How Can I Accurately Calculate Age in Years from a Date of Birth Using SQL?

Calculating Age in Years from a Date of Birth with SQL: A Precise Approach

Accurate age calculation in SQL, based on a date of birth, requires the birthdate column to be of a valid datetime or date data type. If your birthdate column is currently nvarchar(25), you must first convert it to a suitable date format.

Assuming this conversion is complete, let's examine effective age calculation methods:

Method 1: A Simplified Approach (Approximate)

This method uses DATEDIFF to find the difference in hours between the current date and the date of birth, then divides by the approximate number of hours in a year (8766.0):

<code class="language-sql">SELECT DATEDIFF(hour, @dob, GETDATE())/8766.0 AS AgeYearsDecimal</code>

This yields a decimal age, but its accuracy is limited due to variations in the number of days per month and leap years.

Improved and More Accurate Methods:

Integer Age Calculation:

This method calculates the age in whole years:

<code class="language-sql">SELECT (CONVERT(int,CONVERT(char(8),@Now,112))-CONVERT(char(8),@Dob,112))/10000 AS AgeIntYears</code>

Precise Decimal Age Calculation:

For a more precise decimal age, this improved method is recommended:

<code class="language-sql">SELECT 1.0* DateDiff(yy,@Dob,@Now)
    +CASE
         WHEN @Now >= DATEFROMPARTS(DATEPART(yyyy,@Now),DATEPART(m,@Dob),DATEPART(d,@Dob)) THEN
           (
              1.0   
              * DATEDIFF(day,DATEFROMPARTS(DATEPART(yyyy,@Now),DATEPART(m,@Dob),DATEPART(d,@Dob)),@Now)
              / DATEDIFF(day,DATEFROMPARTS(DATEPART(yyyy,@Now),1,1),DATEFROMPARTS(DATEPART(yyyy,@Now)+1,1,1))
           )
         ELSE 
           -1 
           * (       
                * DATEDIFF(day,DATEFROMPARTS(DATEPART(yyyy,@Now),DATEPART(m,@Dob),DATEPART(d,@Dob)),@Now) 
                / DATEDIFF(day,DATEFROMPARTS(DATEPART(yyyy,@Now),1,1),DATEFROMPARTS(DATEPART(yyyy,@Now)+1,1,1))
                )
     END AS AgeYearsDecimal</code>

This advanced method accounts for leap years and provides a highly accurate decimal representation of age in years. It's the preferred method for situations requiring precise age calculations.

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