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 a composite index and what is the leftmost prefix rule?
A composite index covers several columns in a fixed order. It can be used for queries filtering on its leading columns, such as (a) or (a, b), but generally not for a filter on b alone.
IntermediateQuick answer1 min read - What is a covering index?
A covering index contains every column a query needs, so the database answers from the index alone without visiting the table. This is called an index-only scan and avoids extra lookups.
IntermediateQuick answer1 min read - Why might the database ignore an index on a column?
Common reasons are wrapping the column in a function, a leading wildcard in LIKE, a type mismatch, a condition that matches most rows, or outdated statistics. The optimizer then prefers a full scan.
IntermediateQuick answer1 min read