“SQLite3::SQLException: near ".": 语法错误”是什么意思？

Question

I'm writing an ORM (object relational model) and I've been stuck on this error message for a while now. I did narrow it down to the update and the save method (I have a feeling that the error most likely is in the update method specifically), but I can't figure out where the syntax error is happening.

Here is the error message:

SQLite3::SQLException: near ".": syntax error
from /Users/karenlee/.rbenv/versions/2.5.1/lib/ruby/gems/2.5.0/gems/sqlite3-1.4.1/lib/sqlite3/database.rb:147:in `initialize'

And here is my ORM:

require_relative 'questions_database'

class User
  attr_reader :id
  attr_accessor :fname, :lname

  # displays all users
  def self.all
    users = QuestionsDatabase.instance.execute("SELECT * FROM users")
    users.map { | user_info | User.new(user_info) }
  end

  # finds a user by their primary id
  def self.find_by_id(id)
    found_user = QuestionsDatabase.instance.execute(<<-SQL, id: id)
      SELECT
        users.*
      FROM
        users
      WHERE
        users.id = :id
    SQL
    found_user.nil? ? nil : User.new(found_user.first)
  end

  # finds a user by their first and last name
  def self.find_by_name(fname, lname)
    name = {fname: fname, lname: lname}
    found_user = QuestionsDatabase.instance.execute(<<-SQL, name)
      SELECT
        users.*
      FROM
        users
      WHERE
        users.fname = :fname AND users.lname = :lname
    SQL
    found_user.nil? ? nil : User.new(found_user.first)
  end

  # creates a new user instance
  def initialize(options)
    @id, @fname, @lname = options.values_at('id', 'fname', 'lname')
  end

  # saves the user into the database (or updates when needed)
  def save
    if self.id
      update
    else  
      create
    end
  end

  private
  # helper method to update the database
  def update
    values = {id: id, fname: fname, lname: lname}
    QuestionsDatabase.instance.execute(<<-SQL, values)
      UPDATE
        users
      SET
        users.fname = :fname, users.lname = :lname
      WHERE
        users.id = :id
    SQL
  end

  # helper method to create a row in the database
  def create
    name = {fname: fname, lname: lname}
    QuestionsDatabase.instance.execute(<<-SQL, name)
      INSERT INTO
        users (fname, lname)
      VALUES
        (:fname, :lname)
    SQL
    self.id = QuestionsDatabase.instance.last_insert_row_id
  end
end

Here's the QuestionsDatabase class if you need it:

require 'sqlite3'
require 'singleton'

class QuestionsDatabase < SQLite3::Database
    include Singleton  

    def initialize
      super('questions.db')
      self.type_translation = true
      self.results_as_hash = true
    end
end

UPDATE:

I've found where my error was! It turned out that I should have just been saying fname = :fname, lname = :lname instead of saying users.fname = :fname, users.lname = :lname . But I'm still unsure as to why this is. It seems like the update statement doesn't like it when I don't use aliases, but some more explanation on this would be great!

Answer 1

It turns out that aliases are only available in the FROM clause. So if I were to use aliases in the UPDATE statement, it would have to be structured differently; I'd have to use a FROM clause. See this link for more info on restructuring!

https://sqlstudies.com/2013/09/16/dba-myths-you-cant-use-an-alias-in-an-update-statement/

Answer 2

It turned out that I should have just been saying fname = :fname, lname = :lname instead of saying users.fname = :fname, users.lname = :lname. But I'm still unsure as to why this is.

If you look at the UPDATE syntax diagrams you'll see it just takes plain column names. There's no need to specify the table name because you can only update a single table at a time; no room for ambiguity like in a SELECT . Therefore the parser rules don't support an optional table name component there.