Union and Union All in SQL

 Union: The Union operator in SQL is used to combine the results of two or more SELECT QUERIES 

into a single result set and give the unique rows by removing duplicate rows 

To use UNION, keep in mind:

   - For all SELECT commands number of columns must be the same

    - The data type for the corresponding position of columns will be the same

    - columns should be listed in the same order across all SELECT statements

example: 

    SELECT ID FROM CUSTOMERS

    UNION

    SELECT ID FROM ORDERS;


UNION ALL: The Union All operator in SQL is used to combine the results of two or more SELECT QUERIES into a  single result, and gives all rows by not removing duplicate rows 

example:  

    SELECT id from customers

    UNION ALL

    SELECT id from orders 

    

Comments

Post a Comment

Popular posts from this blog

Introduction to DBMS

🚀 Day 1 of My DBMS Learning Journey Quick question: What's the difference between "orange" and "fruit orange"? 🍊 One is DATA. The other is INFORMATION! 🎯 Started my DBMS journey today and mind = blown! 🤯 Here's what I discovered: 📊 Data vs Information • Data = Raw facts like "12", "Orange", "EXY" • Information = Data with context & meaning • Example: "12" is data, but "Age: 12" or "Roll Number: 12" is information! 🗄️ What is a Database? A structured collection of interrelated data that can be: • Stored in tables • Efficiently retrieved & manipulated • Any size (from small to massive!) • Example: College DB with Student, Professor, Timetable tables all connected 💻 What is DBMS? Software that manages databases - acts as an interface between users and data for secure storage, retrieval, and updates! 🗂️ Why File Systems Failed: → Data Redundancy: Same info stored multiple times (wasted m...
Read more

ACID Properties

ACID Properties (Explained Simply) ACID is a set of four properties that help databases execute transactions reliably and correctly , even in tough situations like system crashes or network failures . Together, they make sure your data stays accurate and trustworthy. A — Atomicity (All or Nothing) Atomicity means a transaction is treated as one single unit of work : If all steps succeed → COMMIT If any step fails → ROLLBACK (everything is undone) Example: Transferring money from Account A to Account B should either complete fully or not happen at all—no partial updates. C — Consistency (Rules Must Always Hold) Consistency ensures the database always moves from one valid state to another valid state . This means: Reads return valid data from the database Writes must follow database rules/constraints (like primary keys , foreign keys , checks, etc.) Example: If a foreign key requires a related record to exist, the database won’t allow an update that breaks that relation...
Read more

Understanding First Normal Form (1NF) in Database Design

Understanding First Normal Form (1NF) in Database Design First Normal Form (1NF) is the very first and most fundamental step in the process of database normalization. Applying 1NF helps reduce data redundancy and ensures your data is clean, consistent, and easy to manage. What is 1NF? Atomic Values: Each field in a table contains only a single value—no lists, sets, or multiple values in one cell. No Multivalued Attributes: You shouldn’t have columns that store several items (e.g., a list of courses in one column). Unique Rows: Every row must be uniquely identifiable, often by a primary key. Rules of 1NF Atomicity: Try to ensure each attribute holds a single value. Uniqueness: Every row in the table should be uniquely identifiable (often through a primary key). Examples of Applying 1NF Let’s look at different ways to organize data and whether they follow 1NF: 1. Row Splitting: One Row Per Student-Course Combination How it works: For every unique student-course combination,...
Read more