How to copy tables avoiding cursors in SQL?

Question

I want to write script in SQL that will copy these 2 tables(A,B) to other 2 tables(C,D) with the same structure as A,B accordingly.

IMPORTANT:

1. Tables C,D are NOT necessary empty
2. Several processes may call script simultaneously

Table A has foreign key(fk_a_b) of table B

   ________________________  _________________
   |        Table A       |  |   Table B     |  
   |______________________|  |_______________|
   | id     FK_A_B   name |  | id    visible |
   | ----- -------- ------|  | ----- --------|
   | 1      21       n1   |  | 21     true   |
   | 5      32       n2   |  | 32     false  |
   ------------------------  -----------------

Let say that after copying table B to D this is what I get

   ________________
   |   Table D    |  
   |______________|
   | id   visible |
   | ----- -------|
   | 51    true   |
   | 52    false  |
   ----------------

Now, when I'll copy table A to C I need to know, somehow, that ID=21 maps now to ID=51, and ID=32 to ID=52. Finally, the table C will be:

   ________________________
   |        Table C       |
   |______________________|
   | id     FK_C_D   name |
   | ----- -------- ------|
   | 61      51       n1  |
   | 62      52       n2  |
   ------------------------

Because several processes may call script simultaneously, I CAN'T alter table A,B to add some helper columns. So, to achieve this I used CURSOR. I copied row by row of table B and managed temp table to map OldId to NewId(21->51,32->52) and then used this temp table to copy table A.

I've read that CURSOR is bad practice. So, is there another way to do it?

Thank you

Answer 1

You can use the output clause with the merge statement to get a mapping between source id and target id. Described in this question. Using merge..output to get mapping between source.id and target.id

Here is some code that you can test. I use table variables instead of real tables.

Setup sample data:

-- @A and @B is the source tables
declare @A as table
(
  id int,
  FK_A_B int,
  name varchar(10)
)

declare @B as table
(
  id int,
  visible bit
)  

-- Sample data in @A and @B
insert into @B values (21, 1),(32, 0)
insert into @A values (1, 21, 'n1'),(5, 32, 'n2')


-- @C and @D is the target tables with id as identity columns
declare @C as table
(
  id int identity,
  FK_C_D int not null,
  name varchar(10)
)

declare @D as table
(
  id int identity,
  visible bit
)  

-- Sample data already in @C and @D
insert into @D values (1),(0)
insert into @C values (1, 'x1'),(1, 'x2'),(2, 'x3')

Copy data:

-- The @IdMap is a table that holds the mapping between
-- the @B.id and @D.id (@D.id is an identity column)
declare @IdMap table(TargetID int, SourceID int)

-- Merge from @B to @D.
merge @D as D             -- Target table
using @B as B             -- Source table
on 0=1                    -- 0=1 means that there are no matches for merge
when not matched then
  insert (visible) values(visible)    -- Insert to @D
output inserted.id, B.id into @IdMap; -- Capture the newly created inserted.id and
                                      -- map that to the source (@B.id)

-- Add rows to @C from @A with a join to
-- @IdMap to get the new id for the FK relation
insert into @C(FK_C_D, name)
select I.TargetID, A.name 
from @A as A
  inner join @IdMap as I
    on A.FK_A_B = I.SourceID

Result:

select *
from @D as D
  inner join @C as C
    on D.id = C.FK_C_D

id          visible id          FK_C_D      name
----------- ------- ----------- ----------- ----------
1           1       1           1           x1
1           1       2           1           x2
2           0       3           2           x3
3           1       4           3           n1
4           0       5           4           n2

You can test the code here: https://data.stackexchange.com/stackoverflow/q/101643/using-merge-to-map-source-id-to-target-id