INNER JOIN execution in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does an INNER JOIN do in SQL?

An INNER JOIN returns only the rows where there is a match in both tables based on the join condition.

beginner

In an INNER JOIN, what happens if a row in one table has no matching row in the other table?

That row is not included in the result because INNER JOIN only shows rows with matches in both tables.

intermediate

How does PostgreSQL execute an INNER JOIN internally?

PostgreSQL finds pairs of rows from both tables that satisfy the join condition, often using indexes or scanning tables, then combines those rows into the result.

intermediate

What is the difference between INNER JOIN and LEFT JOIN in terms of execution?

INNER JOIN returns only matching rows from both tables, while LEFT JOIN returns all rows from the left table and matches from the right, filling with NULLs if no match.

intermediate

Why is it important to have indexes on join columns for INNER JOIN execution?

Indexes speed up finding matching rows between tables, making INNER JOIN queries faster and more efficient.

What rows does an INNER JOIN return?

A Only rows with matching values in both tables

B All rows from the left table

C All rows from the right table

D All rows from both tables

INNER JOIN returns only rows where the join condition matches in both tables.

If a row in the left table has no matching row in the right table, what does INNER JOIN do?

A Excludes the row from the result

B Duplicates the row

C Includes the row with NULLs for right table columns

D Returns an error

INNER JOIN excludes rows without matches in the other table.

Which of these helps PostgreSQL speed up INNER JOIN execution?

A Adding more columns to the join condition

B Using SELECT *

C Indexes on join columns

D Using ORDER BY on unrelated columns

Indexes on join columns help PostgreSQL quickly find matching rows.

What is the main difference in output between INNER JOIN and LEFT JOIN?

A INNER JOIN returns unmatched rows, LEFT JOIN does not

B LEFT JOIN returns unmatched rows from the left table, INNER JOIN does not

C INNER JOIN returns all rows, LEFT JOIN returns only matches

D They return the same rows

LEFT JOIN includes all rows from the left table, even if no match exists.

During INNER JOIN execution, what does PostgreSQL do?

A Deletes unmatched rows from tables

B Combines every row from both tables regardless of condition

C Returns only rows from the first table

D Finds matching rows based on join condition and combines them

PostgreSQL finds matching rows that satisfy the join condition and returns combined rows.

Explain how INNER JOIN works when executed in PostgreSQL.

Think about how two lists are matched by a common key.

Describe the difference between INNER JOIN and LEFT JOIN in terms of their output.

Consider what happens when one table has rows without matches in the other.