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

       创建代码:
       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
     

本文由博客一文多发平台 OpenWrite 发布！