How to DELETE on a table using the table's name - sql-server

Question

I'm trying to write a trigger that checks for if a bit is true before deleting a table, and setting it as inactive if said bit is false. My trigger was this:
DELETE Contacts
FROM Contacts
INNER JOIN deleted ON Contacts.ContactID = deleted.ContactID
WHERE deleted.AllowDelete = 1
UPDATE Contacts
SET Active = 0
FROM Contacts
INNER JOIN deleted on Contacts.ContactID = deleted.ContactID
WHERE deleted.allowDelete = 0
Where Contacts is the table someone is trying to delete from. However, using this trigger on every table where it is necessary seemed inefficient, so I'm trying to normalize it with a stored procedure.
The idea is to exec the SP with the tablename as a variable, and the deleted table put into a temptable. Right now the trigger looks like this:
SELECT *
INTO #deleted
FROM deleted
DROP TABLE #deleted
And the SP looks like this:
ALTER PROCEDURE [dbo].[OnDeleteTrigger]
-- Add the parameters for the stored procedure here
#TableToDeleteFrom nvarchar(max) = ''
AS
BEGIN
SET NOCOUNT ON;
DECLARE
DELETE Contacts
FROM Contacts
INNER JOIN #deleted ON Contacts.ContactID = #deleted.ContactID
WHERE #deleted.AllowDelete = 1
Update Contacts
SET Active = 0
FROM Contacts
INNER JOIN #deleted ON Contacts.ContactID = #deleted.ContactID
WHERE #deleted.AllowDelete = 1
END
The deleted temptable seems to work fine, although I can't test it yet as I can't find a way to get the table dbo from the table name, to replace all the 'Contacts'.
Hopefully this is enough information to get an answer, if not I'll edit it later.

Answer 1

Having a separate trigger on each table with static SQL is more efficient, albeit doesn't lend itself well to code reuse. The other tables will have different primary key column names than ContactID so you would need to pass that name as well as the table name.
With the desired tables tables all having standard columns Active and AllowDelete, you could create the needed triggers dynamically during deployment rather than dynamic SQL at run time. Below is an example of this technique, using MERGE instead of separate INSERT and UPDATE statements:
CREATE TABLE dbo.Contacts(
ContactID int NOT NULL CONSTRAINT PK_Contacts PRIMARY KEY
, Active bit NOT NULL
, AllowDelete bit NOT NULL
);
CREATE TABLE dbo.Users(
UserID int NOT NULL CONSTRAINT PK_Users PRIMARY KEY
, Active bit NOT NULL
, AllowDelete bit NOT NULL
);
INSERT INTO dbo.Contacts VALUES(1, 1, 0);
INSERT INTO dbo.Contacts VALUES(2, 1, 0);
INSERT INTO dbo.Contacts VALUES(3, 1, 1);
INSERT INTO dbo.Contacts VALUES(4, 1, 1);
INSERT INTO dbo.Users VALUES(1, 1, 0);
INSERT INTO dbo.Users VALUES(2, 1, 0);
INSERT INTO dbo.Users VALUES(3, 1, 1);
INSERT INTO dbo.Users VALUES(4, 1, 1);
GO
CREATE PROC dbo.CreateSoftDeleteTrigger
#SchemaName sysname
, #TableName sysname
, #JoinCriteria nvarchar(MAX)
AS
SET NOCOUNT ON;
DECLARE #SQL nvarchar(MAX);
SET #SQL = N'CREATE TRIGGER ' + QUOTENAME(N'TRD_' + #TableName) + N'
ON ' + QUOTENAME(#SchemaName) + N'.' + QUOTENAME(#TableName) + N'
INSTEAD OF DELETE
AS
SET NOCOUNT ON;
MERGE ' + QUOTENAME(#SchemaName) + N'.' + QUOTENAME(#TableName) + N' AS target
USING deleted ON ' + #JoinCriteria + N'
WHEN MATCHED AND deleted.AllowDelete = 1 THEN DELETE
WHEN MATCHED AND deleted.AllowDelete = 0 THEN UPDATE SET Active = 0;'
EXECUTE(#SQL);
GO
EXEC dbo.CreateSoftDeleteTrigger
#SchemaName = N'dbo'
, #TableName = N'Contacts'
, #JoinCriteria = N'target.ContactID = deleted.ContactID';
EXEC dbo.CreateSoftDeleteTrigger
#SchemaName = N'dbo'
, #TableName = N'Users'
, #JoinCriteria = N'target.UserID = deleted.UserID ';
GO
--soft delete test
DELETE FROM dbo.Contacts WHERE ContactID = 1;
SELECT * FROM Contacts;
--hard delete test
DELETE FROM dbo.Users WHERE UserID = 4;
SELECT * FROM Users;
GO

Answer 2

I guess you want to use the trigger "instead of":
https://technet.microsoft.com/en-us/library/ms175521(v=sql.105).aspx
CREATE TABLE Contacts
(
id int identity(1,1) primary key clustered,
[name] varchar(50),
isactive bit not null default(1),
SoftDeletion bit not null default(1)
)
insert into Contacts([name]) values ('my'),('myself'),('I');
insert into Contacts([name],SoftDeletion) values ('Killable', 0);
GO
CREATE TRIGGER trgInsteadOfDelete ON Contacts
INSTEAD OF DELETE
AS
BEGIN
UPDATE Contacts
set isactive = 0
where
SoftDeletion = 1
and
id in (SELECT ID from deleted);
DELETE FROM Contacts
WHERE
softdeletion = 0
AND
id in (SELECT ID from deleted);
END
GO
SELECT * from Contacts;
DELETE FROM Contacts;
SELECT * from Contacts;