code: func (p *Parser) parseCondition() (Expr, error) {
  	// Check if the WHERE token exists.
  	if tok, _, _ := p.ScanIgnoreWhitespace(); tok != WHERE {
  		p.Unscan()
  		return nil, nil
  	}
  
  	// Scan the identifier for the source.
  	expr, err := p.ParseExpr()
  	if err != nil {
  		return nil, err
  	}
  
  	return expr, nil
  }
docs: // parseCondition parses the "WHERE" clause of the query, if it exists.