Dynamic Insert Loop - sql-server

Question

As I understand it, the following code:
declare #csvnmbr nvarchar(3)
declare #sql nvarchar(400)
set #csvnmbr = 1
set #sql = 'BULK INSERT #tablename FROM ''C:\TEMP\'+#csvnmbr+'.csv''
WITH (FIELDTERMINATOR = '','')'
while #csvnmbr < 4
begin
print #sql
set #csvnmbr = #csvnmbr + 1
end
GO
Should print:
BULK INSERT #tablename FROM 'C:\TEMP\1.csv' WITH (FIELDTERMINATOR = ',')
BULK INSERT #tablename FROM 'C:\TEMP\2.csv' WITH (FIELDTERMINATOR = ',')
BULK INSERT #tablename FROM 'C:\TEMP\3.csv' WITH (FIELDTERMINATOR = ',')
but it doesn't, it prints:
BULK INSERT #tablename FROM 'C:\TEMP\1.csv' WITH (FIELDTERMINATOR = ',')
BULK INSERT #tablename FROM 'C:\TEMP\1.csv' WITH (FIELDTERMINATOR = ',')
BULK INSERT #tablename FROM 'C:\TEMP\1.csv' WITH (FIELDTERMINATOR = ',')
Does anyone know why the loop won't print the number sequence but just sticks to the first csv number instead?
If I do this though:
declare #csvnmbr nvarchar(3)
declare #sql nvarchar(400)
set #csvnmbr = 1
set #sql = 'HI'
while #csvnmbr < 4
begin
print #sql
print #csvnmbr
set #csvnmbr = #csvnmbr + 1
end
GO
There is no apparent need to write the #sql inside the loop. Any idea why the fact that the #sql statement is dynamic means it needs to be reset each time?

Answer 1

Because it is set once before loop starts. You need to set #sql inside loop:
DECLARE #csvnmbr INT = 1
,#sql NVARCHAR(MAX);
WHILE #csvnmbr < 4
BEGIN
SET #sql = N'BULK INSERT #tablename FROM ''C:\TEMP\'' +
CAST(#csvnmbr AS NVARCHAR(10)) +
'.csv'' WITH (FIELDTERMINATOR = '','')';
PRINT #sql;
SET #csvnmbr += 1;
END
About your question why it needs to be inside loop:
declare #csvnmbr nvarchar(3)
declare #sql nvarchar(400)
set #csvnmbr = 1
/* At this point #sql if fixed string, during concatenation #csvnmbr was
replaced by its actual value */
set #sql = 'BULK INSERT #tablename FROM ''C:\TEMP\'+#csvnmbr+'.csv''
WITH (FIELDTERMINATOR = '','')'
while #csvnmbr < 4
begin
print #sql /* Here sql will be always the same you need to set it again */
set #csvnmbr = #csvnmbr + 1
end
What you trying to achieve is passing string by reference. You can mimic it like this:
DECLARE #csvnmbr NVARCHAR(3) = '1'
,#sql NVARCHAR(MAX);
SET #sql =
N'BULK INSERT #tablename FROM ''C:\TEMP\#csvnmbr.csv'' WITH (FIELDTERMINATOR = '','')';
WHILE #csvnmbr < 4
BEGIN
/* Warning!!! This will execute code in #sql*/
EXEC [dbo].[sp_executesql]
#sql,
N'#csvnmbr nvarchar(3)',
#cvnmbr; /* Will substitute #csvnmbr with passed value */
SET #csvnmbr += 1;
END

Answer 2

Keep your statement inside the while loop
declare #csvnmbr nvarchar(3)
declare #sql nvarchar(400)
set #csvnmbr = 1
while #csvnmbr < 4
begin
set #sql = 'BULK INSERT #tablename FROM ''C:\TEMP\'+#csvnmbr+'.csv''
WITH (FIELDTERMINATOR = '','')'
print #sql
set #csvnmbr = #csvnmbr + 1
end
GO
the while loop iteration happens between the while loop condition begin and end blocks.
WHILE (Some Condition) --<-- first the condition is checked
BEGIN
--<-- if the while condition is evaluated to true
-- code between the BEGIN and END is executed
-- it keeps getting executing until while condition returns false
END

Answer 3

You need to fill #sql in each iteration:
DECLARE #csvnmbr NVARCHAR(3);
DECLARE #sql NVARCHAR(400);
SET #csvnmbr = 1;
SET #sql = 'BULK INSERT #tablename FROM ''C:\TEMP\' + #csvnmbr + '.csv''
WITH (FIELDTERMINATOR = '','')';
WHILE #csvnmbr < 4
BEGIN
PRINT #sql;
SET #csvnmbr = #csvnmbr + 1;
SET #sql = 'BULK INSERT #tablename FROM ''C:\TEMP\' + #csvnmbr + '.csv''
WITH (FIELDTERMINATOR = '','')';
END;
GO