select rowid from dual;
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It will show one row but no column, no value, not even a NULL value.
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1
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0
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NULL
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One row with one column containing the ROWID of DUAL's single row (an 18-character rowid string, e.g. AAAADeAABAAAAgiAAA).
ROWID is a pseudo-column that exists for every row in Oracle tables. Querying it from DUAL returns the actual 18-character ROWID string (e.g., AAAADeAABAAAAgiAAA) of DUAL's single row.
DUAL is a special one-row, one-column table owned by SYS. ROWID is a pseudocolumn that returns the physical address of a row. SELECT ROWID FROM DUAL therefore returns exactly one row containing a real ROWID value — the address of DUAL's single row — displayed as its 18-character ASCII (base-64) representation. The database's marked answer ('one row but no column/value') is incorrect: a genuine rowid string is returned. The values 1, 0, and NULL are also wrong, so the correct rowid answer was added.