Software Systems Development CL4 考试技巧

计算器程序

常见错误

1. 1high涉及分数: 2Normalisation

   Forgetting to mark primary and foreign keys, with asterisks or underlining, at every stage of decomposing a relation into 2NF and 3NF.

   如何避免: Re-mark every key at every step of the normalisation, even for a table that looks unchanged from the previous stage. Losing the key markers loses marks separately from getting the normal form itself right.
2. 2medium涉及分数: 1Structured Query Language (SQL)

   Writing SQL CHECK constraints with incorrect syntax, for example omitting the equality operator when checking that a delivery date is on or after an order date.

   如何避免: Write comparison operators out fully, greater than or equal to, not just greater than, when a business rule allows the two dates to match.
3. 3high涉及分数: 3Structured Query Language (SQL)

   Using an inner join to find parent records with no matching child rows, which silently excludes exactly the records the question is asking for.

   如何避免: Use a LEFT JOIN from the parent table and add a WHERE clause checking that the child table's key IS NULL to isolate unmatched parent rows.
4. 4high涉及分数: 4Managing projects

   Miscalculating a PERT backward pass when more than one successor activity converges on the same predecessor, by using the wrong one of the competing late start times.

   如何避免: When several activities depend on the same predecessor, its late finish time is the smallest of their late start times, not the largest. Work backward one node at a time and write each value down.
5. 5high涉及分数: 3Systems methodologies

   Writing a generic, textbook-style answer to a quality of written communication question, such as Agile versus Waterfall or a feasibility study, without linking it to the pre-release case study.

   如何避免: Name the business and the specific characters or business rule from the case study in your opening sentence, and keep referring back to them through the answer.
6. 6high涉及分数: 2Structured Query Language (SQL)

   Leaving non-aggregate attributes out of the GROUP BY clause when writing an SQL query that sums or counts values across a joined table.

   如何避免: List every selected column that is not inside an aggregate function in the GROUP BY clause, and use COUNT(DISTINCT ...) if a join could produce duplicate rows for the same entity.
7. 7medium涉及分数: 2Testing

   Confusing extreme test data, valid values at the edge of an acceptable range, with erroneous test data, values of the wrong type or outside the range entirely.

   如何避免: Keep the two categories separate in a test plan: extreme data should be accepted by the system, erroneous data should be rejected. State the expected outcome for each row.
8. 8medium涉及分数: 1Structured Query Language (SQL)

   Getting the argument order wrong in SQL date functions such as DATEDIFF or DATEADD, which reverses the sign of the result.

   如何避免: Check the specific date function's parameter order before the exam and write it from memory correctly, start date and end date are not interchangeable in DATEDIFF.