SQL Syntax and Comments: A Clear Beginner Reference

Run this setup once in a new practice database, before the examples. It uses four customers and six sales. Ana has two sales, Ben has one, Cara has two, and Dev has none.

Sale 106 has no customer link; sale 104 has an unknown amount. These deliberate test cases expose missing information and unmatched relationships.

CREATE TABLE customers (
  id INTEGER PRIMARY KEY,
  name VARCHAR(40) NOT NULL,
  city VARCHAR(40),
  manager_id INTEGER
);
CREATE TABLE sales (
  id INTEGER PRIMARY KEY,
  customer_id INTEGER,
  amount DECIMAL(10,2),
  status VARCHAR(20) NOT NULL,
  created_on DATE NOT NULL
);
INSERT INTO customers (id,name,city,manager_id) VALUES
  (1,'Ana','Leeds',NULL),(2,'Ben','Leeds',1),
  (3,'Cara','Porto',1),(4,'Dev',NULL,2);
INSERT INTO sales (id,customer_id,amount,status,created_on) VALUES
  (101,1,10,'paid','2026-01-01'),
  (102,1,30,'pending','2026-01-02'),
  (103,2,20,'paid','2026-01-02'),
  (104,3,NULL,'pending','2026-02-01'),
  (105,3,20,'paid','2026-02-02'),
  (106,NULL,5,'paid','2026-02-03');

An amount of zero would be a known value. The NULL amount here means the value is unknown. A customer with no matching sale differs from a sale whose customer_id is NULL.

The primary keys identify individual records even when amounts or cities repeat. Dates use year-month-day format. The examples specify sorting whenever order matters; insertion order alone does not guarantee display order.

A statement consists of tokens. SELECT and FROM are keywords with a defined role; customers and name are identifiers that refer to your database objects; 'Leeds' is a string literal, meaning a value you wrote directly. Numbers such as 20 are numeric literals. Spaces and line breaks separate these parts, but formatting a clause on a new line does not change its meaning. The semicolon marks the end of a statement, which is especially useful in scripts containing several requests.

Write text values in single quotes. An apostrophe inside a text literal is represented by two apostrophes, as in 'O''Brien'. Do not put ordinary table names inside single quotes: that turns a name into a string value. Quoted identifiers use different punctuation depending on the database. Simple lower-case names without spaces are easiest for a beginner to move between systems. SQL keywords may be written in lower case or upper case; uppercase keywords are a reading convention rather than a requirement.

A line comment begins with two hyphens and continues to the end of that line. Leave a space after the hyphens for portability. A block comment begins with slash-star and ends with star-slash, and can span several lines. Use comments to explain a business rule, such as why a sale is excluded, rather than merely repeating the words of the query. Nesting block comments is not portable, so avoid placing one inside another when sharing a script.

Clause order matters even though indentation does not. A simple reading query writes SELECT, FROM, WHERE, then ORDER BY. Put commas between selected expressions, but not before FROM. Parentheses must balance, and strings must close. When a tool reports an error near a token, inspect both that token and the text immediately before it: an earlier missing quote or comma can make a later keyword appear to be the problem. Execute one complete statement at a time while learning.

The example runs on all four systems. MySQL requires whitespace or a control character after -- to recognise a line comment. MySQL normally quotes identifiers with backticks, PostgreSQL and standard SQL use double quotes, and SQL Server commonly uses brackets. MySQL double-quote behaviour depends on SQL mode; avoid borrowing identifier quoting without checking it.

Check the engine-specific syntax and version requirements in PostgreSQL lexical structure.

Write a query that returns id and name for the customer named Cara, sorted by id. Add a line comment explaining that this checks the Porto customer in the sample data.

Try the task before reading its worked solution. The supplied rows make the expected answer checkable by hand.

-- Check the Porto customer in the sample.
SELECT id, name FROM customers
WHERE name = 'Cara' ORDER BY id;

id | name
3  | Cara

The comma separates the two output columns. The string literal Cara is compared with the name column, so exactly one row remains. The comment is ignored during execution. Removing the comment should produce precisely the same result.

Try SELECT id name FROM customers and inspect the returned heading: name can become an alias for id because the comma is absent. Then restore the comma. Add a trailing comment after the semicolon and verify the values stay unchanged. Finally use a literal such as 'O''Brien' in SELECT to practise escaping an apostrophe without changing a stored name.