Cursor - Declare Select-Statement executed multiple times? - sql

Question

I've got a question regarding cursor in t-sql.
when i do a cursor like that, it will end up in an endless loop.
drop table [dbo].[cursorcheck]
GO
CREATE TABLE [dbo].[cursorcheck](
[ID] [int] IDENTITY(1,1) NOT NULL,
[Wert] [varchar](10) NOT NULL
) ON [PRIMARY]
GO
delete dbo.cursorcheck
GO
insert into dbo.cursorcheck
select 'Insert'
GO
DECLARE #vendor_id int, #vendor_name nvarchar(50);
DECLARE vendor_cursor CURSOR FOR
SELECT ID, wert
FROM dbo.cursorcheck
WHERE wert = 'insert';
OPEN vendor_cursor
FETCH NEXT FROM vendor_cursor
INTO #vendor_id, #vendor_name
WHILE ##FETCH_STATUS = 0
BEGIN
insert into dbo.cursorcheck
select 'Insert'
FETCH NEXT FROM vendor_cursor
INTO #vendor_id, #vendor_name
END
CLOSE vendor_cursor;
DEALLOCATE vendor_cursor;
I'm a bit confused about this behavior. Does that mean that the select script, the cursor was declared for, is executed multiple times? i got the same effect when i insert into that table from another transaction while debugging the cursor. the problem is solved when i use the "static" keyword for declaring the cursor. but i'm confused about the behavior of the cursor when i don't use the static keyword.
any explanation?
Regards,
Reto

Answer 1

The key is you try to read and insert the same table in loop (so it is like "a snake eating own tail"):
Cursor:
DECLARE vendor_cursor CURSOR FOR
SELECT ID, wert
FROM dbo.cursorcheck
WHERE wert = 'insert';
And loop:
WHILE ##FETCH_STATUS = 0
BEGIN
insert into dbo.cursorcheck
select 'Insert'
FETCH NEXT FROM vendor_cursor
INTO #vendor_id, #vendor_name
END
For second question: The problem is solved when i use the "static":
STATIC CURSOR
The complete result set of a static cursor is built in tempdb when the
cursor is opened. A static cursor always displays the result set as it
was when the cursor was opened.
and:
The cursor does not reflect any changes made in the database that
affect either the membership of the result set or changes to the
values in the columns of the rows that make up the result set. A
static cursor does not display new rows inserted in the database after
the cursor was opened, even if they match the search conditions of the
cursor SELECT statement