SQL INSERT but avoid duplicates

Question

I want to do some quick inserts but avoid duplicates into a Table. For argument\'s sake lets call it MarketPrices, I\'ve been experimenting with two ways of doing it but not

Answer 1

EDIT: to prevent race conditions in a concurrent environment, use WITH (UPDLOCK) in the correlated subquery.

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I think this would be the standard method:

INSERT INTO MarketPrices (SecurityCode, BuyPrice, SellPrice, IsMarketOpen)
SELECT @SecurityCode, @BuyPrice,  @SellPrice, @IsMarketOpen
WHERE NOT EXISTS (
  SELECT * FROM MarketPrices WITH (UPDLOCK)
  WHERE SecurityCode = @SecurityCode 
    AND BuyPrice = @BuyPrice 
    AND SellPrice = @SellPrice
  )

If any of your fields are nullable, you would have to add that to the condition.

Your first method is interesting, but the requirements for EXCEPT have you jumping through hoops. This method is essentially the same, but it gets you around the column matching issue.

Alternatively:

INSERT INTO MarketPrices (SecurityCode, BuyPrice, SellPrice, IsMarketOpen)
SELECT SecurityCode, BuyPrice, SellPrice, @IsMarketOpen
FROM (
  SELECT @SecurityCode, @BuyPrice,  @SellPrice
  EXCEPT
  SELECT SecurityCode, BuyPrice, SellPrice FROM MarketPrices WITH (UPDLOCK)
  ) a (SecurityCode, BuyPrice, SellPrice)

The nice thing about EXCEPT in this instance is that it handles NULLs without any extra coding on your part. To achieve the same thing in first example, you would need to test each pair for NULLs as well as equality, long-hand.

Your second method is ok, but you don't need the variable. See Tomalak's solution, he cleaned it up nicely. Also, you would need to explicitly handle the possibility of concurrent inserts, if that were a concern.