How do I create column calculated from another column? - sql

Question

I need to create a column age in a SQL Server database.
The values of this column should be calculated based on the values of the column DOB.
Also its values should increment as Age increases.

Answer 1

You should use a computed column to solve this problem. Something with a definition similar to this:
ALTER TABLE Customers ADD Age AS datediff(year, DOB ,getdate())
Original statement taken from and further information available at BlackWasp.
Edit:
MSDN explains computed columns as:
A computed column is computed from an expression that can use other
columns in the same table. The expression can be a noncomputed column
name, constant, function, and any combination of these connected by
one or more operators. The expression cannot be a subquery.
Unless otherwise specified, computed columns are virtual columns that
are not physically stored in the table. Their values are recalculated
every time they are referenced in a query. The Database Engine uses
the PERSISTED keyword in the CREATE TABLE and ALTER TABLE statements
to physically store computed columns in the table. Their values are
updated when any columns that are part of their calculation change. By
marking a computed column as PERSISTED, you can create an index on a
computed column that is deterministic but not precise. Additionally,
if a computed column references a CLR function, the Database Engine
cannot verify whether the function is truly deterministic. In this
case, the computed column must be PERSISTED so that indexes can be
created on it. For more information, see Creating Indexes on Computed
Columns.
Computed columns can be used in select lists, WHERE clauses, ORDER BY
clauses, or any other locations in which regular expressions can be
used, with the following exceptions:
Computed columns used as CHECK, FOREIGN KEY, or NOT NULL constraints must be marked
PERSISTED. A computed column can be used as a key column in an index or as part of any
PRIMARY KEY or UNIQUE constraint if the computed column value is defined by a
deterministic expression and the data type of the result is allowed in index
columns.
For example, if the table has integer columns a and b, the computed column a + b can be
indexed, but computed column a + DATEPART(dd, GETDATE()) cannot be indexed because the
value may change > in subsequent invocations.
A computed column cannot be the target of an INSERT or UPDATE statement.
The Database Engine automatically determines the nullability of
computed columns based on the expressions used. The result of most
expressions is considered nullable even if only nonnullable columns
are present, because possible underflows or overflows will produce
null results as well. Use the COLUMNPROPERTY function with the
AllowsNull property to investigate the nullability of any computed
column in a table. An expression that is nullable can be turned into a
nonnullable one by specifying ISNULL(check_expression, constant),
where the constant is a nonnull value substituted for any null result.
Source: MSDN - Computed Columns

Answer 2

Code snippet
ALTER TABLE
TheTable
ADD
DOB AS
CASE
WHEN
MONTH(Birth) > MONTH(ISNULL(Death, SYSDATETIME()))
OR (
MONTH(Birth) = MONTH(ISNULL(Death, SYSDATETIME()))
AND DAY(Birth) >= DAY(ISNULL(Death, SYSDATETIME()))
)
THEN
DATEDIFF(YEAR, Birth, ISNULL(Death, SYSDATETIME())) - 1
ELSE
DATEDIFF(YEAR, Birth, ISNULL(Death, SYSDATETIME()))
END

Answer 3

Create Table with auto-generated column,
CREATE TABLE Person2
(Id int IDENTITY(1,1) NOT NULL, Name nvarchar(50),
DOB date, Age AS DATEDIFF(YEAR, DOB ,GETDATE()) )

Answer 4

This is the correct way of getting the age:
alter table <yourtable> add age as datediff(year, DOB, getdate())- case when month(DOB)*32 + day(DOB) > month(getdate()) * 32 + day(getdate()) then 1 else 0 end