Joining Tables using MultiMaps

// C++ program for hashjoin on two tables
#include <iostream>
#include <string>
#include <unordered_map>
#include <vector>
using namespace std;

// Generate two tables to join
using tab_t = vector<vector<string> >;

// Table 1
tab_t tab1{ // Age  Name
            { "32", "Rahul" },
            { "25", "Anshul" },
            { "17", "Lok" },
            { "25", "Akil" },
            { "17", "Anshul" }
};

// Table 2
tab_t tab2{ // Student  Friend
            { "Rahul", "Tim" },
            { "Rahul", "Siva" },
            { "Anshul", "Gary" },
            { "Anshul", "Azhar" },
            { "Lok", "Vamsi" }
};

// Overloading of Output Operator
ostream& operator<<(ostream& o,
                    const tab_t& t)
{
    // Iterate through the table t
    for (size_t i = 0; i < t.size(); ++i) {
        o << i << ":";
        for (const auto& e : t[i])
            o << '\t' << e;
        o << endl;
    }
    return o;
}

// Function that perform join operation
// on the two tables
tab_t Join(const tab_t& a, size_t columna,
           const tab_t& b, size_t columnb)
{
    unordered_multimap<string, size_t> hashmap;

    // Use of Hashmap
    for (size_t i = 0;
         i < a.size(); ++i) {
        hashmap.insert({ a[i][columna], i });
    }

    // Perform Mapping
    tab_t result;
    for (size_t i = 0; i < b.size(); ++i) {

        auto range = hashmap.equal_range(
            b[i][columnb]);

        // Create new joined table
        for (auto it = range.first;
             it != range.second; ++it) {

            tab_t::value_type row;

            // Insert values to row
            row.insert(row.end(),
                       a[it->second].begin(),
                       a[it->second].end());
            row.insert(row.end(),
                       b[i].begin(),
                       b[i].end());

            // Push the row
            result.push_back(move(row));
        }
    }
    return result;
}

// Driver Code
int main(int argc, char const* argv[])
{
    int ret = 0;

    // Given Tables
    cout << "Table A: " << endl
         << tab1 << endl;
    cout << "Table B: " << endl
         << tab2 << endl;

    // Function Call
    auto tab3 = Join(tab1, 1, tab2, 0);

    // Print the joined table
    cout << "Joined tables: " << endl
         << tab3 << endl;
    return ret;
}