SELECT column1, column2, column3

FROM table_name;

SELECT * FROM table_name;

SELECT

first_name AS name,

salary * 12 AS annual_salary,

CONCAT(first_name, ' ', last_name) AS full_name

FROM employees;

SELECT

product_name,

price,

price * 0.9 AS discounted_price,

price * 1.1 AS price_with_tax

FROM products;

SELECT DISTINCT department FROM employees;

SELECT DISTINCT department, job_title FROM employees;

SELECT is required - every query starts with SELECT

FROM clause specifies the table - required except for constant expressions

Column names are comma-separated - no comma after last column

AS keyword for aliases is optional but recommended for readability

DISTINCT applies to entire row, not individual columns

SELECT * means all columns in table definition order

Logical execution order: FROM → JOIN → WHERE → GROUP BY → HAVING → SELECT → DISTINCT → ORDER BY → LIMIT

WHERE cannot use SELECT aliases (WHERE executes before SELECT projection)

Core references in this topic include WHERE, =, <, >, <=, >=. Learn what each one does, when to use it, and the execution or engine rules that matter.

WHERE

Filters rows before projection and sorting. It decides which rows continue through the query pipeline.

SELECT ... FROM table WHERE condition;

Most performance issues start with a weak WHERE clause or a missing supporting index.

=

Returns rows where the left and right values are exactly equal.

column = value

Use with exact matches. Do not use = NULL.

<, >, <=, >=

Range comparison operators for less-than, greater-than, and inclusive boundary checks.

salary >= 80000

BETWEEN

Checks whether a value falls inside an inclusive lower/upper range.

order_total BETWEEN 100 AND 500

ANY / ALL