SQL Coding

Improving Investigation & Information Warehousing With SQL

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SQL (Structured Query Language) 

SQL (Structured Query Language) is a standardized programming language used for managing and manipulating relational databases. It allows users to perform various operations on the data stored in databases, such as querying, updating, deleting, and creating new databases, tables, and structures.

Key Features
  • Data Querying: Retrieve specific data from large datasets using SELECT.
  • Data Manipulation: Insert, update, and delete data using INSERT, UPDATE, and DELETE.
  • Data Definition: Create and modify database structures using CREATE, ALTER, and DROP.
  • Data Control: Manage permissions with GRANT and REVOKE.
  • Transaction Control: Maintain data integrity with COMMIT, ROLLBACK, and SAVEPOINT.

Explanation

  • SELECT: Specifies the columns to retrieve (e.g., name and age).
  • FROM: Indicates the table to query (employees).
  • WHERE: Filters rows where the condition (age > 30) is true.

SELECT name, age 

FROM employees

WHERE age > 30;

Explanation of Core Concepts

1. Database Structure
A database is organized into tables, which consist of rows (records) and columns (attributes).
 
Example Table: employees
 

2. Data Querying

SQL queries retrieve data from tables.

 Explanation

  •  * retrieves all columns. 
  • The output would display every row in the employees table.

SELECT * FROM employees;

 

3. Data Manipulation

Modify data in the database using INSERT, UPDATE, and DELETE.

Explanation:

  • INSERT adds new rows to the table.
  • UPDATE modifies existing records.
  • DELETE removes rows based on conditions.

INSERT INTO employees (id, name, age, position)

VALUES (4, ‘Malik’, 28, ‘bester’);

UPDATE employees

SET age = 29

WHERE name = ‘Malik’;

DELETE FROM employees

WHERE name = ‘Malik’;

4. Joins

SQL joins combine data from two or more tables based on a related column.

employees
01
departments

Example: Inner Join

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SELECT employees.name, departments.dept_name

FROM employees

INNER JOIN departments ON employees.dept_id = departments.dept_id;

Result

INNER JOIN combines rows from both tables where the dept_id matches

5. Aggregations

SQL can summarize data using aggregate functions like COUNT, SUM, AVG, MIN, and MAX.

Explanation
  • COUNT counts the number of rows.
  • AVG calculates the average value in a column.

SELECT COUNT(*) AS total_employees

FROM employees;

SELECT AVG(age) AS average_age

FROM employees;

6. Creating Tables

 Define a table’s structure using CREATE TABLE.
Explanation
  • CREATE TABLE defines a new table.
  • VARCHAR(50) specifies a text column with a maximum length of 50 characters.
  • PRIMARY KEY uniquely identifies each row

CREATE TABLE employees (

    id INT PRIMARY KEY,

    name VARCHAR(50),

    age INT,

    position VARCHAR(50)

);

7. Constraints

Constraints ensure data integrity in a table.
Common Constraints:
PRIMARY KEY: Uniquely identifies rows.
FOREIGN KEY: Links one table to another.
NOT NULL: Ensures a column cannot have NULL values.
UNIQUE: Ensures all values in a column are unique.
CHECK: Validates that column values meet a condition.
 

ALTER TABLE employees

ADD CONSTRAINT age_check CHECK (age > 28);

 

8. Transactions

Transactions group multiple SQL operations into a single unit.
Explanation:

  • BEGIN TRANSACTION starts a transaction.
  • COMMIT saves changes to the database.
  • ROLLBACK undoes changes.

BEGIN TRANSACTION;

UPDATE employees SET age = 26 WHERE name = ‘Alice’;

DELETE FROM employees WHERE name = ‘Bob’;

COMMIT; — Save changes

— ROLLBACK; — Undo changes (if needed)

 

Use Cases

  • Data Retrieval: Reporting and analysis.
  • Data Manipulation: Modifying data for business logic.
  • Database Design: Structuring data with normalization.
  • Data Integration: Linking multiple systems via databases.