SELECT Statement

The SELECT statement is used to fetch data from a database. It is the most frequently used SQL command.

Syntax

SELECT column1, column2, ...
FROM table_name;

You can select all columns using *, but explicitly naming columns is faster and more maintainable in production queries.

SELECT with Expressions

You are not limited to column names. You can use expressions, calculations, and string operations directly in SELECT:

SELECT name, salary * 12 AS annual_salary, UPPER(name) AS upper_name
FROM employees;

Aliases

Aliases rename a column or table for the duration of a query. They make output more readable and are required when using expressions.

SELECT salary AS monthly_income FROM employees;

The AS keyword is optional but recommended for clarity.

DISTINCT

DISTINCT removes duplicate rows from the result set:

SELECT DISTINCT department_id FROM employees;

It operates on all selected columns — if you select two columns, only rows where both columns are identical are collapsed.

SELECT TOP / LIMIT / FETCH

Different databases use different syntax to limit results:

Execution Order (Logical)

Knowing the order in which SQL processes your query helps you write correct and efficient statements:

1. FROM — identify the source table
2. WHERE — filter rows
3. GROUP BY — group rows
4. HAVING — filter groups
5. SELECT — pick columns and apply aliases
6. ORDER BY — sort the final result
7. LIMIT / OFFSET — paginate

This is why you cannot use a column alias defined in SELECT inside the WHERE clause — WHERE runs before SELECT.

SELECT * FROM employees;

SELECT name, salary FROM employees;

SELECT salary AS annual_salary FROM employees;

SELECT * FROM employees
WHERE hire_date > '2022-12-31';

SELECT DISTINCT department_id FROM employees;

SELECT name, salary, salary * 0.1 AS tax FROM employees;

SELECT * FROM employees ORDER BY employee_id LIMIT 3;