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|
