Join types: is an outer join the same as an equi-join except that one duplicate column is omitted in the result?

Introduction / Context:
SQL offers several join flavors: inner, outer (LEFT/RIGHT/FULL), natural, and equi-joins. Confusion often arises between outer joins and natural joins. This statement claims an outer join is the same as an equi-join but with one duplicate column removed, which is not correct.

Given Data / Assumptions:

• Equi-join: join condition uses equality between columns (A.id = B.id).
• Outer join: preserves unmatched rows from one or both sides, filling missing columns with NULLs.
• Natural join: automatically joins by columns with the same names and outputs only one copy of those columns.

Concept / Approach:
The defining feature of outer joins is row preservation, not column projection. Removing duplicate columns is a characteristic of NATURAL JOIN (and sometimes USING syntax), not of outer joins per se. An outer join can be equi-based in its predicate, but it is distinct because it keeps non-matching rows from the preserved side(s).

Step-by-Step Solution:

Verification / Alternative check:
Test with sample data containing non-matching keys; only outer joins preserve those rows; natural join behavior on columns is unrelated to row preservation.

Why Other Options Are Wrong:

• Correct / LEFT OUTER JOIN / primary key: None of these make an outer join intrinsically remove duplicate columns.
• Only true for NATURAL JOIN: This option names the feature (column de-duplication) correctly but confuses it with outer joins; natural joins can be inner or outer but the de-dup relates to column naming, not “outer-ness.”

Common Pitfalls:
Thinking USING/NATURAL removes columns for all join types; forgetting that outer joins are about including unmatched rows.

Final Answer:
Incorrect