node.js + postgres：“$ 1”或附近的语法错误

Question

I am having trouble understanding the npm package pg and its "dynamic statements".

I want to be able to generate INSERT INTO statements but I only get syntax error in return

var pg = require('pg');
var connectionString = 'postgres://postgres@localhost/db';

pg.connect(connectionString, function(err, client, done) {
  if (err) { return console.error(err); }

  var queryConfig = {
    text: 'INSERT INTO users($1, $2, $3) VALUES($4, $5, $6);',
    values: ['username', 'firstname', 'lastname', 'ab', 'A', 'B']
  };

  client.query(queryConfig, function (err, result) {
    if (err) { return console.error(err); }
    done();
  });
});

The error I get is:

{ [error: syntax error at or near "$1"]
  name: 'error',
  length: 84,
  severity: 'ERROR',
  code: '42601',
  detail: undefined,
  hint: undefined,
  position: '19',
  internalPosition: undefined,
  internalQuery: undefined,
  where: undefined,
  file: 'scan.l',
  line: '1053',
  routine: 'scanner_yyerror' }

Can anybody help me to understand what I'm doing wrong?

Answer 1

Databases generally don't let you use placeholders for identifiers (table names, column names, ...). PostgreSQL is complaining because you're trying to use the first three placeholders as column names:

INSERT INTO users($1, $2, $3) ...

You can think of identifiers like variable names in JavaScript: you can't put a variable name into another variable without some sort of eval chicanery. Similarly, you can't put a column name in a placeholder.

You should specify the column names without placeholders:

var queryConfig = {
  text: 'INSERT INTO users(username, firstname, lastname) VALUES($1, $2, $3);',
  values: ['ab', 'A', 'B']
};