Interview Q&A / SQL & DBMS
SQL & DBMS interview questions
Joins, keys, normalisation, indexes and transactions for placement and backend interviews.
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- What is the difference between LEFT JOIN and RIGHT JOIN?
LEFT JOIN keeps every row of the left table and fills NULLs where the right has no match. RIGHT JOIN does the opposite. They are mirror images, and any RIGHT JOIN can be rewritten as a LEFT JOIN by swapping the tables.
BeginnerQuick answer1 min read - What is the difference between putting a filter in ON versus WHERE in a LEFT JOIN?
A filter on the right table in ON limits which rows match while still keeping every left row. The same filter in WHERE runs after the join and removes the NULL-extended rows, turning the LEFT JOIN into an INNER JOIN.
IntermediateQuick answer1 min read