Update a column with it's concatenated previous value? - sql

Question

Hello dear Stackoverflow SQL gurus.
Using this simple data model:
create table test(Id INT, Field1 char(1), Field2 varchar(max));
insert into test (id, Field1) values (1, 'a');
insert into test (id, Field1) values (2, 'b');
insert into test (id, Field1) values (3, 'c');
insert into test (id, Field1) values (4, 'd');
I'm able to update Field2 with Field1 and Field2 concatenated previous value in a simple TSQL anonymous block like this :
BEGIN
DECLARE #CurrentId INT;
DECLARE #CurrentField1 char(1);
DECLARE #Field2 varchar(max) = NULL;
DECLARE cur CURSOR FOR
SELECT id, Field1
FROM test
ORDER BY id;
OPEN cur
FETCH NEXT FROM cur INTO #CurrentId, #CurrentField1;
WHILE ##FETCH_STATUS = 0
BEGIN
SET #Field2 = CONCAT(#Field2, #CurrentId, #CurrentField1);
UPDATE test
SET Field2 = #Field2
WHERE Id = #CurrentId;
FETCH NEXT FROM cur INTO #CurrentId, #CurrentField1;
END
CLOSE cur;
DEALLOCATE cur;
END
GO
Giving me the desired result:
select * from test;
Id Field1 Field2
1 a 1a
2 b 1a2b
3 c 1a2b3c
4 d 1a2b3c4d
I want to achieved the same result with a single UPDATE command to avoid CURSOR.
I thought it was possible with the LAG() function:
UPDATE test set Field2 = NULL; --reset data
UPDATE test
SET Field2 = NewValue.NewField2
FROM (
SELECT CONCAT(Field2, Id, ISNULL(LAG(Field2,1) OVER (ORDER BY Id), '')) AS NewField2,
Id
FROM test
) NewValue
WHERE test.Id = NewValue.Id;
But this give me this:
select * from test;
Id Field1 Field2
1 a 1
2 b 2
3 c 3
4 d 4
Field2 is not correctly updated with Id+Field1+(previous Field2).
The update result is logic to me because when the LAG() function re-select the value in the table this value is not yet updated.
Do you think their is a way to do this with a single SQL statement?

Answer 1

One method is with a recursive Common Table Expression (rCTE) to iterate through the data. This assumes that all values of Id are sequential:
WITH rCTE AS(
SELECT Id,
Field1,
CONVERT(varchar(MAX),CONCAT(ID,Field1)) AS Field2
FROM dbo.test
WHERE ID = 1
UNION ALL
SELECT t.Id,
t.Field1,
CONVERT(varchar(MAX),CONCAT(r.Field2,t.Id,t.Field1)) AS Field2
FROM dbo.test t
JOIN rCTe r ON t.id = r.Id + 1)
SELECT *
FROM rCTe;
If they aren't sequential, you can use a CTE to row number the rows first:
WITH RNs AS(
SELECT Id,
Field1,
ROW_NUMBER() OVER (ORDER BY ID) AS RN
FROM dbo.Test),
rCTE AS(
SELECT Id,
Field1,
CONVERT(varchar(MAX),CONCAT(ID,Field1)) AS Field2,
RN
FROM RNs
WHERE ID = 1
UNION ALL
SELECT RN.Id,
RN.Field1,
CONVERT(varchar(MAX),CONCAT(r.Field2,RN.Id,RN.Field1)) AS Field2,
RN.RN
FROM RNs RN
JOIN rCTe r ON RN.RN = r.RN + 1)
SELECT Id,
Field1,
Field2
FROM rCTe;

Answer 2

Unfortunately, SQL Server does not (yet) support string_agg() as a window function.
Instead, you can use cross apply to calculate the values:
select t.*, t2.new_field2
from test t cross apply
(select string_agg(concat(id, field1), '') within group (order by id) as new_field2
from test t2
where t2.id <= t.id
) t2;
For an update:
with toupdate as (
select t.*, t2.new_field2
from test t cross apply
(select string_agg(concat(id, field1), '') within group (order by id) as new_field2
from test t2
where t2.id <= t.id
) t2
)
update toupdate
set field2 = new_field2;
Here is a db<>fiddle.
Note: This works for small tables, but it would not be optimal on large tables. But then again, on large tables, the string would quickly become unwieldy.