=======
常用sql
=======

逻辑运算
--------

sql 中用 `OR` 和 `AND` 来表示条件的 `或` 和 `且`

.. code-block:: sql

    $ SELECT * FROM World;
    +-----------------+------------+------------+--------------+---------------+
    | name            | continent  | area       | population   | gdp           |
    +-----------------+------------+------------+--------------+---------------+
    | Afghanistan     | Asia       | 652230     | 25500100     | 20343000      |
    | Albania         | Europe     | 28748      | 2831741      | 12960000      |
    | Algeria         | Africa     | 2381741    | 37100000     | 188681000     |
    | Andorra         | Europe     | 468        | 78115        | 3712000       |
    | Angola          | Africa     | 1246700    | 20609294     | 100990000     |
    +-----------------+------------+------------+--------------+---------------+
    
    $ SELECT name,population,area FROM World WHERE area > 3000000 OR population > 25000000;
    +--------------+-------------+--------------+
    | name         | population  | area         |
    +--------------+-------------+--------------+
    | Afghanistan  | 25500100    | 652230       |
    | Algeria      | 37100000    | 2381741      |
    +--------------+-------------+--------------+


分组统计
--------

.. code-block:: sql

    $ SELECT * FROM Person;
    +----+---------+
    | Id | Email   |
    +----+---------+
    | 1  | a@b.com |
    | 2  | c@d.com |
    | 3  | a@b.com |
    +----+---------+

    $ SELECT Email FROM Person GROUP BY Email having count(Email) > 1;
    +---------+
    | Email   |
    +---------+
    | a@b.com |
    +---------+