UPDATE with result from INSERT query - sql-server

Question

I have an existing table, A, defined as follows:
CREATE TABLE dbo.A(
id int IDENTITY(1, 1) NOT NULL PRIMARY KEY CLUSTERED,
name nvarchar(255) NOT NULL UNIQUE NONCLUSTERED
);
This table has 1,000s of existing records.
I now require a new table, B, defined as follows:
CREATE TABLE dbo.B(
id int IDENTITY(1, 1) NOT NULL PRIMARY KEY CLUSTERED,
some_value int NOT NULL
);
I need to create a record in B for each record in A and add a reference from the A record to the corresponding B record. Firstly, I need to add a new field to table A that references table B (which necessarily must be nullable with a default value of NULL, at this stage):
ALTER TABLE dbo.A
ADD b_id int NULL references dbo.B;
How can I create the necessary B records and update the A.b_id field accordingly? I'm looking for something like this:
UPDATE dbo.A
SET b_id = (INSERT INTO dbo.B(some_value) VALUES(5));
such that the value of b_id is the id field of the newly inserted B. (Clearly, this query isn't valid.)
The only solution I can come up with is to write a complex stored procedure using multiple, separate queries. Is there a way to do this with a single query?

Answer 1

IF I am understanding correctly, there's no need for a CURSOR here. What you could do is a MERGE that allows your to OUTPUT both columns from the source data and the target:
CREATE TABLE dbo.A(
id int IDENTITY(1, 1) NOT NULL PRIMARY KEY CLUSTERED,
name nvarchar(255) NOT NULL UNIQUE NONCLUSTERED
);
GO
WITH N AS(
SELECT N
FROM (VALUES(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL),(NULL))N(N))
INSERT INTO dbo.A
SELECT NEWID()
FROM N N1, N N2, N N3;
GO
CREATE TABLE dbo.B(
id int IDENTITY(1, 1) NOT NULL PRIMARY KEY CLUSTERED,
some_value int NOT NULL
);
GO
ALTER TABLE dbo.A
ADD b_id int NULL references dbo.B;
GO
DECLARE #IDs table (a_id int, b_id int)
MERGE dbo.B B
USING dbo.A A ON B.id = A.b_id
WHEN NOT MATCHED BY TARGET THEN
INSERT (some_value)
VALUES(RAND() * 1000)
OUTPUT A.id, inserted.id
INTO #IDs;
UPDATE A
SET b_id = I.b_id
FROM dbo.A A
JOIN #Ids I ON A.id = I.a_id;
GO
SELECT *
FROM dbo.A;
GO
DROP TABLE dbo.A;
DROP TABLE dbo.B;
db<>fiddle

Answer 2

If you are willing (and able) to make a slight adjustment to TableB as well, you can use OUTPUT when inserting. I added a_id to TableB for my example. Updating b_id back to TableA is a bit redundant as you would now have TableA's ID in TableB for joining.
Here's an example.
-- Mock tables.
DECLARE #TableA table ( id int IDENTITY (100,1) PRIMARY KEY, [name] nvarchar(255), b_id int );
DECLARE #TableB table ( id int IDENTITY (1,1) PRIMARY KEY, a_id int, some_value int );
-- Mock data.
INSERT INTO #TableA ( [name] ) VALUES
( 'TableA_1' ), ( 'TableA_2' ), ( 'TableA_3' );
-- Create an OUTPUT table to capture the values inserted into TableB.
DECLARE #out table ( a_id int, b_id int );
INSERT INTO #TableB ( a_id, some_value )
OUTPUT inserted.a_id, inserted.id INTO #out
SELECT
id AS a_id, 999 AS some_value
FROM #TableA;
-- Update TableA's b_id column.
UPDATE #TableA
SET
b_id = o.b_id
FROM #TableA AS a
INNER JOIN #out AS o
ON a.id = o.a_id;
-- Show updated results.
SELECT * FROM #TableA ORDER BY id;
Returns
+-----+----------+------+
| id | name | b_id |
+-----+----------+------+
| 100 | TableA_1 | 1 |
| 101 | TableA_2 | 2 |
| 102 | TableA_3 | 3 |
+-----+----------+------+