In our examples, we’ll consistently assume that our string will be a VARCHAR(8000) data type and we’ll use one of our favorite tally tables that generates exactly 8,000 rows we can use to split the string. To do this we’ll fall back on one of our favorite T-SQL constructs: the Tally table. The best-practice approach is to split that string so each character appears as a separate row. In T-SQL, it is relatively awkward to do this directly to each character in a string, particularly when the string is not of fixed length. Most check digit calculation algorithms require applying some transformation to each digit in a string. Such proofs are beyond the scope of this article. From this, you can arrive at a probability distribution that describes how efficient the algorithm is in avoiding these transcription errors. Numerical methods exist that can generate all of the possible transcription errors of the types described for a given length string, and these can then be assembled into test strings to be applied against the check digit algorithm. Phonetic errors, which can occur when someone tells you a number over the phone, like hearing 17 when the caller said 70.Jump twin errors, such as 282 instead of 181, which can easily occur if both hands are engaged in typing in the number using the number keys above the letters on the keyboard, when one of the hands is offset by one key position.Jump transposition errors, which involve incorrect arrangement of a multi-digit sequence, such as 729 instead of 972.Twin errors, which are like a single digit error but occur in pairs, for example typing 33 when you meant to type 44.Transposition errors (particularly challenging for those who are dyslexic) are when you enter two characters that are reversed in sequence, such as 54 when you meant to enter 45.Single digit errors, such as simply entering a 2 when you meant to enter a 1.Human Transcription Errorsįirst we’ll mention a few common, human-keying errors that can occur, for which check digits were invented to avoid. ![]() Based on the specific examples we’ll provide, you should ultimately be able to apply the techniques we will use in the event that you need to calculate a check digit from a different algorithm using T-SQL.Īll of the check digit calculation routines we’ll present use set-based T-SQL code, so should perform better than an equivalent looping solution (although we will not attempt to prove this). We will also use our calculation techniques to show how they can be applied to validate that the check digit embedded in a string is correct, thus validating that the underlying coded string is at least potentially legitimate. Fortunately for us, since I am no mathematician, we will focus on the more mundane aspect of calculating check digits using some known, good algorithms that have been invented by those much smarter than me. The mathematics behind such a proof is spectacularly complex. ![]() While it is relatively easy to come up with an algorithm to calculate a check digit on a string of numbers and letters, it is quite another thing to prove that it is a good one. If someone is keying in your bank account number so they can send you some money, wouldn’t you like to have some level of confidence that they got that number correct? The idea of including a check digit in a number was invented to avoid common human transcription errors that easily occur when keying from a document into a data processing application. Things like the product code on our box of cereal, many national identity numbers, international bank account numbers and even the vehicle identification number on our personal cars all have a check digit embedded somewhere within that long string of digits and characters. Oftentimes we may not realize that the numbers that occur in many real-life situations contain a check digit. There is no doubt a more elegant solution to this, but this is my first crack at it.Calculating and Verifying Check Digits in T-SQL - Simple Talk Skip to contentĬheck digits have become ubiquitous in the digital age. TotalSUM = oddSumMultipl圓 + evenPositionsSUM ![]() Int SSCCArray = SSCC.ToCharArray().Select(c => (int)char.GetNumericValue(c)).ToArray() You can find more information on: static int getCheckDigit(string SSCC) the check digit is a number between 0-9 and is calculated using a specific algorithm on the preceding 17 digits which are the input for this code. The 18th digit is a check digit which is used to validate the rest of that code and check that it is correct. I'm calculating the GS1 SSCC codes that are 18-digit barcodes.
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