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https://github.com/sanju-srivatsa/sql-interview-q-a-s-simple-revision
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- URL: https://github.com/sanju-srivatsa/sql-interview-q-a-s-simple-revision
- Owner: Sanju-srivatsa
- Created: 2024-08-06T02:18:26.000Z (5 months ago)
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- Readme: README.md
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README
# MySQL Interview Q&A Cheatsheet
## Table of Contents
1. [General Concepts](#general-concepts)
- [SQL vs MySQL](#sql-vs-mysql)
- [What is SQL?](#what-is-sql)
- [What is MySQL?](#what-is-mysql)
2. [Basic Queries](#basic-queries)
- [SELECT](#select)
- [WHERE](#where)
- [AND, OR, NOT](#and-or-not)
- [ORDER BY](#order-by)
- [INSERT INTO](#insert-into)
- [NULL Values](#null-values)
- [UPDATE](#update)
- [DELETE](#delete)
- [LIMIT](#limit)
3. [Aggregate Functions](#aggregate-functions)
- [MIN and MAX](#min-and-max)
- [COUNT, AVG, SUM](#count-avg-sum)
4. [String Functions](#string-functions)
- [LIKE](#like)
- [Wildcards](#wildcards)
5. [Advanced Queries](#advanced-queries)
- [IN](#in)
- [BETWEEN](#between)
- [Aliases](#aliases)
- [Joins](#joins)
- [INNER JOIN](#inner-join)
- [LEFT JOIN](#left-join)
- [RIGHT JOIN](#right-join)
- [CROSS JOIN](#cross-join)
- [Self Join](#self-join)
- [UNION](#union)
- [GROUP BY](#group-by)
- [HAVING](#having)
- [EXISTS](#exists)
- [ANY, ALL](#any-all)
6. [Other Important Concepts](#other-important-concepts)
- [INSERT SELECT](#insert-select)
- [CASE](#case)
- [Null Functions](#null-functions)
- [Comments](#comments)
- [Operators](#operators)
7. [Data Science-Specific Questions](#data-science-specific-questions)
- [Normalization vs Denormalization](#normalization-vs-denormalization)
- [Indexes](#indexes)
- [Views](#views)
- [Stored Procedures](#stored-procedures)
- [Transactions](#transactions)
- [Window Functions](#window-functions)
- [CTEs (Common Table Expressions)](#ctes-common-table-expressions)
- [Temporary Tables](#temporary-tables)
8. [Security Concepts](#security-concepts)
- [SQL Injection](#sql-injection)
9. [Important Data Science Interview Questions](#important-data-science-interview-questions)
## General Concepts
### SQL vs MySQL
**Q:** What is the difference between SQL and MySQL?
**A:** SQL (Structured Query Language) is a standard language for managing and manipulating databases. MySQL is a relational database management system (RDBMS) that uses SQL to query databases.
### What is SQL?
**Q:** What is SQL?
**A:** SQL (Structured Query Language) is a programming language used to manage and manipulate relational databases.
### What is MySQL?
**Q:** What is MySQL?
**A:** MySQL is an open-source relational database management system (RDBMS) that uses SQL to manage databases.
## Basic Queries
### SELECT
**Q:** What does the SELECT statement do?
**A:** The SELECT statement is used to select data from a database.
**Syntax:**
```sql
SELECT column1, column2, ...
FROM table_name;
```
### WHERE
**Q:** What is the purpose of the WHERE clause?
**A:** The WHERE clause is used to filter records.
**Syntax:**
```sql
SELECT column1, column2, ...
FROM table_name
WHERE condition;
```
### AND, OR, NOT
**Q:** How are AND, OR, and NOT used in SQL?
**A:** They are logical operators used to combine multiple conditions in a WHERE clause.
**Syntax:**
```sql
SELECT column1, column2, ...
FROM table_name
WHERE condition1 AND condition2;
```
### ORDER BY
**Q:** What does the ORDER BY clause do?
**A:** The ORDER BY clause is used to sort the result set.
**Syntax:**
```sql
SELECT column1, column2, ...
FROM table_name
ORDER BY column1 [ASC|DESC];
```
### INSERT INTO
**Q:** How do you insert new data into a table?
**A:** Using the INSERT INTO statement.
**Syntax:**
```sql
INSERT INTO table_name (column1, column2, ...)
VALUES (value1, value2, ...);
```
### NULL Values
**Q:** What are NULL values?
**A:** NULL values represent missing or unknown data.
### UPDATE
**Q:** How do you update existing data in a table?
**A:** Using the UPDATE statement.
**Syntax:**
```sql
UPDATE table_name
SET column1 = value1, column2 = value2, ...
WHERE condition;
```
### DELETE
**Q:** How do you delete data from a table?
**A:** Using the DELETE statement.
**Syntax:**
```sql
DELETE FROM table_name
WHERE condition;
```
### LIMIT
**Q:** What does the LIMIT clause do?
**A:** The LIMIT clause specifies the number of records to return.
**Syntax:**
```sql
SELECT column1, column2, ...
FROM table_name
LIMIT number;
```
## Aggregate Functions
### MIN and MAX
**Q:** What do the MIN and MAX functions do?
**A:** MIN returns the smallest value and MAX returns the largest value in a set.
**Syntax:**
```sql
SELECT MIN(column_name) FROM table_name;
SELECT MAX(column_name) FROM table_name;
```
### COUNT, AVG, SUM
**Q:** What do the COUNT, AVG, and SUM functions do?
**A:** COUNT returns the number of rows, AVG returns the average value, and SUM returns the total sum of a numeric column.
**Syntax:**
```sql
SELECT COUNT(column_name) FROM table_name;
SELECT AVG(column_name) FROM table_name;
SELECT SUM(column_name) FROM table_name;
```
## String Functions
### LIKE
**Q:** What does the LIKE operator do?
**A:** The LIKE operator is used for pattern matching in a WHERE clause.
**Syntax:**
```sql
SELECT column1, column2, ...
FROM table_name
WHERE column LIKE pattern;
```
### Wildcards
**Q:** What are SQL wildcards?
**A:** Wildcards are used with the LIKE operator to search for a specified pattern in a column.
**Syntax:**
```sql
SELECT column1, column2, ...
FROM table_name
WHERE column LIKE '%value%';
```
## Advanced Queries
### IN
**Q:** What does the IN operator do?
**A:** The IN operator allows you to specify multiple values in a WHERE clause.
**Syntax:**
```sql
SELECT column1, column2, ...
FROM table_name
WHERE column IN (value1, value2, ...);
```
### BETWEEN
**Q:** What does the BETWEEN operator do?
**A:** The BETWEEN operator selects values within a given range.
**Syntax:**
```sql
SELECT column1, column2, ...
FROM table_name
WHERE column BETWEEN value1 AND value2;
```
### Aliases
**Q:** What are SQL aliases?
**A:** Aliases are used to give a table or a column a temporary name.
**Syntax:**
```sql
SELECT column_name AS alias_name
FROM table_name;
```
### Joins
**Q:** What is a SQL JOIN?
**A:** JOINs are used to combine rows from two or more tables based on a related column.
### INNER JOIN
**Q:** What does the INNER JOIN do?
**A:** The INNER JOIN keyword selects records that have matching values in both tables.
**Syntax:**
```sql
SELECT columns
FROM table1
INNER JOIN table2
ON table1.column = table2.column;
```
### LEFT JOIN
**Q:** What does the LEFT JOIN do?
**A:** The LEFT JOIN keyword returns all records from the left table and the matched records from the right table.
**Syntax:**
```sql
SELECT columns
FROM table1
LEFT JOIN table2
ON table1.column = table2.column;
```
### RIGHT JOIN
**Q:** What does the RIGHT JOIN do?
**A:** The RIGHT JOIN keyword returns all records from the right table and the matched records from the left table.
**Syntax:**
```sql
SELECT columns
FROM table1
RIGHT JOIN table2
ON table1.column = table2.column;
```
### CROSS JOIN
**Q:** What does the CROSS JOIN do?
**A:** The CROSS JOIN returns the Cartesian product of the two tables.
**Syntax:**
```sql
SELECT columns
FROM table1
CROSS JOIN table2;
```
### Self Join
**Q:** What is a Self Join?
**A:** A self join is a regular join, but the table is joined with itself.
**Syntax:**
```sql
SELECT a.column_name, b.column_name
FROM table_name a, table_name b
WHERE condition;
```
### UNION
**Q:** What does the UNION operator do?
**A:** The UNION operator is used to combine the result sets of two or more SELECT statements.
**Syntax:**
```sql
SELECT column_name(s) FROM table1
UNION
SELECT column_name(s
) FROM table2;
```
### GROUP BY
**Q:** What does the GROUP BY statement do?
**A:** The GROUP BY statement groups rows that have the same values into summary rows.
**Syntax:**
```sql
SELECT column1, aggregate_function(column2)
FROM table_name
GROUP BY column1;
```
### HAVING
**Q:** What does the HAVING clause do?
**A:** The HAVING clause is used to filter records after the GROUP BY clause.
**Syntax:**
```sql
SELECT column1, aggregate_function(column2)
FROM table_name
GROUP BY column1
HAVING condition;
```
### EXISTS
**Q:** What does the EXISTS operator do?
**A:** The EXISTS operator is used to test for the existence of any record in a subquery.
**Syntax:**
```sql
SELECT column1, column2, ...
FROM table_name
WHERE EXISTS (SELECT column1 FROM table_name WHERE condition);
```
### ANY, ALL
**Q:** What do the ANY and ALL operators do?
**A:** The ANY and ALL operators are used with a WHERE or HAVING clause to compare a value to a set of values.
**Syntax:**
```sql
SELECT column1, column2, ...
FROM table_name
WHERE column operator ANY (SELECT column FROM table_name WHERE condition);
```
## Other Important Concepts
### INSERT SELECT
**Q:** What does the INSERT SELECT statement do?
**A:** The INSERT INTO SELECT statement copies data from one table and inserts it into another table.
**Syntax:**
```sql
INSERT INTO table2 (column1, column2, ...)
SELECT column1, column2, ...
FROM table1
WHERE condition;
```
### CASE
**Q:** What does the CASE statement do?
**A:** The CASE statement goes through conditions and returns a value when the first condition is met.
**Syntax:**
```sql
SELECT column1,
CASE
WHEN condition1 THEN result1
WHEN condition2 THEN result2
...
ELSE result
END
FROM table_name;
```
### Null Functions
**Q:** What are Null Functions in SQL?
**A:** Functions like ISNULL(), COALESCE() are used to handle NULL values in SQL.
**Syntax:**
```sql
SELECT column_name, ISNULL(column_name, value)
FROM table_name;
```
### Comments
**Q:** How do you add comments in SQL?
**A:** Comments can be added using -- for single line comments and /* */ for multi-line comments.
**Syntax:**
```sql
-- This is a single-line comment
SELECT * FROM table_name;
/*
This is a multi-line comment
SELECT * FROM table_name;
*/
```
### Operators
**Q:** What are SQL Operators?
**A:** SQL operators are used to specify conditions in an SQL statement and to serve as conjunctions for multiple conditions.
**Syntax:**
```sql
SELECT column1, column2
FROM table_name
WHERE column1 = value;
```
## Data Science-Specific Questions
### Normalization vs Denormalization
**Q:** What is the difference between normalization and denormalization?
**A:** Normalization is the process of organizing data to reduce redundancy, while denormalization is the process of combining normalized tables to improve read performance.
### Indexes
**Q:** What are indexes and why are they used?
**A:** Indexes are used to speed up the retrieval of rows by creating a data structure that allows quick lookups.
**Syntax:**
```sql
CREATE INDEX index_name ON table_name (column1, column2, ...);
```
### Views
**Q:** What is a view in SQL?
**A:** A view is a virtual table based on the result-set of an SQL statement.
**Syntax:**
```sql
CREATE VIEW view_name AS
SELECT column1, column2, ...
FROM table_name
WHERE condition;
```
### Stored Procedures
**Q:** What are stored procedures?
**A:** Stored procedures are a set of SQL statements that can be stored and executed on the database server.
**Syntax:**
```sql
CREATE PROCEDURE procedure_name (parameters)
BEGIN
SQL statements;
END;
```
### Transactions
**Q:** What is a transaction in SQL?
**A:** A transaction is a sequence of one or more SQL operations treated as a single unit of work.
**Syntax:**
```sql
START TRANSACTION;
SQL statements;
COMMIT;
```
### Window Functions
**Q:** What are window functions?
**A:** Window functions perform calculations across a set of table rows related to the current row.
**Syntax:**
```sql
SELECT column1,
aggregate_function(column2) OVER (PARTITION BY column3 ORDER BY column4)
FROM table_name;
```
### CTEs (Common Table Expressions)
**Q:** What are Common Table Expressions (CTEs)?
**A:** CTEs are a temporary result set that you can reference within a `SELECT`, `INSERT`, `UPDATE`, or `DELETE` statement.
**Syntax:**
```sql
WITH cte_name AS (
SELECT column1, column2, ...
FROM table_name
WHERE condition
)
SELECT *
FROM cte_name;
```
### Temporary Tables
**Q:** What are temporary tables?
**A:** Temporary tables are tables that are created and used during the session and automatically dropped at the end of the session.
**Syntax:**
```sql
CREATE TEMPORARY TABLE temp_table_name (
column1 datatype,
column2 datatype,
...
);
```
## Security Concepts
### SQL Injection
**Q:** What is SQL injection?
**A:** SQL injection is a type of security vulnerability that allows attackers to interfere with the queries that an application makes to its database.
**Prevention Techniques:**
1. Use prepared statements and parameterized queries.
2. Use stored procedures.
3. Validate and sanitize user inputs.
4. Use ORM (Object Relational Mapping) frameworks.
5. Implement proper error handling.
## Important Data Science Interview Questions
### Pattern Matching
**Q:** What is Pattern Matching in SQL?
**A:** Pattern matching in SQL is done using the LIKE operator to search for a specified pattern in a column.
### Creating Empty Tables
**Q:** How to create empty tables with the same structure as another table?
**A:** Use the CREATE TABLE statement with the AS clause and a false condition.
**Syntax:**
```sql
CREATE TABLE new_table AS SELECT * FROM existing_table WHERE 1=0;
```
### Recursive Stored Procedure
**Q:** What is a Recursive Stored Procedure?
**A:** A recursive stored procedure is a stored procedure that calls itself until a boundary condition is met.
### Collation
**Q:** What is Collation? What are the different types of Collation Sensitivity?
**A:** Collation specifies how data is sorted and compared. Types include case sensitivity, accent sensitivity, kana sensitivity, and width sensitivity.
### OLTP vs OLAP
**Q:** What are the differences between OLTP and OLAP?
**A:** OLTP (Online Transaction Processing) is used for transactional applications, while OLAP (Online Analytical Processing) is used for analytical and data warehousing applications.
### User-defined Function
**Q:** What is a User-defined function? What are its various types?
**A:** A user-defined function is a function created by the user. Types include scalar functions, inline table-valued functions, and multi-statement table-valued functions.
### UNIQUE Constraint
**Q:** What is a UNIQUE constraint?
**A:** A UNIQUE constraint ensures all values in a column are unique.
### Query
**Q:** What is a Query?
**A:** A query is a request for data or information from a database.
### Data Integrity
**Q:** What is Data Integrity?
**A:** Data integrity refers to the accuracy and consistency of data stored in a database.
### Clustered vs Non-clustered Index
**Q:** What is the difference between Clustered and Non-clustered index?
**A:** A clustered index determines the physical order of data in a table, while a non-clustered index does not alter the physical order of data.
### Index
**Q:** What is an Index? Explain its different types.
**A:** An index is a database object used to speed up data retrieval. Types include unique, non-unique, clustered, and non-clustered indexes.
### Cross-Join
**Q:** What is a Cross-Join?
**A:** A cross-join returns the Cartesian product of two tables, combining all rows from the first table with all rows from the second table.
### Self-Join
**Q:** What is a Self-Join?
**A:** A self-join is a join that is used to join a table with itself.
### Foreign Key
**Q:** What is a Foreign Key?
**A:** A foreign key is a column or set of columns that establishes a link between data in two tables.
### Subquery
**Q:** What is a Subquery? What are its types?
**A:** A subquery is a query nested within another query. Types include single-row subquery, multiple-row subquery, and correlated subquery.
### Primary Key
**Q:** What is a Primary Key?
**A:** A primary key is a column or set of columns that uniquely identifies each row in a table.
### Constraints
**Q:** What are Constraints in SQL?
**A:** Constraints are rules applied to table columns to enforce data integrity.
### Tables and Fields
**Q:** What are Tables and Fields?
**A:** Tables are collections of related data entries, and fields are columns in a table.
### RDBMS vs DBMS
**
Q:** What is RDBMS? How is it different from DBMS?
**A:** RDBMS (Relational Database Management System) stores data in related tables, whereas DBMS (Database Management System) does not necessarily use a tabular structure.
### DBMS
**Q:** What is DBMS?
**A:** DBMS is software for creating and managing databases.
### Database
**Q:** What is a Database?
**A:** A database is an organized collection of structured data.
### SELECT Statement
**Q:** What is the SELECT statement?
**A:** The SELECT statement is used to query the database and retrieve data.
### Common Clauses with SELECT
**Q:** What are some common clauses used with SELECT query in SQL?
**A:** Common clauses include WHERE, ORDER BY, GROUP BY, HAVING, and LIMIT.
### UNION, MINUS, INTERSECT
**Q:** What are UNION, MINUS and INTERSECT commands?
**A:** UNION combines results of two queries, MINUS returns the difference, and INTERSECT returns the intersection of two queries.
### Cursor
**Q:** What is a Cursor?
**A:** A cursor is a database object used to retrieve, manipulate, and navigate through a result set row by row.
### Entities and Relationships
**Q:** What are Entities and Relationships?
**A:** Entities are objects stored in tables, and relationships define how entities are related.
### Types of Relationships
**Q:** List the different types of relationships in SQL.
**A:** Types include one-to-one, one-to-many, and many-to-many relationships.
### Alias
**Q:** What is an Alias in SQL?
**A:** An alias is a temporary name for a table or column.
### View
**Q:** What is a View?
**A:** A view is a virtual table based on the result-set of a query.
### Normalization
**Q:** What is Normalization?
**A:** Normalization is the process of organizing data to minimize redundancy.
### Denormalization
**Q:** What is Denormalization?
**A:** Denormalization is the process of combining normalized tables to improve read performance.
### Forms of Normalization
**Q:** What are the various forms of Normalization?
**A:** Forms include 1NF, 2NF, 3NF, BCNF, and 4NF.
### TRUNCATE, DELETE, DROP
**Q:** What are the TRUNCATE, DELETE and DROP statements?
**A:** TRUNCATE removes all rows from a table, DELETE removes rows based on a condition, and DROP removes the table and its data.
### Aggregate vs Scalar Functions
**Q:** What are Aggregate and Scalar functions?
**A:** Aggregate functions operate on a set of values and return a single value. Scalar functions operate on a single value and return a single value.
### Ranking Functions
**Q:** What are Ranking Functions in SQL?
**A:** Ranking functions return a ranking value for each row in a partition.
**Syntax:**
```sql
SELECT column_name, RANK() OVER (PARTITION BY column_name ORDER BY column_name) AS rank
FROM table_name;
```
### DDL, DML, DCL, TCL
**Q:** What are the different types of SQL commands?
**A:** SQL commands can be categorized into four main types: Data Definition Language (DDL), Data Manipulation Language (DML), Data Control Language (DCL), and Transaction Control Language (TCL).
### SQL Injection
**Q:** What is SQL Injection?
**A:** SQL Injection is a security vulnerability that allows attackers to interfere with SQL queries made to a database.
### Recursive Stored Procedure
**Q:** What is a Recursive Stored Procedure?
**A:** A recursive stored procedure calls itself until a boundary condition is met.
### Collation
**Q:** What is Collation?
**A:** Collation determines how data is sorted and compared in a database.
### OLTP
**Q:** What is OLTP?
**A:** OLTP (Online Transaction Processing) is a category of data processing that supports transaction-oriented applications.
### ACID Properties
**Q:** What are ACID properties in the context of database transactions?
**A:** ACID stands for Atomicity, Consistency, Isolation, Durability, which are the key properties that ensure reliable transaction processing.
### Constraints
**Q:** What are Constraints in SQL?
**A:** Constraints are rules enforced on data columns on a table to ensure the validity and integrity of data.
### Transactions
**Q:** What is a Transaction in SQL?
**A:** A transaction is a sequence of SQL operations that are executed as a single unit of work.
### Triggers
**Q:** What is a Trigger in SQL?
**A:** A trigger is a database object that automatically executes a predefined action when certain events occur.
### Events
**Q:** What is an Event in SQL?
**A:** An event is a task that runs according to a schedule.