How to calculate AVG from two tablets

I am creating a database in SSMS and I am trying to calculate AVG from 2 columns from 2 tablets; I wrote :

;WITH combined AS
(
    SELECT Datum_narodenia 
    FROM Zamestnanci 
    UNION ALL
    SELECT Datum_narodenia 
    FROM ObčaniaSR
)
SELECT AVG(Datum_narodenia)
FROM combined;

But I got this error:

Msg 8117, Level 16, State 1
Operand data type date is invalid for avg operator.

I created table 1 :

CREATE TABLE Zamestnanci
(
    id_Zamestnanca int not null,
    Meno varchar(50) null,
    Priezvisko varchar(50)null,
    Adresa varchar(50) null,
    Datum_narodenia date null,

    PRIMARY KEY (id_Zamestnanca)
);

CREATE TABLE ObčaniaSR
(
    id_Občana int not null,
    Meno varchar(50) null,
    Priezvisko varchar(50) null,
    Adresa varchar(50) null,
    Datum_narodenia date null,
    Zápis_v_trestnom_registry varchar(50) null,

    PRIMARY KEY (id_Občana)
);

Datum_narodenia means Date_of_birth from witch I try to calculate AVG.

What is best way to calculate AVG age according to you? Thank you for your answers and advice.

Solution 1:

Pretty close calculation of age (lots of nitty-gritty in this answer) uses the difference in hours between the date and now, divided by the number of hours in a year (8766/24 = 365.25), which tries to roughly account for leap years; it is imperfect, but a pretty good trade-off IMHO between simple and right:

SELECT DATEDIFF(HOUR,'19860201',GETDATE())/8766;

So:

;WITH combined AS
(
    SELECT Datum_narodenia 
    FROM Zamestnanci 
    UNION ALL
    SELECT Datum_narodenia 
    FROM ObčaniaSR
)
SELECT AVG(DATEDIFF(HOUR, Datum_narodenia, GETDATE())/8766)
FROM combined;

Now, this does integer math, so you might instead want:

;WITH combined AS
(
    SELECT Datum_narodenia 
    FROM Zamestnanci 
    UNION ALL
    SELECT Datum_narodenia 
    FROM ObčaniaSR
)
SELECT CONVERT(DECIMAL(5,1), 
  AVG(DATEDIFF(HOUR, Datum_narodenia, GETDATE())/8766.0))
FROM combined;