https://github.com/apollo-level2-web-dev/postgressql-practice-task

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URL: https://github.com/apollo-level2-web-dev/postgressql-practice-task
Owner: Apollo-Level2-Web-Dev
Created: 2023-08-06T11:39:54.000Z (over 2 years ago)
Default Branch: main
Last Pushed: 2023-08-06T11:40:36.000Z (over 2 years ago)
Last Synced: 2025-06-22T21:44:12.594Z (10 months ago)
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Forks: 3
Open Issues: 0
Metadata Files:
- Readme: README.md
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README

          # postgresSQL-practice-task

Task 1: SELECT and WHERE Clause

Create a table named "employees" with columns (emp_id, emp_name, department, salary) and insert the following data:

```sql

CREATE TABLE employees (

    emp_id INT PRIMARY KEY,

    emp_name VARCHAR(50),

    department VARCHAR(50),

    salary DECIMAL(10, 2)

);

INSERT INTO employees (emp_id, emp_name, department, salary)

VALUES

    (1, 'John Doe', 'HR', 50000.00),

    (2, 'Jane Smith', 'IT', 60000.00),

    (3, 'Michael Johnson', 'Finance', 55000.00),

    (4, 'Emily Brown', 'HR', 52000.00);

```

Write an SQL query to retrieve the names and salaries of employees who work in the "HR" department.

Expected Result:

| emp_name | salary |

|----------------|-----------|

| John Doe | 50000.00 |

| Emily Brown | 52000.00 |

Task 2: Aggregation and HAVING Clause

Create a table named "orders" with columns (order_id, customer_id, total_amount) and insert the following data:

```sql

CREATE TABLE orders (

    order_id INT PRIMARY KEY,

    customer_id INT,

    total_amount DECIMAL(10, 2)

);

INSERT INTO orders (order_id, customer_id, total_amount)

VALUES

    (101, 1, 200.00),

    (102, 2, 300.00),

    (103, 1, 150.00),

    (104, 3, 400.00),

    (105, 2, 250.00);

```

Write an SQL query to find the customer IDs and the average total amount of their orders. Display only those customers whose average total amount is greater than or equal to 250.

Expected Result:

| customer_id | average_total_amount |

|----------------|----------------------|

| 2 | 275.00 |

Task 3: JOIN and GROUP BY

Create two tables named "students" and "courses" with columns as follows:

```sql

CREATE TABLE students (

    student_id INT PRIMARY KEY,

    student_name VARCHAR(50),

    age INT,

    gender VARCHAR(10)

);

CREATE TABLE courses (

    course_id INT PRIMARY KEY,

    course_name VARCHAR(50),

    credits INT

);

INSERT INTO students (student_id, student_name, age, gender)

VALUES

    (1, 'Alice', 22, 'Female'),

    (2, 'Bob', 21, 'Male'),

    (3, 'Charlie', 23, 'Male');

INSERT INTO courses (course_id, course_name, credits)

VALUES

    (101, 'Mathematics', 3),

    (102, 'History', 4),

    (103, 'Physics', 3);

-- Enrollment table with student-course relationships

CREATE TABLE enrollment (

    enrollment_id INT PRIMARY KEY,

    student_id INT,

    course_id INT

);

INSERT INTO enrollment (enrollment_id, student_id, course_id)

VALUES

    (1, 1, 101),

    (2, 1, 102),

    (3, 2, 103),

    (4, 3, 101);

```

Write an SQL query to retrieve the student name, course name, and credits for all enrolled courses.

Expected Result:

| student_name | course_name | credits |

|----------------|----------------|---------|

| Alice | Mathematics | 3 |

| Alice | History | 4 |

| Bob | Physics | 3 |

| Charlie | Mathematics | 3 |

Task 4: Multiple Joins and Aggregation

Create three tables named "employees," "departments," and "salaries" with columns as follows:

```sql

CREATE TABLE employees (

    emp_id INT PRIMARY KEY,

    emp_name VARCHAR(50),

    department_id INT

);

CREATE TABLE departments (

    department_id INT PRIMARY KEY,

    department_name VARCHAR(50)

);

CREATE TABLE salaries (

    emp_id INT,

    salary DECIMAL(10, 2)

);

INSERT INTO employees (emp_id, emp_name, department_id)

VALUES

    (1, 'John Doe', 1),

    (2, 'Jane Smith', 2),

    (3, 'Michael Johnson', 1),

    (4, 'Emily Brown', 3);

INSERT INTO departments (department_id, department_name)

VALUES

    (1, 'HR'),

    (2, 'IT'),

    (3, 'Finance');

INSERT INTO salaries (emp_id, salary)

VALUES

    (1, 50000.00),

    (2, 60000.00),

    (3, 55000.00),

    (4, 52000.00);

```

Write an SQL query to retrieve the department name and the average salary of employees working in each department. Sort the results by the average salary in descending order.

Expected Result:

| department_name | average_salary |

|-------------------|----------------|

| IT | 60000.00 |

| HR | 52500.00 |

| Finance | 52000.00 |

Task 5: Aggregation and Grouping

Create a table named "orders" with columns (order_id, customer_id, order_date, total_amount) and insert the following data:

```sql

CREATE TABLE orders (

    order_id INT PRIMARY KEY,

    customer_id INT,

    order_date DATE,

    total_amount DECIMAL(10, 2)

);

INSERT INTO orders (order_id, customer_id, order_date, total_amount)

VALUES

    (101, 1, '2023-01-05', 200.00),

    (102, 2, '2023-01-06', 300.00),

    (103, 1, '2023-02-10', 150.00),

    (104, 3, '2023-02-15', 400.00),

    (105, 2, '2023-03-20', 250.00);

```

Write an SQL query to find the total sales amount for each month, along with the number of orders in that month.

Expected Result:

| Month | Total_Sales | Num_Orders |

|----------|-------------|------------|

| January | 550.00 | 2 |

| February | 550.00 | 2 |

| March | 250.00 | 1 |

Task 6: Using JOINs and Aggregation

Create two tables named "employees" and "salaries" with columns as follows:

```sql

CREATE TABLE employees (

    emp_id INT PRIMARY KEY,

    emp_name VARCHAR(50),

    department_id INT

);

CREATE TABLE salaries (

    emp_id INT,

    salary DECIMAL(10, 2)

);

INSERT INTO employees (emp_id, emp_name, department_id)

VALUES

    (1, 'John Doe', 1),

    (2, 'Jane Smith', 2),

    (3, 'Michael Johnson', 1),

    (4, 'Emily Brown', 3);

INSERT INTO salaries (emp_id, salary)

VALUES

    (1, 50000.00),

    (2, 60000.00),

    (3, 55000.00),

    (4, 52000.00);

```

Write an SQL query to retrieve the department name and the average salary of employees in each department, excluding departments with fewer than two employees.

Expected Result:

| department_id | avg_salary |

|-----------------|--------------|

| 1 | 52500.00 |

Task 7: Using HAVING with Aggregation

Create a table named "products" with columns (product_id, product_name, stock_quantity) and insert the following data:

```sql

CREATE TABLE products (

    product_id INT PRIMARY KEY,

    product_name VARCHAR(50),

    stock_quantity INT

);

INSERT INTO products (product_id, product_name, stock_quantity)

VALUES

    (101, 'Widget A', 20),

    (102, 'Widget B', 10),

    (103, 'Widget C', 15),

    (104, 'Widget D', 5);

```

Write an SQL query to find the product names and their total sales quantity for products with a total sales quantity greater than 5.

Expected Result:

| product_name | total_sales_quantity |

|--------------|---------------------|

| Widget A | 20 |

| Widget C | 15 |

Task 8: Combining Multiple Joins

Create three tables named "customers," "orders," and "order_items" with columns as follows:

```sql

CREATE TABLE customers (

    customer_id INT PRIMARY KEY,

    customer_name VARCHAR(50),

    city VARCHAR(50)

);

CREATE TABLE orders (

    order_id INT PRIMARY KEY,

    customer_id INT,

    order_date DATE

);

CREATE TABLE order_items (

    item_id INT PRIMARY KEY,

    order_id INT,

    product_name VARCHAR(50),

    quantity INT

);

INSERT INTO customers (customer_id, customer_name, city)

VALUES

    (1, 'John Doe', 'New York'),

    (2, 'Jane Smith', 'Los Angeles'),

    (3, 'Michael Johnson', 'Chicago');

INSERT INTO orders (order_id, customer_id, order_date)

VALUES

    (101, 1, '2023-01-05'),

    (102, 2, '2023-02-10'),

    (103, 1, '2023-02-15');

INSERT INTO order_items (item_id, order_id, product_name, quantity)

VALUES

    (1, 101, 'Widget A', 2),

    (2, 101, 'Widget B', 3),

    (3, 102, 'Widget C', 1),

    (4, 103, 'Widget A', 4);

```

Write an SQL query to retrieve the customer name, order date, and the total quantity of items ordered for each order.

Expected Result:

| customer_name | order_date | total_quantity |

|----------------|--------------|----------------|

| John Doe | 2023-01-05 | 5 |

| Jane Smith | 2023-02-10 | 1 |

| John Doe | 2023-02-15 | 4 |

Task 9: Conditional Aggregation

Create a table named "sales" with columns (sale_id, product_id, sale_date, sale_amount) and insert the following data:

```sql

CREATE TABLE sales (

    sale_id INT PRIMARY KEY,

    product_id INT,

    sale_date DATE,

    sale_amount DECIMAL(10, 2)

);

INSERT INTO sales (sale_id, product_id, sale_date, sale_amount)

VALUES

    (1, 101, '2023-01-05', 200.00),

    (2, 102, '2023-01-06', 300.00),

    (3, 101, '2023-02-10', 150.00),

    (4, 103, '2023-02-15', 400.00),

    (5, 102, '2023-03-20', 250.00);

```

Write an SQL query to find the total sales amount for each product. However, if the total sales amount is less than 300, show it as 0.

Expected Result:

| product_id | total_sales_amount |

|--------------|--------------------|

| 101 | 350.00 |

| 102 | 550.00 |

| 103 | 400.00 |

Task 10: Combining Multiple Aggregates

Create a table named "students" with columns (student_id, student_name, age) and insert the following data:

```sql

CREATE TABLE students (

    student_id INT PRIMARY KEY,

    student_name VARCHAR(50),

    age INT

);

INSERT INTO students (student_id, student_name, age)

VALUES

    (1, 'Alice', 22),

    (2, 'Bob', 21),

    (3, 'Charlie', 23);

```

Write an SQL query to find the minimum, maximum, and average age of all students.

Expected Result

:

| min_age | max_age | avg_age |

|---------|---------|---------|

| 21 | 23 | 22.00 |
ecosyste.ms

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https://github.com/apollo-level2-web-dev/postgressql-practice-task

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