フリー問題

問題文

A query must show a null where the current job code equals the previous job code, and must show the current code otherwise. Which expression does that with one function call?

選択肢

1. COALESCE(job_id, previous_job_id), which returns a null when the two arguments are equal and returns the first argument when they differ.
2. NULLIF(job_id, previous_job_id), which returns a null when the two arguments are equal and returns the first argument when they differ.
3. NVL(job_id, previous_job_id), which returns a null when the two arguments are equal and returns the first argument when they differ.
4. LEAST(job_id, previous_job_id), which returns a null when the two arguments are equal and returns the first argument when they differ.