Mysql的查询语句的使用

1. 简单查询

查询所有字段: SELECT * FROM 表名
查询特定字段: SELECT 字段列表 FROM 表名

2. 查询显示行号

- 在字段列表中加入(@rownum := @rownum + 1) AS rownum
- 在FROM子句中加入 (select @rownum:=0)r
- 示例如下:

select (@rownum := @rownum + 1) AS rownum,book_name from (select @rownum:=0)r, book;
查询结果:
+--------+--------------------+
| rownum | book_name |
+--------+--------------------+
| 1 | 数据库系统设计 |
| 2 | 从实模式到保护模式 |
+--------+--------------------+

3. 在查询语句中可以有算术表达式，它将形成一个新列，用于显示计算的结果，通常称为计算列。

实例:select book_price/2 AS new_book_price,author_id from book;

4. 在SELECT子句中使用AS关键字给某个字段起别名

- 基本使用

mysql> select book_price AS New_book_price,author_id as my_idea from book;
+----------------+---------+
| New_book_price | my_idea |
+----------------+---------+
| 214            | 1 |
| 34.5           | 2 |
+----------------+---------+
2 rows in set (0.07 sec)

- 别名中如果出现空格或者()之类的敏感字符,需要使用双引号将别名引起

mysql> select book_name as "the new book name" from book;
+--------------------+
| the new book name |
+--------------------+
| 数据库系统设计 |
| 从实模式到保护模式 |
+--------------------+
2 rows in set (0.06 sec)

mysql> select book_name as the new book name from book;
1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'new book name from book' at line 1

5. 使用DISTINCT消除重复行显示

mysql> select volume FROM book;
+--------+
| volume |
+--------+
| 45 |
| 45 |
+--------+
2 rows in set (0.07 sec)
 
mysql> select DISTINCT volume FROM book;
+--------+
| volume |
+--------+
| 45 |
+--------+
1 row in set (0.06 sec)

6. 查询结果的排序,order by 字段 (asc,desc),asc表示升序,desc表示降序,如果不指明的话,就会默认使用升序

mysql> SELECT * FROM author ORDER BY id;
+----+--------------+---------+
| id | name | address |
+----+--------------+---------+
| 1 | chenchen | |
| 2 | chenqingyang | |
| 5 | 666 | 广州市 |
| 14 | gafv | gfgd |
+----+--------------+---------+
4 rows in set (0.05 sec)
 
mysql> select * from author order by id desc;
+----+--------------+---------+
| id | name | address |
+----+--------------+---------+
| 14 | gafv | gfgd |
| 5 | 666 | 广州市 |
| 2 | chenqingyang | |
| 1 | chenchen | |
+----+--------------+---------+
4 rows in set (0.05 sec)

7. 需要注意的是,在order by子句中,如果该字段是字符类型的话,那将使用字符表的顺序来进行比较

mysql> select * from author order by name ;
+----+--------------+---------+
| id | name | address |
+----+--------------+---------+
| 1 | chenchen | |
| 2 | chenqingyang | |
| 14 | gafv | gfgd |
+----+--------------+---------+
3 rows in set (0.05 sec)

8. 索引的创建与删除

- 创建: CREATE INDEX 索引名 on table_name(字段名)
- 删除: DROP INDEX 索引名 on table_name

创建索引示例:
mysql> create index name_index on author(name);
Query OK, 0 rows affected (0.05 sec)
Records: 0 Duplicates: 0 Warnings: 0
 
删除代码:
mysql> DROP index name_index on author;
Query OK, 0 rows affected (0.02 sec)
Records: 0 Duplicates: 0 Warnings: 0

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