CODEWITHSUNDEEP
mysqldatediff
RominaV
RominaV

Reputation: 3425

Difference when calculating datediff MySQL

I am getting different results when using datediff in a case, and in a where. This is the table users, omitting irrelevant columns, and with all the users:

<table><tbody><tr><th>UserID</th><th>UserTypeID</th><th>UserRegistrationDate</th><th>UserBirthDate</th><th>UserJobID</th><th>UserLocationID</th></tr><tr><td>1</td><td>1</td><td> </td><td> </td><td> </td><td> </td></tr><tr><td>2</td><td>1</td><td>3/12/2015</td><td>30/1/1982</td><td>2</td><td>3</td></tr><tr><td>3</td><td>1</td><td>2/12/2015</td><td>19/9/1987</td><td>2</td><td>4</td></tr><tr><td>12</td><td>1</td><td>2/12/2015</td><td>11/9/1992</td><td>2</td><td>5</td></tr><tr><td>13</td><td>1</td><td>13/12/2015</td><td>15/5/1995</td><td>2</td><td>2</td></tr><tr><td>40</td><td>1</td><td>10/1/2016</td><td>22/7/1975</td><td>2</td><td>2</td></tr><tr><td>47</td><td>1</td><td>16/1/2016</td><td>29/2/2000</td><td>1</td><td>6</td></tr><tr><td>50</td><td> </td><td> </td><td>1/12/1956</td><td>2</td><td>2</td></tr><tr><td>52</td><td> </td><td> </td><td>12/3/1998</td><td>2</td><td>6</td></tr><tr><td>54</td><td>1</td><td>20/1/2016</td><td>11/4/1990</td><td>2</td><td>2</td></tr><tr><td>55</td><td>1</td><td>25/1/2016</td><td> </td><td> </td><td> </td></tr><tr><td>57</td><td> </td><td> </td><td>1/1/1990</td><td> </td><td> </td></tr><tr><td>58</td><td>1</td><td> </td><td> </td><td> </td><td> </td></tr><tr><td>59</td><td>1</td><td>28/1/2016</td><td> </td><td> </td><td> </td></tr><tr><td>60</td><td>1</td><td>13/1/2016</td><td>1/1/1998</td><td>1</td><td>10</td></tr><tr><td>61</td><td>1</td><td>1/12/2015</td><td>22/6/1989</td><td>1</td><td>12</td></tr><tr><td>66</td><td>1</td><td>1/2/2016</td><td> </td><td> </td><td> </td></tr><tr><td>67</td><td>1</td><td>1/2/2016</td><td> </td><td> </td><td> </td></tr><tr><td>68</td><td>1</td><td>1/2/2016</td><td> </td><td> </td><td> </td></tr></tbody></table>

When running this: 

SELECT
  COUNT(users.UserID),
  CASE WHEN users.UserBirthDate IS NULL
    THEN 'Sin Registro'
  WHEN datediff(now(), users.UserBirthDate) / 365.25 > 50
    THEN '> 51'
  WHEN datediff(now(), users.UserBirthDate) / 365.25 > 40 AND datediff(now(), users.UserBirthDate) / 365.25 < 51
    THEN '41 - 50'
  WHEN datediff(now(), users.UserBirthDate) / 365.25 > 30 AND datediff(now(), users.UserBirthDate) / 365.25 < 41
    THEN '31 - 40'
  WHEN datediff(now(), users.UserBirthDate) / 365.25 > 26 AND datediff(now(), users.UserBirthDate) / 365.25 < 31
    THEN '27 - 30'
  WHEN datediff(now(), users.UserBirthDate) / 365.25 > 20 AND datediff(now(), users.UserBirthDate) / 365.25 < 27
    THEN '21 - 26'
  WHEN datediff(now(), users.UserBirthDate) / 365.25 <= 20
    THEN '< 21' END AS AgeGroup
FROM users
GROUP BY AgeGroup

I get this result:

<table><tbody><tr><th>COUNT(users.userid)</th><th>AgeGroup</th></tr><tr><td>3</td><td>21 - 26</td></tr><tr><td>3</td><td>27 - 30</td></tr><tr><td>1</td><td>31 - 40</td></tr><tr><td>1</td><td>41 - 50</td></tr><tr><td>3</td><td>&lt; 21</td></tr><tr><td>1</td><td>&gt; 51</td></tr><tr><td>7</td><td>Sin Registro</td></tr></tbody></table>

But when I run:

SELECT COUNT(users.UserID)
FROM users
WHERE datediff(now(), users.UserBirthDate) / 365.25 > 20 AND datediff(now(), users.UserBirthDate) / 365.25 < 27;

I get this:

<table><tbody><tr><th>COUNT(users.userid)</th></tr><tr><td>5</td></tr></tbody></table>

UPDATE

SQLFiddle with complete tables and data.

Upvotes: 1

Views: 137

Answers (1)

RominaV
RominaV

Reputation: 3425

After manually checking the results, I realized neither was correct. Adding a FLOOR() to the datediff result, ended up giving the correct results.

Final Query:

SELECT COUNT(users.UserID), case   when users.UserBirthDate IS NULL  then  'Sin Registro'  
when FLOOR(datediff(now(), users.UserBirthDate) / 365.25) > 50  then  '> 51' 
when FLOOR(datediff(now(), users.UserBirthDate) / 365.25) > 40 AND  FLOOR(datediff(now(), users.UserBirthDate) / 365.25) < 51 then  '41 - 50'  
when FLOOR(datediff(now(), users.UserBirthDate) / 365.25) > 30 AND  FLOOR(datediff(now(), users.UserBirthDate) / 365.25) < 41 then  '31 - 40'  
when FLOOR(datediff(now(), users.UserBirthDate) / 365.25) > 26 AND  FLOOR(datediff(now(), users.UserBirthDate) / 365.25) < 31 then  '27 - 30'  
when FLOOR(datediff(now(), users.UserBirthDate) / 365.25) > 20 AND  FLOOR(datediff(now(), users.UserBirthDate) / 365.25) < 27 then  '21 - 26'  
when FLOOR(datediff(now(), users.UserBirthDate) / 365.25) <= 20  then  '< 21'  end as AgeGroup FROM users GROUP BY AgeGroup;

Correct Result:

<table><tbody><tr><th>COUNT(users.userid)</th><th>AgeGroup</th></tr><tr><td>4</td><td>21 - 26</td></tr><tr><td>1</td><td>27 - 30</td></tr><tr><td>2</td><td>31 - 40</td></tr><tr><td>4</td><td>&lt; 21</td></tr><tr><td>1</td><td>&gt; 51</td></tr><tr><td>7</td><td>Sin Registro</td></tr></tbody></table>

Checking it with where + floor, I get the correct result too.

SELECT COUNT(users.UserID) 
FROM users
WHERE FLOOR(datediff(now(), users.UserBirthDate) / 365.25) > 20 AND  FLOOR(datediff(now(), users.UserBirthDate) / 365.25) < 27;

Returns 4, like the case.

I suppose the case was differing from the where because of the case order.

Upvotes: 1

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