使用 MultiMaps 连接表

可以使用嵌套循环在匹配的列上连接两个不同的表，但更有效和可扩展的方法是使用多映射。这个想法是从我们想要连接的每个列值映射到包含它的所有行，从两个表中的一个表生成一个多映射。

生成的多重映射必须是基于哈希的。散列本质上是一种将大元素转换为表示相同元素的小元素的技术。因此，为较小的表生成 multimap，从而减少其生成时间和内存大小。

例子：

方法：

• 创建两个表。
• 现在，获取两个表上列的 ID。
• 然后创建并实现一个多映射来映射到表 B 的各个行。
• 打印完以上步骤后的结果。

下面是上述方法的实现：

C++

// C++ program for hashjoin on two tables
#include 
#include 
#include 
#include 
using namespace std;
  
// Generate two tables to join
using tab_t = vector >;
  
// 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 tablt 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 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;
}

Python3

# Python program for hashjoin on two tables
from collections import defaultdict
  
# Function that perform join operation
# on the two tables
def hashJoin(table1, index1, table2, index2):
  
    h = defaultdict(list)
  
    # Hash
    for s in table1:
        h[s[index1]].append(s)
  
    # Perform join operation
    return [(s, r) for r in table2 for s in h[r[index2]]]
  
# Driver Code
  
# Given two tables
table1 = [("32", "Rahul"),
           ("25", "Anshul"),
           ("17", "Lok"),
           ("25", "Akil"),
           ("17", "Anshul")]
  
table2 = [("Rahul", "Tim"),
          ("Rahul", "Siva"),
          ("Anshul", "Gary"),
          ("Anshul", "Azhar"),
          ("Lok", "Vamsi")]
  
# Print the resultant table
for row in hashJoin(table1, 1, table2, 0):
    print(row)

输出：

Table A: 
0:    32    Rahul
1:    25    Anshul
2:    17    Lok
3:    25    Akil
4:    17    Anshul

Table B: 
0:    Rahul    Tim
1:    Rahul    Siva
2:    Anshul    Gary
3:    Anshul    Azhar
4:    Lok    Vamsi

Joined tables: 
0:    32    Rahul    Rahul    Tim
1:    32    Rahul    Rahul    Siva
2:    17    Anshul    Anshul    Gary
3:    25    Anshul    Anshul    Gary
4:    17    Anshul    Anshul    Azhar
5:    25    Anshul    Anshul    Azhar
6:    17    Lok    Lok    Vamsi

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