SQL第7课：创建计算字段

• 字段（field）：基本与column同意。通常表述为计算字段。
• 拼接（concatenate）：将值联结到一起构成单个值。
• 别名（alias）：一个字段或值的替换名。有时也叫 导出列。

拼接字段

mysql> select Concat(vend_name,'(',vend_country,')')
    -> from Vendors
    -> order by vend_name;
+----------------------------------------+
| Concat(vend_name,'(',vend_country,')') |
+----------------------------------------+
| Bear Emporium(USA)                     |
| Bears R Us(USA)                        |
| Doll House Inc.(USA)                   |
| Fun and Games(England)                 |
| Furball Inc.(USA)                      |
| Jouets et ours(France)                 |
+----------------------------------------+
6 rows in set (0.00 sec)

RTRIM() LTRIM() TRIM() 去掉字符串相应位置的空格。

使用别名

mysql> select Concat(vend_name,'(',vend_country,')')
    -> AS vend_title
    -> from Vendors
    -> order by vend_name;
+------------------------+
| vend_title             |
+------------------------+
| Bear Emporium(USA)     |
| Bears R Us(USA)        |
| Doll House Inc.(USA)   |
| Fun and Games(England) |
| Furball Inc.(USA)      |
| Jouets et ours(France) |
+------------------------+
6 rows in set (0.00 sec)

我拼接了一个字段，这个字段放在了哪个表中呢？

执行计算字段

mysql>  select prod_id,quantity,item_price
    -> from OrderItems
    -> where order_num = 20008;
+---------+----------+------------+
| prod_id | quantity | item_price |
+---------+----------+------------+
| RGAN01  |        5 |       4.99 |
| BR03    |        5 |      11.99 |
| BNBG01  |       10 |       3.49 |
| BNBG02  |       10 |       3.49 |
| BNBG03  |       10 |       3.49 |
+---------+----------+------------+
5 rows in set (0.00 sec)
mysql> select prod_id,quantity,item_price,
    -> quantity*item_price AS expanded_price
    -> from OrderItems
    ->  where order_num = 20008;
+---------+----------+------------+----------------+
| prod_id | quantity | item_price | expanded_price |
+---------+----------+------------+----------------+
| RGAN01  |        5 |       4.99 |          24.95 |
| BR03    |        5 |      11.99 |          59.95 |
| BNBG01  |       10 |       3.49 |          34.90 |
| BNBG02  |       10 |       3.49 |          34.90 |
| BNBG03  |       10 |       3.49 |          34.90 |
+---------+----------+------------+----------------+

mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2018-03-10 09:51:19 |
+---------------------+
1 row in set (0.00 sec)

mysql> select trim('abc');
+-------------+
| trim('abc') |
+-------------+
| abc         |
+-------------+
1 row in set (0.00 sec)