How do I compare column values between two rows which share the same value of another column?

Question

I have a table with these columns:

name, start_date, end_date

In the table there are many rows which share the same name. For example, [John Smith, 10/17/17, 12/17/17] and [John Smith, 01/17/18, 02/17/18] can both exist in the table. What I'm trying to do is given a name, find the earliest start date and the latest end date, and get the difference between these two values and display it as a column.

For the example above, the select statement should return this:

[name, date difference in weeks], with the date difference in this case being 02/17/18 - 10/17/17

Answer 1

You need to group your rows by name:

SELECT name, max(end_date) - min(start_date)
FROM table
GROUP BY name

Answer 2

Not sure what DBMS you are using, but if it is SQL Server, and you want to have difference in weeks, then you can write the following

SELECT name
     , [Date difference in weeks] = DATEDIFF(week, min(start_date), max(end_date))
FROM table
GROUP BY name

Answer 3

Try to use below query.

    select name, max(end_date)-min(start_date)
    from table
    group by name;