Tried to Solve The Practice Problems In Complex Way

FunctionInpostgres.sql

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+-- Create a function that takes a student's score and returns a grade (e.g., A, B, C, F)
+CREATE  or replace FUNCTION grab_grade(score numeric)
+RETURNS TEXT
+LANGUAGE plpgsql
+as
+$$
+
+BEGIN
+    IF score >= 90 THEN
+        RETURN 'A';
+    ELSIF score >= 80 THEN
+        RETURN 'B';
+    ELSIF score >= 70 THEN
+        RETURN 'C';
+    ELSE
+        RETURN 'F';
+    END IF;
+END
+
+$$
+
+SELECT name, score, grab_grade(score) FROM students;
+
+select * from students
+
+select * from departments
+
+
+-- Create a function that returns the full name and department of a student by ID.
+
+
+SELECT students.name as student_name, departments.name as department_name 
+FROM students 
+JOIN departments ON departments.id = students.department_id;
+
+CREATE or replace FUNCTION dpt_fn()
+RETURNS  TABLE(student_name TEXT, department_name TEXT)
+LANGUAGE sql
+AS
+$$
+SELECT students.name as student_name, departments.name as department_name FROM students 
+JOIN departments ON departments.id = students.department_id;
+$$
+
+SELECT * FROM dpt_fn();

dateAndGroupingTask.sql

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+CREATE TABLE departments (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(50) NOT NULL
+);
+
+CREATE TABLE students (
+    id SERIAL PRIMARY KEY,
+    name VARCHAR(50) NOT NULL,
+    age INTEGER NOT NULL,
+    score NUMERIC(10,4) ,
+    department_id INTEGER NOT NULL REFERENCES departments(id)
+);
+
+
+CREATE TABLE course_enrollment (
+    id SERIAL PRIMARY KEY,
+    student_id INTEGER NOT NULL REFERENCES students(id),
+    course_title VARCHAR(100) NOT NULL,
+    enroll_on DATE NOT NULL
+);
+
+INSERT INTO departments (name)
+VALUES 
+    ('CSE'),
+    ('EEE'),
+    ('BBA'),
+    ('ME'),
+    ('Civil');
+
+
+INSERT INTO students (name, age, score, department_id)
+VALUES
+('Alice', 20, 91.5, 1),
+    ('Bob', 21, 78.0, 2),
+    ('Charlie', 22, 84.3, 1),
+    ('David', 20, 65.7, 3),
+    ('Eve', 23, 88.9, 2),
+    ('Frank', 24, 92.4, 4),
+    ('Grace', 21, NULL, 5),
+    ('Rachel', 22, 85.2, 1),
+    ('Steve', 24, 60.0, 3),
+    ('Trudy', 20, NULL, 2),         -- score is NULL
+    ('Uma', 21, 92.7, 1),
+    ('Victor', 23, 48.5, 4),
+    ('Wendy', 25, 75.3, 5),
+    ('Xander', 26, 88.8, 3),
+    ('Yasmine', 22, 79.5, 2),
+    ('Zane', 24, NULL, 4),          -- score is NULL
+    ('Abigail', 20, 66.6, 5),
+    ('Heidi', 22, 79.2, 1),
+    ('Ivan', 20, 55.0, 3),
+    ('Judy', 25, 89.5, 2),
+    ('Kevin', 23, 95.0, 1),
+    ('Laura', 24, 70.0, 4),
+    ('Mallory', 21, 67.8, 5),
+    ('Nathan', 22, 82.6, 2),
+    ('Olivia', 20, NULL, 3),  -- Another NULL score
+    ('Peter', 26, 74.1, 4),
+    ('Quinn', 23, 90.3, 1);
+
+INSERT INTO course_enrollment (student_id, course_title, enroll_on)
+VALUES
+(1, 'Database Systems', '2024-01-15'),
+    (1, 'Operating Systems', '2024-02-10'),
+    (2, 'Circuit Analysis', '2024-03-12'),
+    (3, 'Algorithms', '2024-04-01'),
+    (4, 'Marketing Basics', '2024-02-20'),
+    (6, 'Thermodynamics', '2024-05-01'),
+    (7, 'Cybersecurity Fundamentals', '2023-11-15'),
+    (8, 'Data Analytics', '2024-01-20'),
+    (9, 'Digital Marketing', '2023-12-05'),
+    (10, 'Machine Learning', '2024-03-10'),
+    (11, 'Thermal Engineering', '2024-04-18'),
+    (12, 'Fluid Mechanics', '2023-10-08'),
+    (13, 'Econometrics', '2024-05-21'),
+    (14, 'Intro to AI', '2024-02-28'),
+    (15, 'Project Management', '2024-03-02'),
+    (16, 'Database Systems', '2024-04-25'),
+    (17, 'Software Engineering', CURRENT_DATE);
+
+drop Table course_enrollment;
+drop Table students;
+drop Table departments;
+
+-- Query Practice & Subqueries (Based on 10-1 to 10-3)
+-- Retrieve all students who scored higher than the average score.
+-- SELECT avg(score) from students;
+SELECT* from students
+WHERE score > (SELECT avg(score) from students);
+
+-- Find students whose age is greater than the average age of all students.
+-- SELECT avg(age) from students;
+SELECT * FROM students
+WHERE age > (SELECT avg(age) from students);
+
+-- Get names of students who are enrolled in any course (use IN with subquery).
+
+SELECT * from students;
+SELECT * from students 
+WHERE id IN (SELECT student_id FROM course_enrollment);
+
+-- Retrieve departments with at least one student scoring above 90 (use EXISTS).
+
+SELECT * FROM departments
+WHERE EXISTS (
+    SELECT 1 
+    FROM students 
+    WHERE students.department_id = departments.id
+    AND students.score > 90
+);
+
+SELECT* from students;
+SELECT* from departments;
+SELECT* from course_enrollment

functionProcedures.sql

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+
+/*
+                    @Procedural Approach:
+    1. Language Support: Supports procedural languages like PL/pgSQL, PL/Perl, PL/Python, etc.
+    2. Complex Logic: Allows for complex logic using control structures like loops, conditionals, and exception handling.
+    3. Variable Support: Supports variable declarations and manipulation within the procedural code.
+    4. Stored Procedures/Functions: Provides the ability to create stored procedures or functions, 
+
+                    @Non-Procedural Approach:
+    1. Declarative Queries: Focuses on writing declarative SQL queries to retrieve, insert, update, or delete data from the database.
+    2. Simplicity: Emphasizes simplicity by expressing operations in terms of what data is needed.
+    3. SQL Functions: Supports SQL functions, which are single SQL statements that return a value or set of values.
+    4. Performance: Can sometimes offer better performance for simple operations due to the optimized query execution plans generated by the database engine.
+*/
+
+SELECT * from employees;
+
+SELECT count(*) FROM employees
+
+CREATE  Function emp_count()
+RETURNS INT
+LANGUAGE sql -- here can be plpgsql/plperl/PL/Python or etc for PROCEDURAL
+AS
+$$
+-- here will be the function body
+SELECT count(*) FROM employees
+$$
+
+SELECT emp_count();
+
+CREATE or REPLACE function delete_emp()
+RETURNS void
+LANGUAGE SQL
+AS
+$$
+DELETE FROM employees WHERE employee_id = 30;
+$$
+
+SELECT delete_emp();
+
+
+CREATE OR replace FUNCTION delete_emp_by_id(p_emp_id INT)
+RETURNS void
+LANGUAGE SQL
+AS
+$$
+DELETE FROM employees WHERE employee_id = p_emp_id;
+$$
+
+SELECT delete_emp_by_id(29);
+
+CREATE PROCEDURE remove_emp()
+LANGUAGE plpgsql
+AS
+$$
+BEGIN
+-- here we can write multiple sql queries or one single queries
+-- here will exist the works/action that we want to do using procedure.
+DELETE FROM employees WHERE employee_id = 28;
+END
+$$
+
+CALL remove_emp()
+
+CREATE PROCEDURE remove_emp_var()
+LANGUAGE plpgsql
+AS
+$$
+DECLARE
+test_var INT;
+BEGIN
+SELECT employee_id INTO test_var FROM employees WHERE employee_id = 26;
+DELETE FROM employees WHERE employee_id = test_var;
+END
+$$
+
+CALL remove_emp_var()
+
+SELECT * FROM employees
+
+
+
+CREATE PROCEDURE remove_emp_by_p(p_employee_id int)
+LANGUAGE plpgsql
+AS
+$$
+
+DECLARE
+test_var INT;
+-- variable declared. 
+
+BEGIN
+SELECT employee_id INTO test_var FROM employees WHERE employee_id = p_employee_id;
+ -- we are setting the id to the variable test_var and then we are doing the operation
+DELETE FROM employees WHERE employee_id = test_var;
+
+RAISE NOTICE 'Employee Removed Successfully';
+-- this will give a notice if deleted. 
+END
+
+$$;
+
+CALL remove_emp_by_p(24)
+
+SELECT * FROM employees

indexingInPostgres.sql

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+-- Active: 1747459510114@@localhost@5432@ph_2
+CREATE INDEX idx_score ON students(score);
+
+CREATE INDEX idx_enrollment_composite ON course_enrollment(student_id, enroll_on);
+
+SELECT * FROM course_enrollment
+
+
+EXPLAIN ANALYZE 
+SELECT * 
+FROM students 
+WHERE score > 80;
+
+EXPLAIN ANALYZE 
+SELECT * 
+FROM course_enrollment 
+WHERE student_id = 3 AND enroll_on > '2024-01-01';