Validation, verification and the DBMS

Overview

Range check, data that passes every validation check and is still wrong and check digit.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Why does a database enforce validation centrally rather than leaving it to each program?

   Model answer

   A rule defined once in the data dictionary applies to every program that touches the field, so no route into the data can bypass it.

2. 2

   Why can validation never guarantee correctness?

   Model answer

   A value can satisfy every rule and still be the wrong value, as a valid but mistyped date of birth is.

3. 3

   What is a range check?

   Model answer

   A check that a numeric or date value lies between stated limits.

4. 4

   Which validation checks can the database enforce itself rather than a program?

   Model answer

   Data type, field length, permitted range, whether a value is required, and whether it matches a key in a related table.

5. 5

   Give an example of data that passes every validation check and is still wrong.

   Model answer

   A date of birth typed as 1997 instead of 1979 is the right type, in range and correctly formatted, but it is not the person's date of birth.

6. 6

   What is a check digit, and where is it used?

   Model answer

   A digit calculated from the others and appended to a code so that a transcription error is detected; ISBNs and barcodes use one.

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