Data Integrity

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 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…

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 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 Referential Integrity?

Referential integrity is a database rule ensuring that a foreign key value in one table always matches an existing primary key value in the referenced table, p…

Why is Restore Testing Important for Database Backups?

Restore testing is the practice of regularly restoring a backup to a separate environment and verifying the data is complete and usable, because a backup that…

How Do You Choose Numeric Data Type Precision in a Database?

Choosing numeric precision means matching a column’s storage size and rounding behavior to the exact range and accuracy the data needs — using fixed-point type…

DECIMAL vs FLOAT: How Do They Store Numbers Differently?

DECIMAL stores numbers as an exact sequence of base-10 digits with a fixed precision and scale, while FLOAT stores numbers as an approximate base-2 (binary) re…

What Is a CHECK Constraint and When Would You Use One?

A CHECK constraint is a rule attached to a column or table that the database evaluates on every insert or update, rejecting any row whose values do not satisfy…

What Is the Difference Between a UNIQUE Constraint and a UNIQUE Index?

A UNIQUE constraint is a schema-level rule declaring that a column or set of columns must contain no duplicate values, while a UNIQUE index is the physical dat…

How Should You Decide Which Columns Get a NOT NULL Constraint?

A NOT NULL constraint should be applied to any column whose absence would make a row meaningless or would break downstream logic, such as identifiers, foreign…

How Does a DEFAULT Value Constraint Behave in SQL?

A DEFAULT constraint supplies a predetermined value for a column automatically whenever an INSERT statement omits that column entirely, but it never overrides…

What Do ON DELETE CASCADE, SET NULL, and RESTRICT Mean for Foreign Keys?

These are referential actions attached to a foreign key that tell the database what to do to child rows when the referenced parent row is deleted or updated: C…

What Are 1NF, 2NF, and 3NF in Schema Design?

1NF, 2NF, and 3NF are progressive normalization rules that remove repeating groups, partial key dependencies, and transitive dependencies from a table, each le…

How Do You Design a Rollback Strategy for Schema Migrations?

A safe rollback strategy pairs every forward migration with a tested, reversible 'down' script or an expand-contract sequence, so a failed deploy can restore t…

What Makes a Database Migration Idempotent, and Why Does It Matter?

An idempotent migration produces the same final schema and data state no matter how many times it is run or re-run, so retries after a partial failure never du…

What Are Idempotency Keys and How Do They Prevent Duplicate Database Writes?

An idempotency key is a unique client-generated token attached to a write request that a database uses, via a unique constraint, to guarantee the same logical…

How Do You Design an Audit Trail Table in a Relational Database?

An audit trail table records who changed what, when, and how — typically storing the table name, row ID, changed columns, old and new values, the actor, and a…

What is a "God Table" Anti-Pattern and Why is It a Problem?

A god table is a database anti-pattern where a single, overly wide table absorbs responsibility for many unrelated entities or concerns — often via dozens of n…

What is a Class Invariant?

A class invariant is a condition about an object’s internal state that must hold true before and after every public method call, guaranteeing the object is nev…

What is an Encapsulation Violation?

An encapsulation violation occurs when code outside a class can read or modify that class's internal state directly or indirectly, bypassing the validated publ…

What is File System Consistency Checking (fsck)?

File system consistency checking is the process of scanning on-disk metadata — inodes, directory entries, block bitmaps, and free lists — after an unclean shut…

What Is Content-Addressable Storage?

Content-addressable storage (CAS) identifies and retrieves data by a hash computed from its own content rather than by a location-based path or key, so two ide…