How do I select datediff between a date and time column and a date column from two different tables in MySQL?

Question

So I have these two data tables, users and logins. There is a registration_date column for users and a last_login for logins. I am trying to find the average difference between the two columns from the two tables. What I believe to be the problem is that the registration_date is just date while the last_login is both date and time.

I first tried:

select avg(datediff(u.registration_date, l.last_login)) as avg_acttime from logins l 

left join users u on l.userid = u.id and type like '%new%';

My result was null .

So I then tried to convert the date and time last_login column to just date.

select avg(datediff((DATE_FORMAT(u.registration_date,'%d-%m-%y')), (DATE_FORMAT(l.last_login,'%d-%m-%y')))) as avg_acttime from logins l 

left join users u on l.userid = u.id and type like '%new%';

The result was again null . I need some serious help getting the difference in days between this date column from one table and a date and time column from another table.

Answer 1

This is too long for a comment. First, I think you intend for users to be the first table. Then, you don't need a left join , because avg() ignores NULL values anyway. So a join is sufficient:

select avg(datediff(u.registration_date, l.last_login)) as avg_acttime
from users u join
     logins l     
     on l.userid = u.id and type like '%new%';

You would get NULL values either because there are no rows generated by the from clause or all the rows have a NULL value for one of the columns. More specifically, I can think of the following possibilities:

• logins or users has no rows.
• No rows match the on conditions.
• At least one of u.registration_date and l.last_login are always NULL for each login.

Having a datetime and date should not result in NULL values.

Of these, my guess is that no rows match the on condition. The type comparison is the most likely cause of failure. Without sample data, it is hard to say more.

Answer 2

Your date format %d-%m-%y will yield very strange results, if any at all. Change %d-%m-%y to

%Y-%m-%d

Mind the capital Y , or else mysql will format the date with 2 digits for year, what he will then confuse with the day.