SQL select 查询语句,select distinct语句

select 语句

SELECT 语句用于从数据库中选取数据。结果被存储在一个结果表中，称为结果集。

SQL SELECT 语法

SELECT column_name,column_name
FROM table_name;
与
SELECT * FROM table_name;

实例

实例数据：

+----+--------------+---------------------------+-------+---------+
| id | name         | url                       | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1  | Google       | https://www.google.cm/    | 1     | USA     |
| 2  | 淘宝          | https://www.taobao.com/   | 13    | CN      |
| 3  | Facebook     | https://www.facebook.com/ | 3     | USA     |
| 4  | 百度     | https://www.baidu.com/ | 4     | CN     |
+----+--------------+---------------------------+-------+---------+

查询选取列数据

sql语句从表里读取 "name","country"列数据

select name,country from website;

结果：

+----+---------+
 | name          | country |
+----+------------+
| Google      | USA     |
| 淘宝          | CN      |
| Facebook      | USA     |
| 百度     | CN     |
+----+---------+

查询所有数据

mysql > select * from  website;

+----+--------------+---------------------------+-------+---------+
| id | name         | url                       | alexa | country |
+----+--------------+---------------------------+-------+---------+
| 1  | Google       | https://www.google.cm/    | 1     | USA     |
| 2  | 淘宝          | https://www.taobao.com/   | 13    | CN      |
| 3  | Facebook     | https://www.facebook.com/ | 3     | USA     |
| 4  | 百度     | https://www.baidu.com/ | 4     | CN     |
+----+--------------+---------------------------+-------+---------+

SELECT DISTINCT 语句

在表中，一个列可能会包含多个重复值，有时您也许希望仅仅列出不同（distinct）的值。

DISTINCT 关键词用于返回唯一不同的值。

SQL SELECT DISTINCT 语法

SELECT DISTINCT column_name,column_name
FROM table_name;

从表数据中可以看到 country字段有很多重复值，去掉country列的重复值如何查询呢

sql > SELECT DISTINCT country FROM website;