Day 24 of My Data Analytics Journey!

Today was all about Aggregation Functions and a quick revision of Constraints in SQL.

What I Learned Today

Aggregation Functions

• COUNT() → Counts the number of rows.
• SUM() → Adds up the values in a column.
• AVG() → Finds the average of numeric values.
• MIN() → Gets the smallest value.
• MAX() → Gets the largest value.

Example:

SELECT 
    COUNT(*) AS total_records,
    SUM(salary) AS total_salary,
    AVG(salary) AS avg_salary,
    MIN(salary) AS min_salary,
    MAX(salary) AS max_salary
FROM employees;

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SQL Constraints (Revision)

• Primary Key → Uniquely identifies each record in a table.
• Unique Key → Ensures all values in a column are different.
• Foreign Key → Links one table to another.
• References → Used while defining foreign keys.
• Default → Assigns a default value if none is provided.
• NULL → Represents missing or undefined values.

Example:

CREATE TABLE students (
    student_id INT PRIMARY KEY,
    email VARCHAR(100) UNIQUE,
    course_id INT,
    admission_date DATE DEFAULT CURRENT_DATE,
    FOREIGN KEY (course_id) REFERENCES courses(course_id)
);

Key Takeaway

Aggregation functions help summarize data effectively, while constraints ensure data integrity. Both are essential for building reliable databases.