在MySQL中获取不同的数据

Question

At the moment I'm on the process of getting the counts of the unique email address in the database .

Table1
---------------------------------------------
 id    email          name      date_applied
---------------------------------------------
 1    abc@mail.com   ABC       2013-04-17
 2    bcd@mail.com   BCD       2013-04-18
 3    xyz@mail.com   XYZ       2013-04-20
 4    tre@mail.com   TRE       2013-04-28
 5    esf@mail.com   ESF       2013-04-29
 6    bcd@mail.com   BCD       2013-04-29
 7    abc@mail.com   ABC       2013-04-30
---------------------------------------------

Here's the query on how I get the unique emails for the week:

select count(distinct(`A`.`email`)) as total 
from Table1 as A 
where A.date_applied BETWEEN `2013-04-29` and `2013-05-05`

Current Week: 2013-04-29 => 2013-05-05

What I wanted to achieve is to get the count of the new emails in the current week without ignoring the emails in the previous weeks.

I badly needed your help.

Answer 1

You can achieve this by using sub-query like this:

SELECT COUNT(DISTINCT `email`) AS Total FROM Table1 AS A 
  WHERE A.date_applied 
BETWEEN '2013-04-29' AND '2013-05-05'
    AND A.email NOT IN (SELECT email FROM Table1
                         WHERE date_applied < '2013-04-29');

Result:

| TOTAL |
---------
|     1 |

See this SQLFiddle

---

Here is the description of the query:

First of all you need to get email between two dates (ie a week) like this:

SELECT DISTINCT email FROM Table1
 WHERE date_applied BETWEEN '2013-04-29' AND '2013-05-05'

Then search for emails which are not unique (ie already exists on previous date):

SELECT email 
  FROM Table1
 WHERE date_applied < '2013-04-29'

Now exclude those emails from our current query by using NOT IN :

SELECT DISTINCT `email` FROM Table1 AS A 
  WHERE A.date_applied 
BETWEEN '2013-04-29' AND '2013-05-05'
    AND A.email NOT IN (SELECT email FROM Table1
                         WHERE date_applied < '2013-04-29');

Now use COUNT() function to get the count of emails.

See this SQLFiddle

Answer 2

You'll want to aggregate. To get the date a distinct email was added:

SELECT email, MIN(date_applied) as first
FROM Table1
GROUP BY email
HAVING first BETWEEN 2013-04-29 AND 2013-05-05

The number of rows in your result is your answer.

Or use a sub query. I still have to get used to MySQL supporting them. :)

You should think about your schema design, though. I understand this is probably a minimal example, but date_applied is not really the applicationdate in your mindmap. In your mind email is KEY in this table, not Id . Why not have the dbschema congrue with your mind? Then your query will be simpler, and more performant.