tsql nested cursor does not do a properly rollback continue - sql-server

Question

I have the following script which works fine:
DECLARE db_cursor1 CURSOR LOCAL FOR
SELECT ID, Name table_1
OPEN db_cursor1
FETCH NEXT FROM db_cursor1 INTO #ID, #Name
WHILE ##FETCH_STATUS = 0
BEGIN
BEGIN TRANSACTION
BEGIN TRY
<insert into table values>
COMMIT TRANSACTION
END TRY
BEGIN CATCH
PRINT ERROR_MESSAGE();
ROLLBACK TRANSACTION
END CATCH
FETCH NEXT FROM db_cursor1 INTO #ID, #Name
END
CLOSE db_cursor1
DEALLOCATE db_cursor1
The above script works fine in that it rolls back what is in the current iteration of db_cursor1 and then proceeds to the next iteration in case there is an error.
The problem arises when I have a nested cursor. It rolls back what is in the current iteration but DOES NOT proceed to the next iteration of cursor1.
DECLARE db_cursor1 CURSOR LOCAL FOR
SELECT ID, Name table_1
OPEN db_cursor1
FETCH NEXT FROM db_cursor1 INTO #ID, #Name
WHILE ##FETCH_STATUS = 0
BEGIN
BEGIN TRANSACTION
BEGIN TRY
<insert into table values>
--- inner cursor
DECLARE db_cursor2 CURSOR LOCAL FOR
SELECT ID, Name table_2
OPEN db_cursor2
FETCH NEXT FROM db_cursor2 INTO #ID, #Name
WHILE ##FETCH_STATUS = 0
BEGIN
<insert into table values>
FETCH NEXT FROM db_cursor2 INTO #ID, #Name
END
CLOSE db_cursor2
DEALLOCATE db_cursor2
COMMIT TRANSACTION
END TRY
BEGIN CATCH
PRINT ERROR_MESSAGE();
ROLLBACK TRANSACTION
END CATCH
FETCH NEXT FROM db_cursor1 INTO #ID, #Name
END
CLOSE db_cursor1
DEALLOCATE db_cursor1

Answer 1

I was able to create it on my end with basic values, please see SCRIPT #1 - ERROR if you'd like to test it. The issue that you're running into is that when you have an error in db_cursor2, you exit the loop without closing or deallocating the cursor. Then when the code goes to next iteration, it fails with this error A cursor with the name 'db_cursor2' already exists. Please see SCRIPT #2 - SUCCESS for correct results. To give it more color, you needed to add CLOSE db_cursor2; DEALLOCATE db_cursoe2; in your BEGIN CATCH.
SETUP, Designed for SQL Server 2016+
DROP TABLE IF EXISTS #table_1, #table_2
CREATE TABLE #table_1
(
[ID] INT,
[Name] VARCHAR(5)
);
CREATE TABLE #table_2
(
[ID] INT,
[NAME] VARCHAR(5)
);
INSERT INTO #table_1 SELECT 1, 'j';
INSERT INTO #table_1 SELECT 2, 'j';
INSERT INTO #table_2 SELECT 1, 'j';
INSERT INTO #table_2 SELECT 2, 'j';
SCRIPT #1 - ERROR
DECLARE #ID INT;
DECLARE #name VARCHAR(5);
DECLARE db_cursor1 CURSOR LOCAL FOR SELECT [ID], [Name] FROM #table_1;
OPEN db_cursor1;
FETCH NEXT FROM db_cursor1
INTO #ID, #name;
WHILE ##FETCH_STATUS = 0
BEGIN
BEGIN TRANSACTION;
BEGIN TRY
PRINT('trying 1')
--- inner cursor
DECLARE db_cursor2 CURSOR LOCAL FOR SELECT [ID], [Name] FROM #table_2;
OPEN db_cursor2;
FETCH NEXT FROM db_cursor2
INTO #ID, #name;
WHILE ##FETCH_STATUS = 0
BEGIN
PRINT('trying 2')
SELECT 1/0
FETCH NEXT FROM db_cursor2
INTO #ID, #name;
END;
CLOSE db_cursor2;
DEALLOCATE db_cursor2;
COMMIT TRANSACTION;
END TRY
BEGIN CATCH
PRINT ERROR_MESSAGE();
ROLLBACK TRANSACTION;
END CATCH;
FETCH NEXT FROM db_cursor1
INTO #ID, #name;
END;
CLOSE db_cursor1;
DEALLOCATE db_cursor1;
SCRIPT #2 - SUCCESS
DECLARE #ID INT;
DECLARE #name VARCHAR(5);
DECLARE db_cursor1 CURSOR LOCAL FOR SELECT [ID], [Name] FROM #table_1;
OPEN db_cursor1;
FETCH NEXT FROM db_cursor1
INTO #ID, #name;
WHILE ##FETCH_STATUS = 0
BEGIN
BEGIN TRANSACTION;
BEGIN TRY
PRINT('trying 1')
--- inner cursor
DECLARE db_cursor2 CURSOR LOCAL FOR SELECT [ID], [Name] FROM #table_2;
OPEN db_cursor2;
FETCH NEXT FROM db_cursor2
INTO #ID, #name;
WHILE ##FETCH_STATUS = 0
BEGIN
PRINT('trying 2')
SELECT 1/0
FETCH NEXT FROM db_cursor2
INTO #ID, #name;
END;
CLOSE db_cursor2;
DEALLOCATE db_cursor2;
COMMIT TRANSACTION;
END TRY
BEGIN CATCH
PRINT ERROR_MESSAGE();
-- was missing in above script
CLOSE db_cursor2
DEALLOCATE db_cursor2
ROLLBACK TRANSACTION;
END CATCH;
FETCH NEXT FROM db_cursor1
INTO #ID, #name;
END;
CLOSE db_cursor1;
DEALLOCATE db_cursor1;

Answer 2

Because you are using a TRY-CATCH, if there is an error within the TRY, code execution will begin in the CATCH.
Inside your catch you would have to handle the error, and depending on the error, close and deallocate db_cursor2. Or if the error is benign, perhaps return execution back to the TRY with a GOTO. GOTO statements cannot be used to enter a TRY or CATCH block. GOTO statements can be used to jump to a label inside the same TRY or CATCH block or to leave a TRY or CATCH block.

Answer 3

As #paul-wehland stated, it's because your Try-Catch isn't disposing the nested cursor. As such, your next iteration is going to initialize a cursor by name that already exists. I've provided an example of code that will run your basic scenario with an intended failure condition at iteration 11 of each cursor.
In the example, I've commented out the part of the code that addresses the issue. Where you choose to place that block is totally up to you, but it would make sense to check either before the nested cursor declaration, or inside the Catch block.
declare
#id tinyint,
#parent_id tinyint,
#name varchar(255),
#parent_name varchar(255);
declare
#table
table
(
id tinyint not null primary key,
[name] varchar(255) not null
);
declare
#target
table
(
parent_id tinyint not null,
child_id tinyint not null,
parent_name varchar(10) not null,
child_name varchar(10) not null,
primary key(parent_id, child_id)
);
with cteNumber
as (
select top 11
[id] = row_number() over (order by [object_id])
from
sys.objects
)
insert into
#table
select
id,
[name] = replicate('a', id)
from
cteNumber;
declare
db_cursor1
cursor
local keyset read_only forward_only
for
select
0,
id,
'Initial',
[name]
from
#table;
open
db_cursor1;
fetch
next
from
db_cursor1
into
#id,
#parent_id,
#name,
#parent_name;
while ##FETCH_STATUS = 0
begin
begin transaction;
begin try
insert into #target
(parent_id, child_id, parent_name, [child_name])
values
(#parent_id, #id, #parent_name, #name);
--- inner cursor
/*
if CURSOR_STATUS('local', 'db_cursor2') = 1
begin
close
db_cursor2;
deallocate
db_cursor2;
end;
-- */
declare
db_cursor2
cursor
local keyset read_only forward_only
for
select
id,
[name]
from
#table;
open
db_cursor2;
fetch
next
from
db_cursor2
into
#id,
#name;
while ##FETCH_STATUS = 0
begin
insert into #target
(parent_id, child_id, parent_name, [child_name])
values
(#parent_id, #id, #parent_name, #name);
fetch
next
from
db_cursor2
into
#id,
#name;
end;
close
db_cursor2;
deallocate
db_cursor2;
commit transaction
end try
begin catch
print ERROR_MESSAGE();
rollback transaction;
end catch;
fetch
next
from
db_cursor1
into
#id,
#parent_id,
#name,
#parent_name;
end;
close
db_cursor1;
deallocate
db_cursor1;
select
[Last #id] = #id,
[Last #name] = #name,
[Last #parent_id] = #parent_id,
[Last #parent_name] = #parent_name;
select
*
from
#table;
select
*
from
#target;
EDITED
You could also use the creation of a Cursor Variable, and assign the nested cursor declaration to it, which would negate the problem of dealing with the duplicate names. See below:
declare
#id tinyint,
#parent_id tinyint,
#name varchar(255),
#parent_name varchar(255);
declare
#table
table
(
id tinyint not null primary key,
[name] varchar(255) not null
);
declare
#target
table
(
parent_id tinyint not null,
child_id tinyint not null,
parent_name varchar(10) not null,
child_name varchar(10) not null,
primary key(parent_id, child_id)
);
with cteNumber
as (
select top 11
[id] = row_number() over (order by [object_id])
from
sys.objects
)
insert into
#table
select
id,
[name] = replicate('a', id)
from
cteNumber;
declare
#db_cursor2 cursor;
declare
db_cursor1
cursor
local keyset read_only forward_only
for
select
0,
id,
'Initial',
[name]
from
#table;
open
db_cursor1;
fetch
next
from
db_cursor1
into
#id,
#parent_id,
#name,
#parent_name;
while ##FETCH_STATUS = 0
begin
begin transaction;
begin try
insert into #target
(parent_id, child_id, parent_name, [child_name])
values
(#parent_id, #id, #parent_name, #name);
--- inner cursor
set #db_cursor2 = cursor
local keyset read_only forward_only
for
select
id,
[name]
from
#table;
open
#db_cursor2;
fetch
next
from
#db_cursor2
into
#id,
#name;
while ##FETCH_STATUS = 0
begin
insert into #target
(parent_id, child_id, parent_name, [child_name])
values
(#parent_id, #id, #parent_name, #name);
fetch
next
from
#db_cursor2
into
#id,
#name;
end;
close
#db_cursor2;
deallocate
#db_cursor2;
commit transaction
end try
begin catch
print ERROR_MESSAGE();
rollback transaction;
end catch;
fetch
next
from
db_cursor1
into
#id,
#parent_id,
#name,
#parent_name;
end;
close
db_cursor1;
deallocate
db_cursor1;
select
[Last #id] = #id,
[Last #name] = #name,
[Last #parent_id] = #parent_id,
[Last #parent_name] = #parent_name;
select
*
from
#table;
select
*
from
#target;