Create Sqlite Table with a Column having Autocalculation - android-sqlite

Question

I m using below code for creating SQLite table.
db.execSQL("Create table if not exists " + datausage + "(Id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, mainentry text, stime integer , endtime integer, usedtime integer)");
But I want usedtime should autocalculate when I insert stime and etime. usedtime will be difference of etime and stime.
so can we create a column which do calculation automatically. I got this for sql as stated below.
CREATE TABLE dbo.Products
(
ProductID int IDENTITY (1,1) NOT NULL
, QtyAvailable smallint
, UnitPrice money
, InventoryValue AS QtyAvailable * UnitPrice
)
;
https://learn.microsoft.com/en-us/sql/relational-databases/tables/specify-computed-columns-in-a-table?view=sql-server-2017
but this does not work in case of SQLite and I m getting invalid sql statement
can we do this ?

Answer 1

But I want usedtime should autocalculate when I insert stime and
etime.
Assuming that you mean endtime rather than etime. Then you can add a TRIGGER that will automatically UPDATE the usedtime AFTER the row has been INSERTed.
e.g.
CREATE TRIGGER IF NOT EXISTS datausage_timedifference AFTER INSERT ON datausage
BEGIN
UPDATE datausage SET usedtime = endtime - stime WHERE id = new.id;
END
;
Example
Consider the following demonstration:-
DROP TABLE IF EXISTS datausage;
Create table if not exists datausage (Id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, mainentry text, stime integer , endtime integer, usedtime integer);
CREATE TRIGGER IF NOT EXISTS datausage_timedifference AFTER INSERT ON datausage
BEGIN
UPDATE datausage SET usedtime = endtime - stime WHERE id = new.id;
END
;
INSERT INTO datausage VALUES (null,'mainentry1',10,30,0),(null,'mainentry2',50,75,0);
SELECT *, endtime - stime AS alternative_usedtime FROM datausage;
This will :-
drop the table (for ease of rerunning/adjusting, you wouldn't do this normally)
create the table
create the trigger named datausage_timedifference that will automatically update the usedtime after a row is inserted.
add some rows where the usedtime is 0
select the rows from the table showing that usedtime has been automatically calculated
The result being :-
Note that the above also shows that there is no real need to store a value that is derived from other values, as this can be calculated when extracting the value from the database as per
endtime - stime AS alternative_usedtime
This generates an extra column in the result set (Cursor) called alternative_usedtime that is the difference between stime and endtime. As such the usedtime column isn't required.
Deriving a value in this way also does away with the potential need for a second AFTER UPDATE trigger to handle any changes made to the stime or endtime columns.