数据库笔记9--利用SQL语句实现关系代数操作

た入场券 2022-12-15 12:40 129阅读 0赞

# 利用SQL语言实现关系代数操作 #

## （1）并-交-差的处理 ##

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*  Union Intersect Except ALL是对子查询结果的的操作

### SQL并运算 ###

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### SQL交运算 ###

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### SQL差运算 ###

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## （2）用SQL处理空值 ##

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## （3）内连接、外连接 ##

*  [SQL内外连接的区别][SQL]  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NTY5MTIyNw_size_16_color_FFFFFF_t_70_pic_center 8]  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80NTY5MTIyNw_size_16_color_FFFFFF_t_70_pic_center 9]  
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 *  [自然连接和内连接(θ-连接)的区别][-]
 *  on<连接条件> 用于内连接
 *  using一般可以用于同一个表的自身连接

### 例子 ###

内连接是按照「相等」的原则进行连接，并不能保持某一个关系中的所有元组，所有元组能连接的保留，不能连接的去掉  
为满足该题题意应用外连接  
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# SQL-SELECT小结： #

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## **SELECT语句的完整语法**：[link][] ##

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[SQL]: https://www.cnblogs.com/buxingzhelyd/p/7853454.html
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[-]: https://blog.csdn.net/chendongqaq/article/details/75212559
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[link]: https://blog.csdn.net/younghaiqing/article/details/59482071