フリー問題

問題文

A join must treat two absent readings as a pair so that missing data lines up on both sides. Which operator gives that behavior?

選択肢

1. The join should use the integer divide operator for the two absent readings.
2. The join should use the simple equality operator for the two absent readings.
3. The join should use the bitwise shift operator for the two absent readings.
4. The join should use the null safe operator for the two absent readings.