Waliullah Aggregate = “Total / Summary”. In SQL, aggregate functions are used to calculate values from multiple rows together (instead of checking each row one by one). Example: In a class of students, instead of checking every student’s marks separately, we calculate total marks, average marks, highest marks, etc. Common Aggregate Functions: Real-Life Analogy (Cricket Example): … Read more
Imagine you have a big dictionary .If you want to find the meaning of the word “Cricket”, In SQL, Index = shortcut for the database to find data faster. What are Indexes? Types of Indexes in SQL: Quick Recap for Freshers:
Constraints are like rules we put on a table’s columns to control what kind of data can go inside. Think of it like traffic rules: Types of Constraints (with Simple Examples) NOT NULL Means the column cannot be empty.Like in college admission form → Name is mandatory. Example: You must enter a name, otherwise error. … Read more
What is Denormalization? Example: Imagine you have a normalized database with these two tables: 1. Students Table StudentID Name ClassID 1 Rahul 101 2 Priya 102 2. Classes Table ClassID ClassName 101 Maths 102 Science his is normalized (no repeated data). But if you want to show a list of students with their class name, … Read more
Normalization is the process of organizing data in a database so that: Think of it like arranging your room: Why Do We Normalize? Types of Normalization (Normal Forms) Normalization is done in steps called Normal Forms (NF). First Normal Form (1NF) Example: Bad table: student_id name subjects 1 Ramesh Maths, Science 2 Priya Arts, Maths … Read more
DELETE removes rows (data) from a table, but the table structure remains. Example: TRUNCATE removes all rows from a table in one go. Example: Removes all students, but the students table is still there (empty). DROP deletes the entire table (structure + data). Example: Removes the table itself. Now students doesn’t exist anymore. Quick Comparison … Read more
INNER JOIN returns only the rows where matching values exist in both tables. LEFT JOIN returns all rows from the left table, and the matching rows from the right table. Example Let’s say we have two tables: students student_id name 1 Ramesh 2 Priya 3 Amit marks mark_id student_id subject marks 1 1 Maths 90 … Read more
A Foreign Key is a column in one table that is used to connect it with another table. Think of it like a relationship: So: Rules of Foreign Key Example in MySQL Table 1: students Data: student_id name 1 Ramesh 2 Priya Table 2: marks Here: Insert Data sqlINSERT INTO marks VALUES (1, 1, ‘Maths’, … Read more
A subquery is a query (SQL statement) inside another query.It’s like asking one question inside another question to get your final answer. Syntax: Example of a Subquery Imagine two tables: students id name class_id 1 Ramesh 101 2 Priya 102 3 Amit 101 classes class_id class_name 101 Science 102 Arts Question: Find students who belong … Read more
Here’s a simple and clear explanation of the difference between WHERE and HAVING in SQL: Quick Explanation Difference Between WHERE and HAVING Feature WHERE HAVING Purpose Filters rows before grouping (used on individual rows) Filters groups after grouping (used with GROUP BY) Used With Any table or query Only with GROUP BY or aggregate functions … Read more