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# 基本的SQL语句

* **+** 号的使用

  ```
  SELECT  'a11'+11;

  ## 11

  SELECT  '1111'+11;

  ## 1122

  SELECT  null+'aaa';

  ## 为空

  SELECT  null+null;

  ##  为空
  ```
* 调节运输符
  * `<`
  * `>`
  * `<>` 或者`!=`
  * `<=` `>=`&#x20;
* 逻辑运算符
  * `and` 或者`&&`
  * `or`或者`||`
  * `not`或者`!`

    ```
    # id 小于5 并且大于2
    SELECT * FROM websites WHERE  NOT(id>=5) AND id>2 ;
    ```
* 模糊查询
  * like 一般与通配符搭配使用

    `%`匹配任意多个字符，并且`like`不区分大小写

    `_`匹配任意一个字符,类似占位符的功能

    ```
    SELECT * FROM websites WHERE name like 'g%' ;
    # alexa    country    id    name    url
    # 1    USA    1    Google    https://www.google.cm/
    ```
*