2个联合查询之间的区别

以下2个查询有什么区别？（它们都产生相同的结果）2个联合查询之间的区别

select * 
from (
    select * from (
      select * 
      from phppos_items 
      where name like 'AB10LA2%' and deleted = 0 
      order by `name` limit 16 
      ) t 
    union 
    select * from (
      select * 
      from phppos_items 
      where item_number like 'AB10LA2%' and deleted = 0 
      order by `name` limit 16 
      ) t 
    union 
    select * from (
      select * 
      from phppos_items 
      where category like 'AB10LA2%' and deleted = 0 
      order by `name` limit 16 
      ) t 
) as top_rows 
order by `name` limit 16

select * 
from (
      (select * 
      from phppos_items 
      where name like 'AB10LA2%' and deleted = 0 
      order by `name` limit 16) 
    union 
      (select * 
      from phppos_items 
      where item_number like 'AB10LA2%' and deleted = 0 
      order by `name` limit 16) 
    union 
      (select * 
      from phppos_items 
      where category like 'AB10LA2%' and deleted = 0 
      order by `name` limit 16) 
) as top_rows 
order by `name` limit 16

来源

2011-05-30 Chris Muench

第一个版本还有另一组临时表，这是无用的，资源在这种特殊情况下的浪费。
以下所有会产生相同的结果：

SELECT * FROM T1; 

SELECT * FROM (SELECT * FROM T1); 


SELECT * FROM (SELECT * FROM (SELECT * FROM T1)); 

... 
...

到无穷远:-)

并在您的非常具体的情况下，这将是不够的：

select * 
      from phppos_items 
      where 
       (name like 'AB10LA2%' OR item_number like 'AB10LA2%' OR category like 'AB10LA2%') 
      and deleted = 0 
      order by `name` limit 16

来源

2011-05-30 16:50:31

临时表？我很确定两个查询都会产生相同的执行计划。 – Magnus 2011-05-30 16:56:24

原件不应该优先考虑按名称找到的物品。 – 2011-05-30 17:01:37

@Jonathan Leffler你是对的，编辑 – 2011-05-30 17:04:21

第一个查询嵌入了一个额外的一套SELECT * FROM (sub-query)。优化器需要稍微长一些的时间来整理它。

在功能上，查询之间没有真正的区别。第二个更简单，因此更可取。

来源

2011-05-30 16:50:50

我看到的唯一区别是在顶部，UNION的子查询组件被命名。显然，由于第二个版本没有错误，所以MySQL在这个构造中不需要子查询名称。顺便说一句，你有没有测试过这个版本比使用OR的相应单个查询快？

来源

2011-05-30 16:52:56

2个联合查询之间的区别

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