Checking some columns for specific values before insert/update using a trigger

问题

I have a database FOO with several columns, among those I have one column "Url". I need to write a trigger before insert/update that will check the Url columns whether the newer value matches any existing values, i.e. "hello" except some predefined value. That means if "hello" is inserted or updated multiple times no error will happen otherwise it will check for duplicity. And if it finds some aborts the insertion update. This will also return some code so that my script calling for the insertion/update will know a failure has occurred. I know there might be other workarounds but I will need to have it this way. I am pretty new to SQL.

Foo {
    Url
}

Here is the algorithm

Before update insert
if new value of Url is not "hello1" o "hello 2" 
    check if new value of Url already exists in Foo.Url if so abort otherwise allow update/insert
     return something if aborted/success

回答1:

try something like this.. you'll need to index your table..

    IF EXISTS(SELECT URL FROM  Foo.Url)
      BEGIN 
       SELECT 'URL Exists Already'
      END
   ELSE 
     BEGIN 

      INSERT/UPDATE

     END

回答2:

A unique constraint wouldn't do what you want but you could create an instead of trigger with content something like as:

Create TRIGGER [dbo].[Trig_Insert_XXX]
ON [dbo].[XXX]
INSTEAD OF INSERT
AS
BEGIN
  SET NOCOUNT ON;
  INSERT INTO xxx ([url], field1, field2, fieldN)
  SELECT [url], field1, field2, fieldN
  FROM inserted i
  WHERE i.url = 'hello' OR NOT EXISTS (SELECT * FROM xxx t2 WHERE t2.url = i.url);
END;

回答3:

I suppose you're looking for a UNIQUE constraint & a CHECK constraint as

CREATE TABLE Foo(
  Url VARCHAR(250) NOT NULL,
  CONSTRAINT UQ_Url UNIQUE(Url),
  CONSTRAINT CHK_Url CHECK (Url NOT IN ('hello1', 'hello2'))
);

See how it's working online.

回答4:

If you are using SQL Server 2008 or newer version you can use MERGE as well, the syntax is like the following :

MERGE [TableName] AS TARGET 
USING ( SELECT @UrlName ) AS SOURCE (UrlName) ON SOURCE.UrlName = TARGET.UrlName
WHEN MATCHED THEN
UPDATE SET ...
WHEN NOT MATCHED THEN INSERT ()
VALUES ();

来源：https://stackoverflow.com/questions/60286195/checking-some-columns-for-specific-values-before-insert-update-using-a-trigger

标签

sql

sql-server

database

database-trigger