Creating a date column from multiple columns? - sql

Question

If a table has 3 columns for a person's date of birth (ex: BirthMonth, BirthDay, BirthYear), what is the best way to combine them into one column with a format of MM/dd/yy and (obviously) a date datatype? Thank you.

Answer 1

You say "...put them into one field with a format of MM/dd/yy and (obviously) a date datatype? " but a date type does not have a display format. That's a presentation concern.
SQL Server 2012 onwards:
DATEFROMPARTS(year, month, day)
or
cast((DATEFROMPARTS(year, month, day) as date) to cast to pure date and remove default time portion
e.g.
SELECT DATEFROMPARTS(2010, 12, 31) AS Result;

Answer 2

Simply, combine all 3 columns into YYYY-MM-DD and then convert this string into Datetime.
For example:
Select
cast(cast(Year as string) + '-' +
cast(Month as string) + '-' +
cast(Day as string) as datetime)
Now you can format it any date format.

Answer 3

For SQL Server 2012, use the DATEFROMPARTS() function as suggested by Mitch. For prior versions:
Assuming BirthMonth, BirthDay, and BirthYear are all integral values, you can cast them:
CAST(CAST(BirthYear AS varchar) + '-' + CAST(BirthMonth AS varchar) + '-' + CAST(BirthDay AS varchar) AS DATETIME)
Alternatively, you can do it like this:
DATEADD(MONTH, BirthMonth - 1, DATEADD(YEAR, BirthYear - 1900, BirthDay - 1));
This has the advantage of not doing any string conversions and it's not dependent on any date format. This utilizes the fact that SQL Server's internal representation for the date is the number of days since 1 January 1900.
If BirthMonth, BirthDay, and BirthYear are valid string values, then you can cast them directly:
CAST(BirthYear + '-' + BirthMonth + '-' + BirthDay AS DATETIME)
And if they are guaranteed to have 2 digits for both the month and day (and 4 or 2 digits for the year), then you can cast them in a more compact way without hyphens:
CAST(BirthYear + BirthMonth + BirthDay AS DATETIME)