Only inserting a row if it’s not already there

Posted on

Only inserting a row if it’s not already there – This article will take you through the common SQL errors that you might encounter while working with sql, sql-server,  tsql. The wrong arrangement of keywords will certainly cause an error, but wrongly arranged commands may also be an issue. SQL keyword errors occur when one of the words that the SQL query language reserves for its commands and clauses is misspelled. If the user wants to resolve all these reported errors, without finding the original one, what started as a simple typo, becomes a much bigger problem.

SQL Problem :

I had always used something similar to the following to achieve it:

INSERT INTO TheTable
SELECT
    @primaryKey,
    @value1,
    @value2
WHERE
    NOT EXISTS
    (SELECT
        NULL
    FROM
        TheTable
    WHERE
        PrimaryKey = @primaryKey)

…but once under load, a primary key violation occurred. This is the only statement which inserts into this table at all. So does this mean that the above statement is not atomic?

The problem is that this is almost impossible to recreate at will.

Perhaps I could change it to the something like the following:

INSERT INTO TheTable
WITH
    (HOLDLOCK,
    UPDLOCK,
    ROWLOCK)
SELECT
    @primaryKey,
    @value1,
    @value2
WHERE
    NOT EXISTS
    (SELECT
        NULL
    FROM
        TheTable
    WITH
        (HOLDLOCK,
        UPDLOCK,
        ROWLOCK)
    WHERE
        PrimaryKey = @primaryKey)

Although, maybe I’m using the wrong locks or using too much locking or something.

I have seen other questions on stackoverflow.com where answers are suggesting a “IF (SELECT COUNT(*) … INSERT” etc., but I was always under the (perhaps incorrect) assumption that a single SQL statement would be atomic.

Does anyone have any ideas?

Solution :

What about the “JFDI” pattern?

BEGIN TRY
   INSERT etc
END TRY
BEGIN CATCH
    IF ERROR_NUMBER() <> 2627
      RAISERROR etc
END CATCH

Seriously, this is quickest and the most concurrent without locks, especially at high volumes.
What if the UPDLOCK is escalated and the whole table is locked?

Read lesson 4:

Lesson 4: When developing the upsert proc prior to tuning the indexes, I first trusted that the If Exists(Select…) line would fire for any item and would prohibit duplicates. Nada. In a short time there were thousands of duplicates because the same item would hit the upsert at the same millisecond and both transactions would see a not exists and perform the insert. After much testing the solution was to use the unique index, catch the error, and retry allowing the transaction to see the row and perform an update instead an insert.

I added HOLDLOCK which wasn’t present originally. Please disregard the version without this hint.

As far as I’m concerned, this should be enough:

INSERT INTO TheTable 
SELECT 
    @primaryKey, 
    @value1, 
    @value2 
WHERE 
    NOT EXISTS 
    (SELECT 0
     FROM TheTable WITH (UPDLOCK, HOLDLOCK)
     WHERE PrimaryKey = @primaryKey) 

Also, if you actually want to update a row if it exists and insert if it doesn’t, you might find this question useful.

You could use MERGE:

MERGE INTO Target
USING (VALUES (@primaryKey, @value1, @value2)) Source (key, value1, value2)
ON Target.key = Source.key
WHEN MATCHED THEN
    UPDATE SET value1 = Source.value1, value2 = Source.value2
WHEN NOT MATCHED BY TARGET THEN
    INSERT (Name, ReasonType) VALUES (@primaryKey, @value1, @value2)

Firstly, huge shout out to our man @gbn for his contributions to the community. Can’t even begin to explain how often I find myself following his advice.

Anyway, enough fanboy-ing.

To add slightly to his answer, perhaps “enhance” it. For those, like me, left feeling unsettled with what to do in the <> 2627 scenario (and no an empty CATCH is not an option). I found this little nugget from technet.

    BEGIN TRY
       INSERT etc
    END TRY
    BEGIN CATCH
        IF ERROR_NUMBER() <> 2627
          BEGIN
                DECLARE @ErrorMessage NVARCHAR(4000);
                DECLARE @ErrorSeverity INT;
                DECLARE @ErrorState INT;

                SELECT @ErrorMessage = ERROR_MESSAGE(),
                @ErrorSeverity = ERROR_SEVERITY(),
                @ErrorState = ERROR_STATE();

                    RAISERROR (
                        @ErrorMessage,
                        @ErrorSeverity,
                        @ErrorState
                    );
          END
    END CATCH

I don’t know if this is the “official” way, but you could try the INSERT, and fall back to UPDATE if it fails.

In addition to the accepted answer JFDI pattern, you probably want to ignore 2601 errors too (in addition to 2627) which is “Violation of unique index”.

...
IF ERROR_NUMBER() NOT IN (2601, 2627) THROW
...

P.S. And if you’re already using C# and .NET here’s how you can neatly handle this without complicated SQL code using a simple C# 6.0 when statement:

try
{
    connection.Execute("INSERT INTO etc");
}
catch (SqlException ex) when (ex.Number == 2601 || ex.Number == 2627)
{
    //ignore "dup key" errors
}

By the way, here’s a good read on the subject: https://michaeljswart.com/2017/07/sql-server-upsert-patterns-and-antipatterns/

I’ve done a similar operation in past using a different method. First, I declare a variable to hold the primary key. Then I populate that variable with the output of a select statement which looks for a record with those values. Then I do and IF statement. If primary key is null, then do insert, else, return some error code.

     DECLARE @existing varchar(10)
    SET @existing = (SELECT primaryKey FROM TABLE WHERE param1field = @param1 AND param2field = @param2)

    IF @existing is not null
    BEGIN
    INSERT INTO Table(param1Field, param2Field) VALUES(param1, param2)
    END
    ELSE
    Return 0
END

Finding SQL syntax errors can be complicated, but there are some tips on how to make it a bit easier. Using the aforementioned Error List helps in a great way. It allows the user to check for errors while still writing the project, and avoid later searching through thousands lines of code.

Leave a Reply

Your email address will not be published. Required fields are marked *