
    -> name varchar(30),
    -> password varchar(32),
    -> email varchar(50),
    -> age int，
    -> createTime date,
    -> deleteTime date
    -> );
Query OK, 0 rows affected (0.12 sec)

mysql> show tables;
+-------------------+
| Tables_in_newtest |
+-------------------+
| person            |
| user              |
+-------------------+
2 rows in set (0.00 sec)

mysql> desc person;
+------------+-------------+------+-----+---------+-------+
| Field      | Type        | Null | Key | Default | Extra |
+------------+-------------+------+-----+---------+-------+
| name       | varchar(30) | YES  |     | NULL    |       |
| password   | varchar(32) | YES  |     | NULL    |       |
| email      | varchar(50) | YES  |     | NULL    |       |
| age        | int(11)     | YES  |     | NULL    |       |
| createTime | date        | YES  |     | NULL    |       |
| deleteTime | date        | YES  |     | NULL    |       |
+------------+-------------+------+-----+---------+-------+
6 rows in set (0.00 sec)

5.往空数据表中添加数据的两种方式
　　 针对上面创建的person表的字段，我们向person.txt填入的数据格式如下：
 　 evan  abc123_     \N        14    \N       \N         (\N表示值为null)

mysql> LOAD DATA LOCAL INFILE '/{path}/people.txt' INTO TABLE person  
//mysql 5.7的版本支持
<pre class="programlisting">
//If you created the file on Windows with an editor that uses <code class="literal">\r\n</code> as a line terminator, you should use this statement instead:
mysql> LOAD DATA LOCAL INFILE '/{path}/people.txt' INTO TABLE person
    -> LINES TERMINATED BY '\r\n';

mysql> source /home/user/Desktop/newFile/person.sql;
//使用Source的方式导入.sql文件 

6.表的创建和简单查询操作以及对时间的操作

//基本的模版
SELECT what_to_select
FROM which_table
WHERE conditions_to_satisfy;

　　基本操作：插入语句， 搜索所有的数据，根据条件搜索特定的行，根据条件搜索特定的列。

mysql> insert into person values('evan','123456','[email protected]',21);
Query OK, 1 row affected (0.03 sec)
//搜索所有的数据
mysql> select * from person;
+------+----------+-----------------+------+------------+------------+
| name | password | email           | age  | createTime | deleteTime |
+------+----------+-----------------+------+------------+------------+
| evan | 123456   | [email protected] |   21 | 2010-01-01 | 2010-10-10 |
| ovan | 123456   | [email protected] |   23 | 2012-03-15 | NULL       |
| kvan | 123456   | [email protected] |   22 | 2012-03-18 | NULL       |
| kvan | 123456   | [email protected] |   18 | 2012-02-15 | NULL       |
| kvan | 123456   | [email protected] |   26 | 2012-04-18 | NULL       |
+------+----------+-----------------+------+------------+------------+
5 rows in set (0.00 sec)
//根据条件搜索特定的行
mysql> select * from person where name = 'evan';
+------+----------+-----------------+------+------------+------------+
| name | password | email           | age  | createTime | deleteTime |
+------+----------+-----------------+------+------------+------------+
| evan | 123456   | [email protected] |   21 | 2010-01-01 | 2010-10-10 |
+------+----------+-----------------+------+------------+------------+
1 row in set (0.00 sec)
//根据条件搜索特定的列
mysql> select name,password from person where name = 'evan';
+------+----------+
| name | password |
+------+----------+
| evan | 123456   |
+------+----------+
1 row in set (0.00 sec)
//对搜索记录进行降序排序，也可对多列进行排序，desc只对它最近的那个列名起作用
mysql> select * from person order by age desc;
+------+----------+-----------------+------+------------+------------+
| name | password | email           | age  | createTime | deleteTime |
+------+----------+-----------------+------+------------+------------+
| kvan | 123456   | [email protected] |   26 | 2012-04-18 | NULL       |
| ovan | 123456   | [email protected] |   23 | 2012-03-15 | NULL       |
| kvan | 123456   | [email protected] |   22 | 2012-03-18 | NULL       |
| evan | 123456   | [email protected] |   21 | 2010-01-01 | 2010-10-10 |
| kvan | 123456   | [email protected] |   18 | 2012-02-15 | NULL       |
+------+----------+-----------------+------+------------+------------+
5 rows in set (0.00 sec)
//CURDATE(获取当前的时间)，返回当前时间； 

//TIMESTAMPDIFF(xxx,xxx,xxx)可以用来计算两个时间段的年/月/日等的天数差，比如计算Q龄。还可以对下面的durationTime进行排序！
mysql> select name,age,createTime,CURDATE(),TIMESTAMPDIFF(YEAR,createTime,CURDATE()) AS durationTime from person order by age; 
+------+------+------------+------------+--------------+
| name | age  | createTime | CURDATE()  | durationTime |
+------+------+------------+------------+--------------+
| kvan |   18 | 2012-02-15 | 2014-10-27 |            2 |
| evan |   21 | 2010-01-01 | 2014-10-27 |            4 |
| kvan |   22 | 2012-03-18 | 2014-10-27 |            2 |
| ovan |   23 | 2012-03-15 | 2014-10-27 |            2 |
| kvan |   26 | 2012-04-18 | 2014-10-27 |            2 |
+------+------+------------+------------+--------------+
5 rows in set (0.00 sec)
//month函数的使用，获取指定时间的月份; MONTH()函数返回的数字是1-12;

//MOD(MONTH(CURDATE()),12) 返回的数据是0－11;
mysql> select name,age,createTime  from person where month(createTime)=2; 
+------+------+------------+
| name | age  | createTime |
+------+------+------------+
| kvan |   18 | 2012-02-15 |
+------+------+------------+
1 row in set (0.01 sec)
//还有很多关于操作时间的函数，再次不一一赘述，可查找官网了解更多。

7.mysql对空值的操作以及mysql的模糊匹配

mysql> select 1 is  null,1 is not null, 0 is null, 0 is not null, '' is null, '' is not null,  1 = null, 1 <> null, 1 > null, 1 < null;
+------------+---------------+-----------+---------------+------------+----------------+----------+-----------+----------+----------+
| 1 is  null | 1 is not null | 0 is null | 0 is not null | '' is null | '' is not null | 1 = null | 1 <> null | 1 > null | 1 < null |
+------------+---------------+-----------+---------------+------------+----------------+----------+-----------+----------+----------+
|          0 |             1 |         0 |             1 |          0 |              1 |     NULL |      NULL |     NULL |     NULL |
+------------+---------------+-----------+---------------+------------+----------------+----------+-----------+----------+----------+
1 row in set (0.00 sec)

　　可以看出NULL意味着“失踪的未知的值”，在mysql中，0 or NULL意味着 false ，anything else 意味着 true；
　　mysql> select * from person order by null desc;
　　mysql> select * from person order by null; 　
　　mysql> select * from person；//三者输出结果一致

　　SQL模式匹配中，使用“_”匹配任何单个字符“%”匹配任意数量的字符（包括零个字符）

mysql> select * from person where name LIKE '__p%m%q__';
//表示，匹配任意两个单字符开头，第三个字符是p,倒数第三个字符是q,最后两个字符是任意两个单字符，字符p和字符q之间包含字符m的任意符合条件的字符串。

　　使用REGEXP和not REGEXP 进行sql匹配:

• "." 匹配任何单个字符。
• [ABC],字符数组匹配任何在里面的单个字符。 [a-z]匹配小写字母，[0-9]匹配任意的数字
• "*" 表示匹配0个或多个，例如[0-9]*,匹配任意数量的数字，".*" 匹配anything
• "^" 匹配以......开头，例如 "^b" 表示匹配的内容以字符b开头，"$"匹配以......结束，例如"mn$" 表示匹配的内容以字符串mn结束

mysql> select * from person where name REGEXP '^.{4}$';
+------+----------+-----------------+------+------------+------------+
| name | password | email           | age  | createTime | deleteTime |
+------+----------+-----------------+------+------------+------------+
| evan | 123456   | [email protected] |   21 | 2010-01-01 | 2010-10-10 |
| ovan | 123456   | [email protected] |   23 | 2012-03-15 | NULL       |
| kvan | 123456   | [email protected] |   18 | 2012-02-15 | NULL       |
| kvan | 123456   | [email protected] |   26 | 2012-04-18 | NULL       |
+------+----------+-----------------+------+------------+------------+
4 rows in set (0.00 sec)
mysql> select * from person where name REGEXP '^....$';//和上面的查询语句是等价的
+------+----------+-----------------+------+------------+------------+
| name | password | email           | age  | createTime | deleteTime |
+------+----------+-----------------+------+------------+------------+
| evan | 123456   | [email protected] |   21 | 2010-01-01 | 2010-10-10 |
| ovan | 123456   | [email protected] |   23 | 2012-03-15 | NULL       |
| kvan | 123456   | [email protected] |   18 | 2012-02-15 | NULL       |
| kvan | 123456   | [email protected] |   26 | 2012-04-18 | NULL       |
+------+----------+-----------------+------+------------+------------+
4 rows in set (0.00 sec)

8. mysql的count()函数的使用

mysql> select count(*) from person;
+----------+
| count(*) |
+----------+
|        5 |
+----------+
1 row in set (0.43 sec)

　　count(*)是对行数目进行计数。

　　count(item)，其中item表示列名，count(item)表示对列中不为空的行进行计数。

　　在不加where条件限制的情况下，COUNT(*)与COUNT(COL)基本可以认为是等价的；但是在有Where条件限制的情况下，count(*)会比count(col)快非常多。

　　网上关于count的优化策略，对于MYISAM表来说：
　　1.任何情况下SELECT COUNT(*) FROM tablename是最优选择；
　　2.尽量减少SELECT COUNT(*) FROM tablename WHERE COL = 'value' 这种查询；
　　3.杜绝SELECT COUNT(COL) FROM tablename WHERE COL2 = 'value' 的出现。

mysql> select name,age,count(*) from person;
+------+------+----------+
| name | age  | count(*) |
+------+------+----------+
| evan |   21 |        5 |
+------+------+----------+
1 row in set (0.00 sec)

mysql> set SQL_MODE = "ONLY_FULL_GROUP_BY";
Query OK, 0 rows affected (0.06 sec)

mysql> select name,age,count(*) from person;
ERROR 1140 (42000): Mixing of GROUP columns (MIN(),MAX(),COUNT(),...) with no GROUP columns is illegal if there is no GROUP BY clause
mysql> set sql_mode = "";
Query OK, 0 rows affected (0.00 sec)

mysql> select name,age,count(*) from person;
+------+------+----------+
| name | age  | count(*) |
+------+------+----------+
| evan |   21 |        5 |
+------+------+----------+
1 row in set (0.00 sec)

//默认sql_mode为空，select name,age,count(*) from person;这个查询即使不加group by子句也不会报错，但是当我们设置sql_mode的模式之后，这个查询会报错。很明显需要加上group by name,age子句

//有人这样实现sql语句,在函数中添加一个三目运算作为参数判断是否为空。
SELECT owner, COUNT(*) AS total_pets, COUNT(IF(dead = '0', 1, NULL)) AS alive_pets, COUNT(IF(dead = '1', 1, NULL)) AS dead_pets FROM pet GROUP BY owner;