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读书笔记《MySQL必知必会》之第十七章组合查询

java持续实践发布时间：2018-11-25 19:06:18 ，浏览量：6

文章目录

- 什么是组合查询
- 创建组合查询
- - 使用union
  - 使用union的规则
  - union all的使用
  - 组合查询的结果的排序
- 总结

什么是组合查询

把多个select语句的结果作为单个查询的结果集返回,就称为并(union)或复合查询(compound query) 有两种情况需要使用组合查询

在单个查询中从不同的表返回类似结构的数据
对单个表执行多个查询, 按单个查询返回结果

任何具有多个where子句的select语句,都可以作为一个组合查询给出.

创建组合查询使用union

需要查询价格小于等于5的所有物品的一个列表，而且还想包括供应商1001和1002生产的所有物品（不考虑价格）。如果是用两条where查询那么语句如下

SELECT vend_id,prod_id,prod_price from products WHERE prod_price


    
        
            
                
                
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