When performing a LEFT OUTER JOIN, what is returned?

When performing a LEFT OUTER JOIN, what is returned?

• A. Returns all rows from both tables regardless of match.
• B. Returns all rows from left table and matched rows from right.
• C. Returns only matched rows.
• D. Returns only rows from right table.

Answer: (B)

In a LEFT OUTER JOIN, you preserve every row from the left table and bring in matching data from the right table when it exists. For each left row, the query looks for a right row that satisfies the join condition. If a match is found, you get the right-table values; if not, the right-side columns are filled with NULLs. This is why all left-table rows appear, plus any corresponding right-table data. For example, if you have a list of customers (left) and their orders (right), you’ll see every customer, and their order details only when they’ve placed something. If a customer hasn’t placed an order, the order fields will be NULL. The other descriptions don’t fit this behavior: returning all rows from both tables regardless of match describes a full outer join (or something similar), not a left-only preservation. Returning only matched rows describes an inner join. Returning only rows from the right table isn’t aligned with the left-outer goal, which prioritizes the left table’s rows.

In a LEFT OUTER JOIN, you preserve every row from the left table and bring in matching data from the right table when it exists. For each left row, the query looks for a right row that satisfies the join condition. If a match is found, you get the right-table values; if not, the right-side columns are filled with NULLs. This is why all left-table rows appear, plus any corresponding right-table data.

For example, if you have a list of customers (left) and their orders (right), you’ll see every customer, and their order details only when they’ve placed something. If a customer hasn’t placed an order, the order fields will be NULL.

The other descriptions don’t fit this behavior: returning all rows from both tables regardless of match describes a full outer join (or something similar), not a left-only preservation. Returning only matched rows describes an inner join. Returning only rows from the right table isn’t aligned with the left-outer goal, which prioritizes the left table’s rows.