DBI202Xuất hiện trong 1 đề thi
Relation $R(x,y)$ currently consists of only one tuple $(\mathrm{NULL},\mathrm{NULL})$. Which of the following queries will produce a nonempty output? That is, at least one tuple will be produced, although the tuple(s) may have NULLs.
Các lựa chọn
A```sql
SELECT * FROM R WHERE x IS NULL
```
B```sql
SELECT * FROM R WHERE x = NULL
```
C```sql
SELECT * FROM R WHERE x = y
```
D```sql
SELECT * FROM R WHERE x <> 10
```