INNER JOIN vs LEFT OUTER JOIN explanation?

• A. INNER JOIN returns all rows from the left table; LEFT JOIN returns only matching rows
• B. INNER JOIN returns non-matching rows; LEFT JOIN returns nothing
• C. INNER JOIN returns rows with matching keys in both tables; LEFT JOIN returns all rows from the left table and matching right-side rows, with nulls when no match
• D. INNER JOIN returns all rows from both tables, regardless of match

Answer: (C)

Understanding how joins include or exclude rows based on matches. An inner join keeps only rows where there is a corresponding row in both tables, based on the join condition. In other words, you get the intersection of the two tables.

A left outer join starts with every row from the left table and attaches matching rows from the right table. If a left-row has no match on the right, the right-side columns appear as NULL. If there are multiple matches on the right, you’ll see multiple rows for that left-row.

This matches the correct description: you get rows with matching keys in both tables, and a left join expands to include all left-side rows with the right-side data filled in where a match exists (and NULLs where it doesn’t).

The other options don’t fit because an inner join does not return non-matching rows, a left join does not return nothing, and an inner join does not return all rows from both tables regardless of match.