SQL Fundamentals
SQL-SQL-SELECT-Guide
The SELECT statement is one of the most fundamental and powerful commands in MySQL. This guide will help you master SELECT queries from basic to advanced concepts.
You'll Learn
- Basic SELECT statement syntax
- How to filter and sort data
- Working with multiple tables
- Query optimization techniques
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1. Basic SELECT Structure
Every SELECT statement follows this basic structure:
SELECT column1, column2, ...
FROM table_name
WHERE condition;Key Point
While we write SELECT first, MySQL actually processes the FROM clause first during execution.
2. Working with Sample Data
Let's use this sample employees table:
employees table
id | first_name | last_name | salary | department ----+-----------+-----------+---------+------------ 1 | John | Doe | 60000 | Sales 2 | Jane | Smith | 65000 | Marketing 3 | Bob | Johnson | 55000 | Sales 4 | Alice | Williams | 70000 | Engineering
Basic Queries
-- Select specific columns
SELECT first_name, last_name, salary
FROM employees;
-- Using WHERE clause
SELECT *
FROM employees
WHERE salary > 60000;
-- Using ORDER BY
SELECT first_name, last_name, salary
FROM employees
ORDER BY salary DESC;3. Advanced Queries
Aggregations
-- Average salary by department
SELECT
department,
COUNT(*) as employee_count,
AVG(salary) as avg_salary
FROM employees
GROUP BY department;Result: department | employee_count | avg_salary -------------+---------------+----------- Sales | 2 | 57500 Marketing | 1 | 65000 Engineering | 1 | 70000