SQL Data Retrieval

Overview of SQL

• SQL (Structured Query Language) is used to search for data in databases.
• Data is stored in records, each containing multiple fields.
• Data manipulation is done using programming languages (example: Python).

Basic SQL Commands

Key Commands:

1. SELECT
   • Specifies which fields to return.
   • * can be used to select all fields.
2. FROM
   • Indicates which table to extract information from.
3. WHERE
   • Sets conditions to filter records.
   • Can use wildcards for pattern matching (e.g., %).

Examples of SQL Queries

1. Simple SELECT

   • Example: SELECT population FROM world WHERE name = 'Germany';
   • Retrieves population of Germany from the world table.

2. Selecting Multiple Fields

   • Query: SELECT name, continent, area, population, GDP, capital FROM world WHERE name = 'Algeria';
   • Returns all specified fields for Algeria.

3. Selecting All Fields

   • Use: SELECT * FROM world;
   • Equivalent to selecting all fields in the table.

4. Complex WHERE Clause

   • Query: SELECT * FROM world WHERE name LIKE 'A%' AND population > 1000000;
   • Returns all records starting with 'A' and having a population greater than 1 million.

5. Using Nested SELECT Statements

   • Example: SELECT name FROM world WHERE population > (SELECT population FROM world WHERE name = 'Algeria');
   • Break down:
     • Inner: SELECT population FROM world WHERE name = 'Algeria'; returns 387,000.
     • Outer: Selects names of countries with a population greater than 387,000.
   • Result: Returns Argentina.

Conclusion

• SQL provides powerful tools to retrieve and manipulate data.
• Understanding the structure and commands is essential for effective data management.