MySQL学习足迹记录08--创建计算字段--Concat(),AS
本文所用到的表格数据
mysql> SELECT * FROM vendors; +---------+----------------+-----------------+-------------+------------+----------+--------------------------------------------+ | vend_id | vend_name | vend_address | vend_city | vend_state | vend_zip | vend_country | +---------+----------------+-----------------+-------------+------------+----------+---------------------------------------------+ | 1001 | Anvils R Us | 123 Main Street | Southfield | MI | 48075 | USA | | 1002 | LT Supplies | 500 Park Street | Anytown | OH | 44333 | USA | | 1003 | ACME | 555 High Street | Los Angeles | CA | 90046 | USA | | 1004 | Furball Inc. | 1000 5th Avenue | New York | NY | 11111 | USA | | 1005 | Jet Set | 42 Galaxy Road | London | NULL | N16 6PS | England | | 1006 | Jouets Et Ours | 1 Rue Amusement | Paris | NULL | 45678 | France | +---------+----------------+-----------------+-------------+------------+----------+-----------------------------------------------+ 6 rows in set (0.00 sec)
1.计算字段并不实际存在于
数据库表中，计算字段是在运行时在SELECT语句内创建的。
2.拼接字段Concat()
*拼接：将值连结到一起构成单个值
拼接两个列
eg: mysql> SELECT Concat(vend_name,'(',vend_country,')') FROM vendors -> ORDER BY vend_name; +-------------------------------------------------------+ | Concat(vend_name,'(',vend_country,')') | +--------------------------------------------------------+ | ACME(USA) | | Anvils R Us(USA) | | Furball Inc.(USA) | | Jet Set(England) | | Jouets Et Ours(France) | | LT Supplies(USA) | +---------------------------------------------------------+ 6 rows in set (0.00 sec)
3.删除右侧多余的空格RTrim()
删除左侧多余的空格LTrim()
删除两侧多余的空格Trim()
以RTrim()为例：
eg:
mysql> SELECT Concat(RTrim(vend_name),'(',RTrim(vend_country),')')
-> FROM vendors
-> ORDER BY vend_name;
+------------------------------------------------------+
| Concat(RTrim(vend_name),'(',RTrim(vend_country),')') |
+------------------------------------------------------+
| ACME(USA) |
| Anvils R Us(USA) |
| Furball Inc.(USA) |
| Jet Set(England) |
| Jouets Et Ours(France) |
| LT Supplies(USA) |
+------------------------------------------------------+
6 rows in set (0.00 sec)
4.使用别名（alias),关键字AS
eg:
mysql> SELECT Concat(Trim(vend_name),'(',Trim(vend_country),')')
-> AS vend_title
-> FROM vendors
-> ORDER BY vend_name;
+------------------------+
| vend_title |
+------------------------+
| ACME(USA) |
| Anvils R Us(USA) |
| Furball Inc.(USA) |
| Jet Set(England) |
| Jouets Et Ours(France) |
| LT Supplies(USA) |
+------------------------+
6 rows in set (0.00 sec)
5.执行算术计算
先检索订单号20005中的所有物品
eg:
mysql> SELECT prod_id,quantity,item_price
-> FROM orderitems
-> WHERE order_num = 20005;
+---------+----------+------------+
| prod_id | quantity | item_price |
+---------+----------+------------+
| ANV01 | 10 | 5.99 |
| ANV02 | 3 | 9.99 |
| TNT2 | 5 | 10.00 |
| FB | 1 | 10.00 |
+---------+----------+------------+
4 rows in set (0.00 sec)
再汇总物品的价格（单价*数量）
eg:
mysql> SELECT prod_id,quantity,item_price,
-> quantity*item_price AS expanded_price
-> FROM orderitems
-> WHERE order_num = 20005;
+---------+----------+------------+----------------+
| prod_id | quantity | item_pric