MYSQL：计算具有相同日期的行

Question

I got a table called event_details summarized as below:

Id creation_date
 1 2015-03-18 18:59
 2 2015-03-18 17:59
 3 2015-03-18 16:59
 4 2015-03-17 14:59
 5 2015-03-17 18:59
 6 2015-03-17 19:59
 7 2015-03-16 11:59
 8 2015-03-15 03:59
 9 2015-03-15 02:59
10 2015-03-15 08:59
11 2015-03-14 09:59

I want to count rows with the same date and get result like below?

How can I do this? Is it possible using nested queries or I should use joins?

cnt creation_date
  3 2015-03-18
  3 2015-03-17
  1 2015-03-16
  3 2015-03-15
  1 2015-03-14

And a final question: Is this naturally a heavy SQL task and impose high loads to database-server or not?

Answer 1

This could be done with group by and count

select 
count(*) as cnt,
date(creation_date) as creation_date_date
from event_details
group by creation_date_date

Answer 2

You can try this.

SELECT COUNT(*), creation_date FROM table_name GROUP BY creation_date

This first groups all your rows with the same creation_date and then counts the number of rows in each group. You can furthermore order the rows as follows:

SELECT COUNT(*), creation_date FROM table_name GROUP BY creation_date ORDER BY creation_date

Answer 3

This does it.

SELECT COUNT(*) as cnt,
       date(creation_date) as creation_date 
FROM event_details 
GROUP BY 2