How to Accurately Calculate Age in Years from a Date of Birth in SQL Server?

Accurately calculate age by date of birth in SQL Server

Question:

How to convert a date of birth stored as nvarchar(25) to a date in SQL Server and subsequently calculate its corresponding age in years?

Sample data:

ID	Name	DOB
1	John	1992-01-09 00:00:00
2	Sally	1959-05-20 00:00:00

Expected output:

ID	Name	AGE	DOB
1	John	31	1992-01-09 00:00:00
2	Sally	64	1959-05-20 00:00:00

Answer: Calculate age accurately

The originally proposed method faced challenges in accounting for leap years and irregularities in the number of days in the month. Modern SQL Server versions provide us with improved techniques to resolve these complexities.

The best way to calculate age in whole numbers:

This method is simple and direct, but the accuracy may be slightly lower than the decimal method.

SELECT
    (YEAR(GETDATE()) - YEAR(DOB)) - CASE WHEN MONTH(GETDATE()) < MONTH(DOB) OR (MONTH(GETDATE()) = MONTH(DOB) AND DAY(GETDATE()) < DAY(DOB)) THEN 1 ELSE 0 END AS AgeIntYears
FROM YourTable;

The best way to calculate age as a decimal:

This method uses the DATEDIFF function and the CASE statement to accurately calculate age and considers whether the birthday has passed in the current year.

SELECT
    DATEDIFF(year, DOB, GETDATE()) - CASE WHEN MONTH(GETDATE()) < MONTH(DOB) OR (MONTH(GETDATE()) = MONTH(DOB) AND DAY(GETDATE()) < DAY(DOB)) THEN 1 ELSE 0 END AS AgeYearsDecimal
FROM YourTable;

Please note that the above code assumes that the DOB column has been converted to the DATE type. If DOB is still of type nvarchar(25), you need to add type conversion in the query, for example: CONVERT(DATE, DOB, 120). Choosing the appropriate transformation style depends on the specific format of your DOB data. For example, CONVERT(DATE, DOB, 120) is for 'yyyy-mm-dd hh:mi:ss' format. You may need to adjust this section to suit your data format. The final query should look something like:

SELECT
    DATEDIFF(year, CONVERT(DATE, DOB, 120), GETDATE()) - CASE WHEN MONTH(GETDATE()) < MONTH(CONVERT(DATE, DOB, 120)) OR (MONTH(GETDATE()) = MONTH(CONVERT(DATE, DOB, 120)) AND DAY(GETDATE()) < DAY(CONVERT(DATE, DOB, 120))) THEN 1 ELSE 0 END AS AgeYearsDecimal
FROM YourTable;

Replace YourTable with your table name. Choosing the integer or decimal method depends on your precision needs. The decimal method is more precise, but the integer method is more concise.

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