Update and Insert When Condition is Matched in TSQL-Merge - sql-server

Question

I have been trying to Write a Stored Procedure where i can perform UpSert using Merge with the Following Condition
If Record is Present then change EndDate of Target to Yesterday's day i.e., Present Day - 1
If Record is not Present then Insert New Record
Here is the Table tblEmployee i used in SP
CREATE TABLE tblEmployee
(
[EmployeeID] [int] IDENTITY(1,1) NOT NULL,
[Name] [varchar](10) NOT NULL,
[StartDate] [date] NOT NULL,
[EndDate] [date] NOT NULL
)
Here is my SP which Takes UDTT as Input parameter
CREATE PROCEDURE [dbo].[usp_UpsertEmployees]
#typeEmployee typeEmployee READONLY -- It has same column like tblEmployye except EmployeeID
AS
BEGIN
SET NOCOUNT ON;
MERGE INTO tblEmployee AS TARGET
USING #typeEmployee AS SOURCE
ON TARGET.Name = SOURCE.Name
WHEN MATCHED and TARGET.StartDate < SOURCE.StartDate
THEN
--First Update Existing Record EndDate to Previous Date as shown below
UPDATE
set TARGET.EndDate = DATEADD(day, -1, convert(date, SOURCE.StartDate))
-- Now Insert New Record
--INSERT VALUES(SOURCE.Name, SOURCE.StartDate, SOURCE.EndDate);
WHEN NOT MATCHED by TARGET
THEN
INSERT VALUES(SOURCE.Name, SOURCE.StartDate, SOURCE.EndDate);
SET NOCOUNT OFF;
END
How can i perform both Updating Existing Record and Adding New Record When Column is matched
Can Please someone Explain me the Execution Flow of Merge in TSQL i.e.,
WHEN MATCHED --Will this Execute Everytime
WHEN NOT MATCHED by TARGET -- Will this Execute Everytime
WHEN NOT MATCHED by SOURCE -- Will this Execute Everytime
Will all above 3 condition get executed everytime in Merge or only Matching condition is executed Everytime
Thanks in Advance

Answer 1

This isn't what MERGE is meant to do (update and insert in same clause). To accomplish this, you can use the OUTPUT clause to get all the updated records only. The MERGE/OUTPUT combo is very picky. Your OUTPUT updates are really the TARGET records that got updated, so you have to start the TARGET records in a temp/table variable. Then you match those back against the SOURCE to do the INSERT. You won't be allowed to join the output results directly back to source or even use as a correlated subquery within the WHERE.
Setup some sample data
The code below just sets up some sample data.
-- Setup sample data
DECLARE #typeEmployee TABLE (
[Name] [varchar](10) NOT NULL,
[StartDate] [date] NOT NULL,
[EndDate] [date] NOT NULL
)
DECLARE #tblEmployee TABLE (
[EmployeeID] [int] IDENTITY(1,1) NOT NULL,
[Name] [varchar](10) NOT NULL,
[StartDate] [date] NOT NULL,
[EndDate] [date] NOT NULL
)
INSERT #tblEmployee VALUES ('Emp A', '1/1/2016', '2/1/2016')
INSERT #typeEmployee VALUES ('Emp A', '1/5/2016', '2/2/2016'), ('Emp B', '3/1/2016', '4/1/2016')
Updates to Stored Procedure
You can use OUTPUT at the end of a MERGE to have it return the modified records of the target records, and by including $action, you will also get whether it was an insert, update, or delete.
However, the result set from MERGE / OUTPUT cannot be directly joined against the SOURCE table so you can do your INSERT since you only get the TARGET records back. You can't use the results of the OUTPUT within correlated sub-query from the SOURCE table either. Easiest thing is to use a temp table or table variable to capture the output.
-- Logic to do upsert
DECLARE #Updates TABLE (
[Name] [varchar](10) NOT NULL,
[StartDate] [date] NOT NULL,
[EndDate] [date] NOT NULL
)
INSERT #Updates
SELECT
Name,
StartDate,
EndDate
FROM (
MERGE INTO #tblEmployee AS TARGET
USING #typeEmployee AS SOURCE
ON TARGET.Name = SOURCE.Name
WHEN MATCHED AND TARGET.StartDate < SOURCE.StartDate
THEN
--First Update Existing Record EndDate to Previous Date as shown below
UPDATE SET
EndDate = DATEADD(DAY, -1, CONVERT(DATE, SOURCE.StartDate))
WHEN NOT MATCHED BY TARGET -- OR MATCHED AND TARGET.StartDate >= SOURCE.StartDate -- Handle this case?
THEN
INSERT VALUES(SOURCE.Name, SOURCE.StartDate, SOURCE.EndDate)
OUTPUT $action, INSERTED.Name, INSERTED.StartDate, INSERTED.EndDate
-- Use the MERGE to return all changed records of target table
) AllChanges (ActionType, Name, StartDate, EndDate)
WHERE AllChanges.ActionType = 'UPDATE' -- Only get records that were updated
Now that you've captured the output of the MERGE and filtered to only get updated TARGET records, you can then do your outstanding INSERT by filtering only the SOURCE records that were part of the MERGE update.
INSERT #tblEmployee
SELECT
SOURCE.Name,
SOURCE.StartDate,
SOURCE.EndDate
FROM #typeEmployee SOURCE
WHERE EXISTS (
SELECT *
FROM #Updates Updates
WHERE Updates.Name = SOURCE.Name
-- Other join conditions to ensure 1:1 match against SOURCE (start date?)
)
Ouput
This is the output of the sample records after the change. Your intended TARGET changes were made.
-- Show output
SELECT * FROM #tblEmployee

Answer 2

Following the idea from the accepted answer, this works as well in Sql server 2008 r2:
create table Test1 (
Id int, FromDate date, ThruDate date, Value int
)
insert into dbo.Test1
(Id, FromDate, ThruDate, [Value])
select
t.Id, t.FromDate, T.ThruDate, t.Value * 100
from (
MERGE dbo.Test1 AS Target
USING (
select 1 as Id, '2000-01-01' as FromDate, '2000-12-31' as ThruDate, 2 as Value
) AS Source
ON ( target.id = source.Id
)
WHEN MATCHED
THEN
UPDATE SET Target.[Id] = Source.[Id]
, Target.[FromDate] = Source.[FromDate]
, Target.[ThruDate] = Source.[ThruDate]
, Target.[Value] = Source.[Value]
WHEN NOT MATCHED BY TARGET
THEN
INSERT ([Id]
, [FromDate]
, [ThruDate]
, [Value])
VALUES (Source.[Id]
, Source.[FromDate]
, Source.[ThruDate]
, Source.[Value])
OUTPUT $ACTION as Act, Inserted.*
) t
where t.Act = 'Update'
You can play with different values.