MairDB计算和别名 (二)

第一部分计算和别名

1.1 计算字段

之前我们的操作都是直接筛选,那么计算字段考虑到了其他的可能,比如

1.选择特定的单词或数值;

2.对单个或多个列进行计算;

3.把列和直接量组合在一起.

1.2直接量

MariaDB [study_db]> SELECT 'BOOK_NAME: ', book_name FROM books;

+-------------+------------------+

|BOOK_NAME:|book_name|

+-------------+------------------+

|BOOK_NAME:|跟我学MariaDB|

|BOOK_NAME:|跟我学NodeJS|

|BOOK_NAME:|HTTP权威指南|

1.3 算术运算

列直接的操作: + - * /等;直接上例子,两个列相乘,

MariaDB[study_db]>SELECTclassify, book_price, 'result: ', classify * book_pricefrombooks;

+----------+------------+----------+-----------------------+

| classify | book_price | result:  | classify * book_price |

+----------+------------+----------+-----------------------+

|18|44.00| result:  |792|

|18|44.44| result:  |799.92|

|18|79.00| result:  |1422|

|9|26.00| result:  |234|

1.4 连接字段

连接(CONCAT)就是将不同的列组合在一起;

MariaDB [study_db]> SELECT book_author_name, book_name, 'RESULT:', CONCAT(book_author_name,'出的�  是:  ',  book_name) FROM books;

+------------------+------------------+---------+-----------------------------------------------------------+

|book_author_name|book_name|RESULT:|CONCAT(book_author_name,'出的�  是:  ',  book_name)|

+------------------+------------------+---------+-----------------------------------------------------------+

|hhw|跟我学MariaDB|RESULT:|hhw出的�  是:跟我学MariaDB|

|hhw|跟我学NodeJS|RESULT:|hhw出的�  是:跟我学NodeJS|

|O'Reilly|HTTP权威指南|RESULT:|O'Reilly出的�  是:  HTTP权威指南|

1.5列/表的别名--AS

AS关键词,将旧列以新列名显示;

MariaDB [study_db]> SELECT book_id , book_id AS 'id' FROM books;

+---------+----+

|book_id|id|

+---------+----+

|1|1|

|2|2|

|3|3|