SQLInterview Questions

What is Normalization in Database?

Normalization is the process of organizing data in a database to reduce redundancy, avoid anomalies, and improve data integrity.

What are ACID Properties in DBMS?

ACID stands for Atomicity, Consistency, Isolation, and Durability — the four guarantees a database transaction must satisfy so that data stays correct even und…

Difference Between SQL and NoSQL Databases

SQL databases are relational, store data in structured tables with a fixed schema, and use SQL for querying; NoSQL databases are non-relational, store data in…

What is an Index in a Database?

A database index is a data structure that speeds up read queries by letting the database find rows without scanning the entire table, at the cost of extra stor…

What is a JOIN in SQL?

A SQL JOIN combines rows from two or more tables based on a related column, letting you query normalized data as if it were a single table.

Difference Between Primary Key and Foreign Key

A primary key uniquely identifies each row in a table and cannot be NULL, while a foreign key is a column that references the primary key of another table to l…

Difference Between DELETE, TRUNCATE and DROP

DELETE removes selected rows and can be rolled back, TRUNCATE quickly removes all rows but keeps the table structure, and DROP removes the entire table includi…

What is a View in SQL?

A view is a virtual table defined by a stored SQL query; it holds no data of its own but presents the result of that query as if it were a table whenever you s…

What is a Transaction in a Database?

A transaction is a sequence of one or more database operations that executes as a single, indivisible unit of work — either every operation succeeds and commit…

What is Database Sharding?

Database sharding is a horizontal partitioning technique that splits a large dataset across multiple independent database servers (shards), where each shard ho…

Clustered vs Non-Clustered Index: What is the Difference?

A clustered index determines the physical order in which table rows are stored on disk, while a non-clustered index is a separate structure that stores pointer…

What is a Stored Procedure in SQL?

A stored procedure is a precompiled, named block of SQL statements saved inside the database that can be executed repeatedly with a single call, optionally acc…

What is Database Replication?

Database replication is the process of continuously copying data from one database server (the primary) to one or more other servers (replicas), so multiple co…

What is a Deadlock in Database?

A deadlock in a database happens when two or more transactions each hold a lock the other needs, so every transaction involved waits forever and none can proce…

What is Database Partitioning?

Database partitioning is splitting a large table into smaller, more manageable pieces called partitions, while the table is still queried as one logical unit.

Optimistic vs Pessimistic Locking: What is the Difference?

Pessimistic locking locks a row before reading it so no one else can change it, while optimistic locking allows concurrent reads and only checks for conflicts,…

What is a Trigger in SQL?

A trigger in SQL is a stored piece of code that automatically executes in response to a specific event, such as an INSERT, UPDATE, or DELETE, on a table.

What is Denormalization in Database?

Denormalization is the deliberate process of combining data from multiple related tables back into fewer, wider tables to reduce joins and speed up read querie…

How Does a B-Tree Index Work Internally?

A B-tree index is a balanced, sorted tree of fixed-size nodes where every leaf sits at the same depth, letting the database find any key in O(log n) disk reads…

Hash Index vs B-Tree Index: What is the Difference?

A hash index maps each key to a bucket via a hash function, giving O(1) average-case exact-match lookups but no support for range queries or ordering, while a…

What is a Covering Index in SQL?

A covering index is an index that includes every column a query needs — both the filter/sort columns and the selected columns — so the database engine can answ…

What is a Partial Index and When Should You Use One?

A partial index is an index built over only the subset of rows that satisfy a WHERE condition, rather than every row in the table, which keeps it smaller, fast…

What Are Index Selectivity and Cardinality?

Cardinality is the number of distinct values in a column, and selectivity is the ratio of distinct values to total rows — a high-selectivity column (close to 1…

What is the Read Uncommitted Isolation Level?

Read Uncommitted is the weakest transaction isolation level, allowing a transaction to see changes made by other transactions even before those changes are com…