The difference between PRINT ##ROWCOUNT and OUTPUT $ACTION in sql server - tsql

Question

Apologies if my question seems to be naive:
I cannot get my head around the 2 statements below, can someone please explain the difference:
OUTPUT $ACTION, INSERTED.BuildRequestID, ..... and
PRINT ##ROWCOUNT
Apparently, they both can be used to print something on the window, with output in the example above, the records that have been inserted will be displayed. And, PRINT ##ROWCOUNT returns the number of rows affected by the last executed statement in the batch, so, if the function was insert, then it will show the inserted records?
Thank you,

Answer 1

In its simplest terms, OUTPUT will give you the actual records affected by a DML statement (INSERT, UPDATE, DELETE, MERGE), ##ROWCOUNT will just tell you how many rows were affected by the previous Statement (not limited to DML).
This is probably easiest understood with a working example that you can run yourself and see both in action:
IF OBJECT_ID(N'tempdb..#T', 'U') IS NOT NULL
DROP TABLE #T;
-- CHECK ##ROWCOUNT
DECLARE #RowCountFromDropTable INT = ##ROWCOUNT;
-- CREATE A TABLE
CREATE TABLE #T (ID INT NOT NULL PRIMARY KEY, Col CHAR(1) NOT NULL);
-- INSERT SOME VALUES AND CHECK THE OUTPUT
INSERT #T (ID, Col)
OUTPUT inserted.*
VALUES (1, 'A'), (2, 'B'), (3, 'C');
-- CHECK ##ROWCOUNT
DECLARE #RowCountFromInsert INT = ##ROWCOUNT;
-- DELETE A VALUE AND INSPECT THE DELETED RECORD WITH OUTPUT
DELETE #T
OUTPUT deleted.*
WHERE ID = 3;
-- CHECK ##ROWCOUNT
DECLARE #RowCountFromDelete INT = ##ROWCOUNT;
-- UPDATE A RECORD AND VIEW BEFORE AND AFTER VALUES
UPDATE #T
SET Col = 'X'
OUTPUT inserted.ID AS ID,
inserted.Col AS UpdatedTo,
deleted.Col AS UpdatedFrom
WHERE ID = 2;
-- CHECK ##ROWCOUNT
DECLARE #RowCountFromUpdate INT = ##ROWCOUNT;
-- USE MERGE, AND CAPTURE ACTION:
MERGE #T AS t
USING (VALUES (2, 'B'), (3, 'C')) AS s (ID, Col)
ON s.ID = t.ID
WHEN NOT MATCHED THEN INSERT (ID, Col) VALUES (s.ID, s.Col)
WHEN MATCHED THEN UPDATE SET Col = s.Col
WHEN NOT MATCHED BY SOURCE THEN DELETE
OUTPUT $Action AS DMLAction,
inserted.ID AS InsertedID,
inserted.Col AS InsertedCol,
deleted.ID AS DeletedID,
deleted.Col AS DeletedCol;
-- CHECK ##ROWCOUNT
DECLARE #RowCountFromMerge INT = ##ROWCOUNT;
SELECT RowCountFromDropTable = #RowCountFromDropTable,
RowCountFromInsert = #RowCountFromInsert,
RowCountFromDelete = #RowCountFromDelete,
RowCountFromUpdate = #RowCountFromUpdate,
RowCountFromMerge = #RowCountFromMerge;
The recordsets output from each of the DML are:
INSERT
ID Col
-------
1 A
2 B
3 C
DELETE
ID Col
-------
3 C
UPDATE
ID UpdatedTo UpdatedFrom
---------------------------
2 X B
MERGE
DMLAction InsertedID InsertedCol DeletedID DeletedCol
------------------------------------------------------------
INSERT 3 C NULL NULL
DELETE NULL NULL 1 A
UPDATE 2 B 2 X
INSPECT ##ROWCOUNTS
RowCountFromDropTable RowCountFromInsert RowCountFromUpdate RowCountFromMerge
--------------------------------------------------------------------------------
0 3 1 3
A quick point on some wording in the qeustion too: You cannot use OUTPUT directly to print something to the window, it returns records much like a SELECT statement. ##ROWCOUNT can be used like any scalar function, so you could use this in consecutive statements. So you could do something like this:
SELECT TOP (1) *
FROM (VALUES (1), (2), (3)) AS t (ID);
SELECT TOP (##ROWCOUNT + 1) *
FROM (VALUES (1), (2), (3)) AS t (ID);
SELECT TOP (##ROWCOUNT + 1) *
FROM (VALUES (1), (2), (3)) AS t (ID);
Which returns 1, 1,2 and 1,2,3 respectively. I have no idea why you would want to do this, but it demonstrates the scope of ##ROWCOUNT a bit better than the above, and how it can be used elsewhere.