Question

以下2个查询之间有什么区别？（它们都产生相同的结果）

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

VS

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

Answer 1

第一个版本有另一组临时表，在这种特定情况下没用，浪费资源以下所有内容都会产生相同的结果：

SELECT * FROM T1;

SELECT * FROM (SELECT * FROM T1);


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

...
...

到无穷大： - ）

在你的具体案例中，这就足够了：

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

Answer 2

第一个查询会嵌入一组额外的SELECT * FROM (sub-query)。优化器需要稍微长一点才能对其进行排序。

从功能上讲，查询之间没有真正的区别。第二个更简单，因此更可取。

Answer 3

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，MySQL在这种结构中不需要子查询名称。顺便说一句，你是否测试过这个版本比使用OR的相应单个查询更快？

2个联合查询之间的区别

3 个答案: