Why doesn't auto_ptr construction work using = syntax

Question

I ran into a compiler error that didn't make much sense to me:

#include <memory>
using namespace std;

auto_ptr<Table> table = db->query("select * from t");

error: conversion from 'Table*' to non-scalar type 'std::auto_ptr< Table>' requested

However, the following line does work:

auto_ptr<Table> table(db->query("select * from t"));

What is it about this definiton of the constructor that prevents it from working as I expect? I thought that initialized declarations used the constructors.

Here's my auto_ptr's constructor (from the SGI STL):

explicit
auto_ptr(element_type* __p = 0) throw() : _M_ptr(__p) { }