把最后一行的关于a的定义放到main之前不会报错了，请问为何不这样做呢？

MyISAM设计之初，没有想到日后自己会成为知名的数据存储引擎之一，也就没有考虑支持事务

5.7之后MySQL默认的存储引擎已经是InnoDB，个人认为，多了解InnoDB特性即可

排序集的原因，详情可以看这里：https://zhuanlan.zhihu.com/p/...
修改表的排序集为utf8mb4_0900_as_ci就查询不出来了

node-mysql文档是这么写的：

Connections are also cycled round-robin style, with connections being taken from the top of the pool and returning to the bottom.

因此推测一个connection用完之后，它马上被放到池的底部，后续的查询用不到它，因此变成sleep状态。你测试时候再多进行几次查询，直到将池里面的连接都耗尽之后再观察一下，应该会发现之前处于sleep的connection被复用了。
另外我认为node-mysql这种类型的库在连接池的实现已经非常完善了，无需自己再去造轮子，例如你说的自己再去写一个定时器。

HotSpot的文档是这样解释safepoint的：

A point during program execution at which all GC roots are known and all heap object contents are consistent. From a global point of view, all threads must block at a safepoint before the GC can run. (As a special case, threads running JNI code can continue to run, because they use only handles. During a safepoint they must block instead of loading the contents of the handle.) From a local point of view, a safepoint is a distinguished point in a block of code where the executing thread may block for the GC. Most call sites qualify as safepoints. There are strong invariants which hold true at every safepoint, which may be disregarded at non-safepoints. Both compiled Java code and C/C++ code be optimized between safepoints, but less so across safepoints. The JIT compiler emits a GC map at each safepoint. C/C++ code in the VM uses stylized macro-based conventions (e.g., TRAPS) to mark potential safepoints.

出处：https://openjdk.java.net/grou...

Keep in mind that there is a distinction between 'active' and 'consumed' memory. Linux tends to store large volumes of data in memory in case it's needed in the future. But processes may not actively be using this memory - in which case they report a lower volume of 'used' memory.
...
You should familiarize yourself with the free command. I also like using htop instead of top, as it displays these differences.

https://serverfault.com/quest...

简单来说就是系统会事先多申请一些内存，所以used中的量是系统申请了的内存，包括已经分配给进程的和没有分配给进程的。你可以试试htop命名，看看是否符合你的预期。

• 首先，Chunked Transfer Encoding是用于server应答client的，不是用于client向server发送数据的。
• 然后，你问的是client如何上传大文件，答案是断点续传，或者你可以参考这里：https://juejin.cn/post/684490...

mysql 8上测试通过

select 
    year.year, 
    sum(case when month=1 then amount end) as m1,
    sum(case when month=2 then amount end) as m2,
    sum(case when month=3 then amount end) as m3,
    sum(case when month=4 then amount end) as m4
from year
group by year 

假设每天的成绩表结构都一样，则可以

select 
    总成绩.学生name , sum(总成绩.分数)
from (
   select * from 第1天的考试成绩 union all
   select * from 第2天的考试成绩 union all
    -- ...
   select * from 第m天的考试成绩 
) as 总成绩

group by 总成绩.学生name
order by sum(总成绩.分数)
limit 20

可行，mongodb本来就是设计用来存储非结构化的数据的