Using table values in DATEDIFF function - sql-server

Question

So I am learning about TSQL and have been given an assignment to do an App that details stuff about a certain series of movies. One of the things I would like to add to the finished product is a DATEDIFF where I have the birthdate of the main actor and the Premiere date of the movie but would like to return the age of the actor at the time of the Premiere.
My question is, HOW do I specify the dates, given a DATEDIFF statement if those dates already exist in a table?
Examples of the tables follow.
Bond
BondID FirstName LastName Birthdate
1 Sean Connery 8/25/1930
2 George Lazenby 9/5/1939
3 Roger Moore 10/14/1927
4 Timothy Dalton 3/21/1946
5 Pierce Brosnan 5/16/1953
6 Daniel Craig 3/2/1968
Film
ID BondID FilmTitle Premiere
1 1 Dr. No 1963-05-08
2 1 From Russia With Love 1974-04-08
3 1 Goldfinger 1964-12-22
4 1 Thunderball 1965-12-29
Here is what I have presently.
SELECT b.Birthdate, f.Premiere, b.FirstName, b.LastName, f.FilmTitle
FROM Bond b INNER JOIN FilmID f ON b.BondID = f.BondID
WHERE b.BondID = 1
SELECT DATEDIFF(YEAR, GETDATE(), GETDATE()) AS "YearDif"
WHERE GETDATE() = Bond.Birthdate AND GETDATE() = f.Premiere
Data
Birthdate Premiere FirstName LastName FilmTitle
1 1930-08-25 1963-05-08 Sean Connery Dr. No
2 1930-08-25 1974-04-08 Sean Connery From Russia With Love
3 1930-08-25 1964-12-22 Sean Connery Goldfinger
4 1930-08-25 1965-12-29 Sean Connery Thunderball
I would like to add a column that states the age of the actor at the time of the premiere...I think I have a handle on that one... But...
I get that the second part of the DATEDIFF part isn't correct, but how do I tell the DATEDIFF where to get the date values?
Any help would be wonderous!

Answer 1

Your query:
SELECT DATEDIFF(YEAR, GETDATE(), GETDATE()) AS "YearDif"
WHERE GETDATE() = Bond.Birthdate AND GETDATE() = f.Premiere
will not work for what you want. GETDATE() will only return today's date so in essence the above will only return a row if the birthdate and the premiere were today at the very moment you ran the query.
What you need to do is is the QUERY section below:
SAMPLE DATA:
IF OBJECT_ID('tempdb..#Bond') IS NOT NULL
DROP TABLE #Bond
CREATE TABLE #Bond
(BondID INT ,FirstName VARCHAR(20),LastName VARCHAR(20),Birthdate DATE)
INSERT INTO #Bond
VALUES
(1 ,'Sean' ,'Connery' ,'8/25/1930')
,(2 ,'George' ,'Lazenby' ,'9/5/1939')
,(3 ,'Roger' ,'Moore' ,'10/14/1927')
,(4 ,'Timothy' ,'Dalton' ,'3/21/1946')
,(5 ,'Pierce' ,'Brosnan' ,'5/16/1953')
,(6 ,'Daniel' ,'Craig' ,'3/2/1968')
IF OBJECT_ID('tempdb..#Film') IS NOT NULL
DROP TABLE #Film
CREATE TABLE #Film
(ID INT , BondID INT , FilmTitle VARCHAR(50), Premiere DATE)
INSERT INTO #Film
VALUES (1 , 1,'Dr. No' ,'1963-05-08')
,(2 , 1,'From Russia With Love','1974-04-08')
,(3 , 1,'Goldfinger' ,'1964-12-22')
,(4 , 1,'Thunderball' ,'1965-12-29')
QUERY:
SELECT b.Birthdate, f.Premiere, b.FirstName, b.LastName, f.FilmTitle , DATEDIFF(YEAR, b.Birthdate, Premiere) AS AgeOfActor
FROM #Bond b INNER JOIN #Film f ON b.BondID = f.BondID
RESULT:
RESPONSE TO COMMENT:
you can always count the days and then devide by 365:
SELECT b.Birthdate, f.Premiere, b.FirstName, b.LastName, f.FilmTitle , DATEDIFF(day, b.Birthdate, Premiere)/365 AS AgeOfActor
FROM #Bond b INNER JOIN #Film f ON b.BondID = f.BondID
or for more accuracy you can do it in seconds :P
SELECT b.Birthdate, f.Premiere, b.FirstName, b.LastName, f.FilmTitle , DATEDIFF(second, b.Birthdate, Premiere)/(365.25*24*60*60) AS AgeOfActor
FROM #Bond b INNER JOIN #Film f ON b.BondID = f.BondID

Answer 2

Such things can be done in SQL in just one step. There is no need to store the intermediate results anywhere to pass them later on.
You just need to pass your column values from the table into the function call like that:
SELECT
b.Birthdate, f.Premiere, b.FirstName, b.LastName, f.FilmTitle,
DATEDIFF(YEAR, b.Birthdate, f.Premiere) AS actor_age_at_premiere
FROM
Bond b
INNER JOIN FilmID f
ON b.BondID = f.BondID
Assuming that an actor was born before the premiere date of a film he/she played in (which is a fair assumption) the result will be a positive Integer.
If you'd like to include the actor age as of now, then your third argument would be GETDATE() instead of Premiere column:
DATEDIFF(Year, b.Birthdate, GETDATE()) AS actor_age_current