How to update a table using while loops and waitfor delay to insert current date with a one second delay between records? - sql

Question

I'd like to create a table that contains two columns (id int, today datetime) and, using while loops, to insert the current date every 1 second. However, the resulting table shows the same time for all rows. Below is my code. Can anyone help me understand what I'm doing wrong, please? Thank you!
declare #mytable table (id int, today datetime)
declare #id int=1
declare #today datetime=getdate()
while #id<10
begin
waitfor delay '00:00:01'
insert into #mytable values (#id,#today)
set #id=#id+1
end

Answer 1

The reason every row has the same value is because you aren't setting the value of #Today anywhere apart from before your WHILE loop. GETDATE() returns a scalar value, and setting a variable to that value means it will be set the value that GETDATE() returned at the time the SET was run. The value of the variable won't change after time has passed. For example:
DECLARE #d datetime;
SET #d = GETDATE();
SELECT #d, GETDATE(); --Will return very similar values
WAITFOR DELAY '00:00:05';
SELECT #d, GETDATE(); --#d will have the same value as before, as its value is static, but GETDATE()'s value will have changed.
To do what you're after, I don't see any need for the variable for #Today, this would work fine:
DECLARE #mytable table (id int,
today datetime);
DECLARE #id int = 1;
WHILE #id < 10
BEGIN
WAITFOR DELAY '00:00:01';
INSERT INTO #mytable
VALUES (#id, GETDATE());
SET #id = #id + 1;
END;
However a loop is a bad choice anyway, as an RDBMS excels at set based operations, not iterative. You would be far better to achieve what you're after by doing:
DECLARE #mytable table (id int,
today datetime);
DECLARE #id int = 1;
WITH N AS (
SELECT N
FROM (VALUES(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL)) N(N)),
Tally AS(
SELECT ROW_NUMBER() OVER (ORDER BY (SELECT NULL)) -1 AS I
FROM N N1
CROSS JOIN N N2 --Not actually eneded here, but shows how to increase row count
)
INSERT INTO #mytable (id,
today)
SELECT TOP 10
T.I + #ID,
DATEADD(SECOND, T.I, GETDATE())
FROM Tally T
ORDER BY T.I;
This builds an inline tally table, and then inserts a value for a row for 10 ID, and adds 1 second to each incremented ID.