格式：
select A, count(*) as num from test2 group by A
应用：
统计超时接口出现的次数，并按出现次数降序
select api_purpose,count(api_purpose) as count_time_out from response_time
where run_time like "%2020-05-09%"
group by api_purpose
order by count_time_out desc
效果：
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