- 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)
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