SELECT, FROM, WHERE

Every query starts with SELECT. It's the verb that says 'give me back this data'. The simplest possible query:

SELECT * FROM customers;

Translation: give me every column (* means all columns) from the customers table. The output is the full table, every row.

Picking specific columns

SELECT first_name, last_name, country FROM customers;

Now we get only three columns. Always prefer naming columns over SELECT *. It's faster, more explicit, and survives schema changes.

WHERE: filtering rows

WHERE adds a condition. Only rows where the condition is true come back.

SELECT first_name, last_name
FROM customers
WHERE country = 'Kenya';

Conditions can be combined with AND, OR, and NOT. Use parentheses when in doubt — operator precedence is real but not worth memorizing.

SELECT *
FROM customers
WHERE country = 'Kenya'
  AND signed_up_at > '2024-01-01'
  AND (plan = 'pro' OR plan = 'enterprise');

Common operators in WHERE

• = and != (or <>) for equality / inequality
• <, <=, >, >= for comparisons
• IN ('a', 'b', 'c') — membership in a list
• BETWEEN x AND y — inclusive range
• LIKE '%pattern%' — pattern matching with % (any chars) and _ (one char)
• IS NULL / IS NOT NULL — never use = NULL; NULL is the absence of a value, not a value itself