MySQL学习笔记（五）

1、拼接字段

　　拼接 (Concatenate)：将值联结到一起构成单个值。

 1 mysql> SELECT Concat(vend_name, '(', vend_country, ')')

 2     -> FROM vendors

 3     -> ORDER BY vend_name;

 4 +-------------------------------------------+

 5 | Concat(vend_name, '(', vend_country, ')') |

 6 +-------------------------------------------+

 7 | ACME(USA)                                 |

 8 | Anvils R Us(USA)                          |

 9 | Furball Inc.(USA)                         |

10 | Jet Set(England)                          |

11 | Jouets Et Ours(France)                    |

12 | LT Supplies(USA)                          |

13 +-------------------------------------------+

14 6 rows in set (0.01 sec)

2、使用别名

　　别名是一个字段或值的替换名

 1 mysql> SELECT Concat(RTrim(vend_name), '(', RTrim(vend_country), ')') AS vend_title

 2     -> FROM vendors

 3     -> ORDER BY vend_name;

 4 +------------------------+

 5 | vend_title             |

 6 +------------------------+

 7 | ACME(USA)              |

 8 | Anvils R Us(USA)       |

 9 | Furball Inc.(USA)      |

10 | Jet Set(England)       |

11 | Jouets Et Ours(France) |

12 | LT Supplies(USA)       |

13 +------------------------+

14 6 rows in set (0.00 sec)

3、执行算术计算

 1 mysql> SELECT prod_id,

 2     ->        quantity,

 3     ->        item_price,

 4     ->        quantity*item_price AS price

 5     -> FROM orderitems

 6     -> WHERE order_num =20005;

 7 +---------+----------+------------+-------+

 8 | prod_id | quantity | item_price | price |

 9 +---------+----------+------------+-------+

10 | ANV01   |       10 |       5.99 | 59.90 |

11 | ANV02   |        3 |       9.99 | 29.97 |

12 | TNT2    |        5 |      10.00 | 50.00 |

13 | FB      |        1 |      10.00 | 10.00 |

14 +---------+----------+------------+-------+

15 4 rows in set (0.00 sec)

4、使用函数

　　文本处理函数

 1 mysql> SELECT vend_name, Upper(vend_name) AS vend_name_upcase

 2     -> FROM vendors

 3     -> ORDER BY vend_name;

 4 +----------------+------------------+

 5 | vend_name      | vend_name_upcase |

 6 +----------------+------------------+

 7 | ACME           | ACME             |

 8 | Anvils R Us    | ANVILS R US      |

 9 | Furball Inc.   | FURBALL INC.     |

10 | Jet Set        | JET SET          |

11 | Jouets Et Ours | JOUETS ET OURS   |

12 | LT Supplies    | LT SUPPLIES      |

13 +----------------+------------------+

14 6 rows in set (0.00 sec)

　　常用的文本处理函数：Left()　　返回串左边的字符，Length()　　返回串的长度，Locate()　　找出串的一个字串，

　　Lower()　　将串转换为小写，LTrim()　　去掉串左边的空格，Right()　　返回串右边的字符，RTrim()　　去掉串

　　右边的空格，Soundex()　　返回串的SOUNDEX值，SubString()　　返回子串的字符，Upper()　　将串转换为大写。

1 mysql> SELECT cust_name, cust_contact

2     -> FROM customers

3     -> WHERE Soundex(cust_contact) = Soundex('Y Lie');

4 +-------------+--------------+

5 | cust_name   | cust_contact |

6 +-------------+--------------+

7 | Coyote Inc. | Y Lee        |

8 +-------------+--------------+

9 1 row in set (0.00 sec)

　　日期和时间处理函数

　　常用日期和时间处理函数：AddDate()　　增加一个日期（天、周等），AddTime　　增加一个时间（时，分等），CurDate()

　　　　返回当前日期，CurTime()　　返回当前时间，Date()　　返回日期时间的日期部分，DateDiff()　　计算两个日期之差，

　　Date_Add()　　高度灵活的日期运算函数，Date_Format()　　返回一个格式化的日期或时间串，Day()　　返回一个日期的天数部分

　　DayOfWeek()　　对于一个日期，返回对应的星期几，Hour()　　返回一个时间的小时部分，Minute()　　返回一个时间的分钟部分，

　　Month()　　返回一个时间的月份部分，Now()　　返回当前日期和时间Second()　　返回一个时间的秒部分，Time()　　返回一个日期

　　时间的时间部分，Year()　　返回一个时间的年份部分

1 mysql> SELECT cust_id, order_num

2     -> FROM orders

3     -> WHERE Date(order_date) = '2005-09-01';

4 +---------+-----------+

5 | cust_id | order_num |

6 +---------+-----------+

7 |   10001 |     20005 |

8 +---------+-----------+

9 1 row in set (0.00 sec)

 1 mysql> SELECT *

 2 FROM orders

 3 WHERE Year(order_date) = 2005 AND Month(order_date) = 9;

 4 +-----------+---------------------+---------+

 5 | order_num | order_date          | cust_id |

 6 +-----------+---------------------+---------+

 7 |     20005 | 2005-09-01 00:00:00 |   10001 |

 8 |     20006 | 2005-09-12 00:00:00 |   10003 |

 9 |     20007 | 2005-09-30 00:00:00 |   10004 |

10 +-----------+---------------------+---------+

11 3 rows in set (0.00 sec)

5、汇总数据

　　聚集函数 (aggregate function) 运行在行组上，计算和返回单个值的函数。

　　AVG()　　　　　　　　　　返回某列的平均值

　　COUNT()　　　　　　　　  返回某列的函数

　　MAX()　　　　　　　　　　返回某列的最大值

　　MIN()　　　　　　　　　　 返回某列的最小值

　　SUM()　　　　　　　　　   返回某列值之和

 1 mysql> SELECT AVG(price) AS avg_price

 2     -> FROM products;

 3 ERROR 1054 (42S22): Unknown column 'price' in 'field list'

 4 mysql> SELECT AVG(prod_price) AS avg_price FROM products;

 5 +-----------+

 6 | avg_price |

 7 +-----------+

 8 | 16.133571 |

 9 +-----------+

10 1 row in set (0.00 sec)

11 

12 mysql> SELECT COUNT(*) AS count

13     -> FROM customers;

14 +-------+

15 | count |

16 +-------+

17 |     5 |

18 +-------+

19 1 row in set (0.00 sec)

20 

21 mysql> SELECT * FROM customers;

22 +---------+----------------+---------------------+-----------+------------+----------+--------------+--------------+---------------------+

23 | cust_id | cust_name      | cust_address        | cust_city | cust_state | cust_zip | cust_country | cust_contact | cust_email          |

24 +---------+----------------+---------------------+-----------+------------+----------+--------------+--------------+---------------------+

25 |   10001 | Coyote Inc.    | 200 Maple Lane      | Detroit   | MI         | 44444    | USA          | Y Lee        | ylee@coyote.com     |

26 |   10002 | Mouse House    | 333 Fromage Lane    | Columbus  | OH         | 43333    | USA          | Jerry Mouse  | NULL                |

27 |   10003 | Wascals        | 1 Sunny Place       | Muncie    | IN         | 42222    | USA          | Jim Jones    | rabbit@wascally.com |

28 |   10004 | Yosemite Place | 829 Riverside Drive | Phoenix   | AZ         | 88888    | USA          | Y Sam        | sam@yosemite.com    |

29 |   10005 | E Fudd         | 4545 53rd Street    | Chicago   | IL         | 54545    | USA          | E Fudd       | NULL                |

30 +---------+----------------+---------------------+-----------+------------+----------+--------------+--------------+---------------------+

31 5 rows in set (0.01 sec)

32 

33 mysql> SELECT MAX(cust_zip) AS max_zip

34     -> FROM customers;

35 +---------+

36 | max_zip |

37 +---------+

38 | 88888   |

39 +---------+

40 1 row in set (0.00 sec)

 1 mysql> SELECT SUM(order_item) AS sum_order

 2     -> FROM orderitems;

 3 +-----------+

 4 | sum_order |

 5 +-----------+

 6 |        23 |

 7 +-----------+

 8 1 row in set (0.00 sec)

 9 

10 mysql> SELECT SUM(item_price*quantity) AS sum_price FROM orderitems;

11 +-----------+

12 | sum_price |

13 +-----------+

14 |   1368.34 |

15 +-----------+

16 1 row in set (0.00 sec)

 1 mysql> SELECT COUNT(*) AS num_items,

 2                                MIN(prod_price) AS min_price,        

 3                                MAX(prod_price) AS max_price,        

 4                                SUM(prod_price) AS sum,       

 5                                AVG(DISTINCT prod_price) AS avg_price FROM products;

 6 +-----------+-----------+-----------+--------+-----------+

 7 | num_items | min_price | max_price | sum    | avg_price |

 8 +-----------+-----------+-----------+--------+-----------+

 9 |        14 |      2.50 |     55.00 | 225.87 | 17.780833 |

10 +-----------+-----------+-----------+--------+-----------+

11 1 row in set (0.29 sec)

12