SELECT, WHERE, ORDER BY & LIMIT: Reading Rows

SELECT ... FROM names the columns you want and the table they come from. SELECT name, city FROM customers returns two columns for every row; SELECT * returns every column. Prefer naming columns: it is clearer, and the query does not silently change shape when someone adds a column.

WHERE keeps only the rows for which the condition is true. Comparisons use =, <> (or !=), <, <=, > and >=. Text goes in single quotes, numbers do not. You can compare a column to another column the same way: WHERE paid_amount > invoice_amount finds rows where two stored numbers disagree, a classic way to spot a doctored record.

SELECT name, city
FROM customers
WHERE city = 'Austin';

Combine conditions with AND (both must be true), OR (either) and NOT. AND binds tighter than OR, so whenever you mix them, add parentheses to say what you mean:

SELECT id, amount
FROM orders
WHERE status = 'paid'
  AND (amount > 1000 OR customer_id = 7);

Shortcuts for common tests. IN ('a', 'b', 'c') matches any value in a list and replaces a chain of ORs. BETWEEN 100 AND 500 includes both ends. LIKE matches text patterns: % means any run of characters (including none) and _ means exactly one character, so LIKE 'Al%' finds names starting with Al and LIKE '%invoice%' finds the word anywhere.

SELECT id, amount, status
FROM orders
WHERE status IN ('paid', 'refunded')
  AND amount BETWEEN 100 AND 500;

DISTINCT removes duplicate rows from the result: SELECT DISTINCT city FROM customers lists each city once, however many customers live there. It applies to the whole combination of selected columns, not just the first one.

ORDER BY sorts the result (ascending by default, add DESC for biggest or newest first) and accepts several columns: ORDER BY city, created_at DESC. LIMIT n keeps only the first n rows after sorting, so ORDER BY created_at DESC LIMIT 1 is the standard way to ask for the most recent row. Without ORDER BY, 'first' has no defined meaning: LIMIT alone returns arbitrary rows.

Written order vs. run order. You write SELECT first, but the database logically works through FROM, then WHERE, then SELECT, then ORDER BY, then LIMIT. That is why, in standard SQL, a WHERE clause cannot use a column alias you only defined in SELECT, and why WHERE is the first lever to reach for when a result is too big.