Parsing and Formatting SQL in JavaScript — Without a Grammar File

Building a SQL formatter sounds like it requires a full parser. For most practical cases, it doesn't. Here's a keyword-based approach that handles 90% of real-world SQL.

The Core Idea

SQL formatting is mostly about:

1. Uppercasing reserved words
2. Adding newlines before clauses
3. Indenting sub-clauses

A full AST parser is overkill for formatting. Instead, tokenize by keywords and apply rules.

Tokenization

const KEYWORDS = [
  'SELECT', 'FROM', 'WHERE', 'JOIN', 'LEFT JOIN', 'RIGHT JOIN',
  'INNER JOIN', 'ON', 'GROUP BY', 'ORDER BY', 'HAVING', 'LIMIT',
  'INSERT INTO', 'VALUES', 'UPDATE', 'SET', 'DELETE FROM',
  'CREATE TABLE', 'DROP TABLE', 'ALTER TABLE', 'WITH', 'UNION'
];

function tokenize(sql) {
  // Sort by length descending to match longer keywords first
  const sorted = [...KEYWORDS].sort((a, b) => b.length - a.length);
  const pattern = sorted.map(k => k.replace(/ /g, '\s+')).join('|');
  const regex = new RegExp(`(${pattern})`, 'gi');
  return sql.split(regex).filter(t => t.trim());
}

Formatting Rules

const NEWLINE_BEFORE = new Set([
  'SELECT', 'FROM', 'WHERE', 'GROUP BY', 'ORDER BY',
  'HAVING', 'LIMIT', 'UNION'
]);

const INDENT_BEFORE = new Set(['AND', 'OR', 'ON']);

function format(sql) {
  const tokens = tokenize(sql);
  let result = '';
  let indent = 0;

  for (const token of tokens) {
    const upper = token.trim().toUpperCase();

    if (NEWLINE_BEFORE.has(upper)) {
      result += `
${upper}`;
    } else if (INDENT_BEFORE.has(upper)) {
      result += `
  ${upper}`;
    } else if (upper === 'JOIN' || upper.endsWith(' JOIN')) {
      result += `
${upper}`;
    } else {
      result += ` ${token.trim()}`;
    }
  }

  return result.trim();
}

Handling Strings and Comments

The trickiest part: don't uppercase keywords inside string literals or comments.

function stripStrings(sql) {
  const placeholders = [];
  const stripped = sql.replace(/'([^']*)'/g, (match) => {
    const idx = placeholders.length;
    placeholders.push(match);
    return `__STR${idx}__`;
  });
  return { stripped, placeholders };
}

function restoreStrings(sql, placeholders) {
  return sql.replace(/__STR(\d+)__/g, (_, i) => placeholders[i]);
}

Minification

The reverse operation — remove all whitespace and produce a single line — is simple:

function minify(sql) {
  return sql.replace(/\s+/g, ' ').trim();
}

Try the formatter at toolzip.app/tools/sql-formatter.