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https://github.com/programmershahjalal/postgres-sql-assignment
https://github.com/programmershahjalal/postgres-sql-assignment
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- Host: GitHub
- URL: https://github.com/programmershahjalal/postgres-sql-assignment
- Owner: ProgrammerShahJalal
- Created: 2024-10-23T13:53:53.000Z (2 months ago)
- Default Branch: main
- Last Pushed: 2024-10-23T17:46:20.000Z (2 months ago)
- Last Synced: 2024-10-24T01:54:50.465Z (2 months ago)
- Size: 8.79 KB
- Stars: 0
- Watchers: 1
- Forks: 0
- Open Issues: 0
-
Metadata Files:
- Readme: README.md
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README
# Postgres SQL Assignment
## 1. Explain the primary key and foreign key concepts in PostgreSQL.
### Primary Key:
- **Primary Key**: A unique identifier for each row in a table. It ensures that no two rows have the same value and that the value is always present.
- **Foreign Key**: Links two tables by referring to the primary key in another table. It ensures that the data is related and consistent.
Example:
```sql
CREATE TABLE customers (
id SERIAL PRIMARY KEY,
name VARCHAR(100)
);
CREATE TABLE orders (
order_id SERIAL PRIMARY KEY,
customer_id INT REFERENCES customers(id)
);
```
## 2. What is the difference between the VARCHAR and CHAR data types?
- **VARCHAR**: Stores variable-length character strings, optimizing storage for varying text lengths.
- **CHAR**: Stores fixed-length character strings, requiring a predefined maximum length. If the data is shorter, it fills in the extra space with blank spaces.
## 3. Explain the purpose of the WHERE clause in a SELECT statement.
The WHERE clause filters records when querying data. It allows us to get only the rows that meet a condition.
Example:
```sql
SELECT * FROM products WHERE price > 100;
```
## 4. What are the LIMIT and OFFSET clauses used for?
- **LIMIT**: Limits the number of rows returned in the result set.
- **OFFSET**: Skips a specified number of rows before starting to return results.
Example:
```sql
SELECT * FROM products LIMIT 10 OFFSET 20;
```
## 5. How can you perform data modification using UPDATE statements?
The UPDATE statement is used to change data in a table. We can update specific columns for selected rows using a WHERE clause.
Example:
```sql
UPDATE products SET price = 150 WHERE id = 1;
```
## 6. What is the significance of the JOIN operation, and how does it work in PostgreSQL?
A JOIN combines rows from two or more tables based on a related column. It’s useful when we need to get data from multiple tables in one query.
Example:
```sql
SELECT customers.name, orders.order_id
FROM customers
JOIN orders ON customers.id = orders.customer_id;
```
## 7. Explain the GROUP BY clause and its role in aggregation operations.
The GROUP BY clause groups rows that have the same values in a specified column. It’s used with aggregate functions like SUM, COUNT, MAX, MIN, and AVG.
Example:
```sql
SELECT category, COUNT(*) FROM products GROUP BY category;
```
## 8. How can you calculate aggregate functions like COUNT, SUM, and AVG in PostgreSQL?
Aggregate functions are used to perform calculations on sets of data. For example:
- COUNT(): Counts rows.
- SUM(): Adds up values.
- AVG(): Calculates the average.
Example:
```sql
SELECT COUNT(*), SUM(price), AVG(price) FROM products;
```
## 9. What is the purpose of an index in PostgreSQL, and how does it optimize query performance?
An index is like a shortcut for searching data. It speeds up queries by allowing the database to quickly find rows instead of scanning the entire table.
## 10. Explain the concept of a PostgreSQL view and how it differs from a table.
A view is like a saved query. It looks like a table but doesn’t store data. It displays results from other tables and updates automatically when the underlying data changes.
Example:
```sql
CREATE VIEW active_customers AS
SELECT * FROM customers WHERE active = true;
```
## License
All rights reserved by **Md Shah Jalal**.