Insert current date time in a column of and keep it constant - sql

Question

i am using sql server in making my project with javafx. Their i have a table of purchase and sale. One of the column of both of them is date having current date and time to store them as a record that this transaction has been saved in this time.
Now i am using the that date column with varchar datatype and have using computed column specification with following function:
(CONVERT([varchar](25),getdate(),(120)))
but when i select records from that table using query
SELECT pr.Date, p.Name, pr.Quantity, s.Name, p.Pur_Price
FROM (([Product] AS p
INNER JOIN [Purchase] AS pr ON pr.Product_id=p.Product_id)
INNER JOIN [Supplier] AS s ON s.Supplier_Id=p.Supplier_Id)
WHERE pr.Date>= dateadd(dd, 0, datediff(dd, 0, getdate()-30))
but it selects all the records keeping all date records to current date and time. Thanks in advance.
Looking forward for your good replies.

Answer 1

The problem is that your Date column is computed on the fly and not actually stored in the table. So each time you SELECT from that table, the expression of the computed column is calculated (CONVERT([varchar](25),getdate(),(120))) thus resulting in the same value for all rows.
A fix would be using a PERSISTED computed column so that values are actually stored with the table when inserting or updating:
CREATE TABLE Product (
OtherColumns INT,
[Date] AS (CONVERT([varchar](25), getdate(), 120)) PERSISTED)
The problem with this is that non-deterministic expressions can't be persisted, as this error message pops up:
Msg 4936, Level 16, State 1, Line 1 Computed column 'Date' in table
'Product' cannot be persisted because the column is non-deterministic.
You have several other options for this. Please use DATE or DATETIME columns to store and handle dates and avoid using VARCHAR for this as it brings many problems. The following examples use DATETIME:
Use a DEFAULT constraint linked to the column with the expression you want:
CREATE TABLE Product (
OtherColumns INT,
[Date] DATETIME DEFAULT GETDATE())
INSERT INTO Product (
OtherColumns) -- Skip the Date column on the INSERT
VALUES
(1)
SELECT * FROM Product
OtherColumns Date
1 2018-12-14 08:49:08.347
INSERT INTO Product (
OtherColumns,
Date)
VALUES
(2,
DEFAULT) -- Or use the keyword DEFAULT to use the default value
SELECT * FROM Product
OtherColumns Date
1 2018-12-14 08:49:08.347
2 2018-12-14 08:50:10.070
Use a trigger to set the value. This will override any inserted or updated value that the original operation set (as it will execute after the operation, as stated in it's definition).
CREATE TRIGGER utrProductSetDate ON Product
AFTER INSERT, UPDATE
AS
BEGIN
SET NOCOUNT ON
UPDATE P SET
Date = GETDATE()
FROM
inserted AS I
INNER JOIN Product AS P ON I.OtherColumns = P.OtherColumns -- Assuming PK or Unique columns join
END

Answer 2

Thanks all of you. But i solved my problem by putting my date Column of that table into datetime data type and i my query i have entered the date using getdate() method. It worked for me to save current date and time in my purchase and sale table.