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 BCNF and how is it stricter than 3NF?
A table is in BCNF if, for every non-trivial dependency X -> Y, X is a superkey. 3NF also lets Y be a prime attribute, which BCNF does not allow, so BCNF removes a few anomalies that 3NF misses.
AdvancedQuick answer1 min read - What is MVCC in databases?
MVCC keeps multiple versions of a row so readers see a consistent snapshot while writers create new versions. This lets reads and writes proceed without blocking each other. PostgreSQL, MySQL InnoDB and Oracle use it.
AdvancedQuick answer1 min read - What is the default window frame, and why can LAST_VALUE give a surprising answer?
With ORDER BY, the default frame runs from the first row to the current row, so LAST_VALUE returns the current row's value. To get the true last value, extend the frame with ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING.
AdvancedQuick answer1 min read