SQL · EASY CODEX

SQL Interview Questions — Easy

Q: What is SQL and how does it differ from NoSQL?

SQL (Structured Query Language): Relational database management system (RDBMS) language. Uses structured schemas, tabular relations (tables/columns/rows), and enforces ACID properties. NoSQL (Not Only

Q: Categorize standard SQL commands into their sub-languages. What are DDL, DML, TCL, and DCL?

DDL (Data Definition Language): Defines/modifies schema structures. Commands: CREATE, ALTER, DROP, TRUNCATE. DML (Data Manipulation Language): Manipulates and queries data. Commands: SELECT, INSERT, U

Q: What is a Primary Key and how does it differ from a Unique Key?

Primary Key (PK): Uniquely identifies a record in a table. Does not allow NULLs. Only one PK allowed per table. Creates a clustered index by default. Unique Key (UK): Ensures all values in a column ar

Q: What is a Foreign Key, and how does it maintain referential integrity?

Foreign Key (FK): A column or group of columns in one table that references the PK/UK of another table. Referential Integrity: Ensures relationships between tables remain consistent. Prevents orphaned

Q: What are the different types of Joins in SQL?

INNER JOIN: Returns records with matching values in both tables. LEFT (OUTER) JOIN: Returns all records from the left table, and matching records from the right; unmatched right side returns NULL. RIG

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THEORY QUESTIONS & SOLUTIONSShowing 5 of 50 questions

Q1What is SQL and how does it differ from NoSQL?

SQL (Structured Query Language): Relational database management system (RDBMS) language. Uses structured schemas, tabular relations (tables/columns/rows), and enforces ACID properties.
NoSQL (Not Only SQL): Non-relational database system. Uses flexible schemas (document, key-value, wide-column, graph), scales horizontally, and prioritizes BASE properties (Basically Available, Soft state, Eventual consistency).

Q2Categorize standard SQL commands into their sub-languages. What are DDL, DML, TCL, and DCL?

DDL (Data Definition Language): Defines/modifies schema structures. Commands: CREATE, ALTER, DROP, TRUNCATE.
DML (Data Manipulation Language): Manipulates and queries data. Commands: SELECT, INSERT, UPDATE, DELETE.
TCL (Transaction Control Language): Manages database transactions. Commands: COMMIT, ROLLBACK, SAVEPOINT.
DCL (Data Control Language): Manages access permissions. Commands: GRANT, REVOKE.

Q3What is a Primary Key and how does it differ from a Unique Key?

Primary Key (PK): Uniquely identifies a record in a table. Does not allow NULLs. Only one PK allowed per table. Creates a clustered index by default.
Unique Key (UK): Ensures all values in a column are distinct. Allows NULL values (single or multiple depending on SQL dialect). Multiple UKs allowed per table. Creates a non-clustered index by default.

Q4What is a Foreign Key, and how does it maintain referential integrity?

Foreign Key (FK): A column or group of columns in one table that references the PK/UK of another table.
Referential Integrity: Ensures relationships between tables remain consistent. Prevents orphaned rows by restricting invalid inserts, and handles deletes/updates via rules like ON DELETE CASCADE or ON DELETE SET NULL.

Q5What are the different types of Joins in SQL?

INNER JOIN: Returns records with matching values in both tables.
LEFT (OUTER) JOIN: Returns all records from the left table, and matching records from the right; unmatched right side returns NULL.
RIGHT (OUTER) JOIN: Returns all records from the right table, and matching records from the left; unmatched left side returns NULL.
FULL (OUTER) JOIN: Returns all records when there is a match in either left or right table; unmatched rows return NULL.
CROSS JOIN: Returns the Cartesian product of both tables (every combination of rows).

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