---
id: 5956795bc9e2c415eb244de1
title: ハッシュ結合 (hash join)
challengeType: 1
forumTopicId: 302284
dashedName: hash-join
---

# --description--

An inner join is an operation that combines two data tables into one table, based on matching column values. The simplest way of implementing this operation is the nested loop join algorithm, but a more scalable alternative is the hash join algorithm.

「hash join」アルゴリズムは次の 2 つのステップで構成されています。

<ol>
  <li><strong>Hash phase:</strong> Create a multimap from one of the two tables, mapping from each join column value to all the rows that contain it.</li>
  <ul>
    <li>multimapは単純な線形探索よりもスケーラブルなハッシュベースの探索をサポートする必要があります。それこそが、このアルゴリズムの肝要な点だからです。</li>
    <li>理想としては、小さい方のテーブルのmultimapを作成することを推奨します。これにより、作成時間とメモリサイズを最小化できます。</li>
  </ul>
  <li><strong>join phase:</strong> もう一方のテーブルをスキャンし、作成しておいたmultimapを参照して一致する行を見つけます。</li>
</ol>

擬似コードでは、このアルゴリズムは以下のように表現できます。

<pre><strong>let</strong> <i>A</i> = the first input table (or ideally, the larger one)
<strong>let</strong> <i>B</i> = the second input table (or ideally, the smaller one)
<strong>let</strong> <i>j<sub>A</sub></i> = the join column ID of table <i>A</i>
<strong>let</strong> <i>j<sub>B</sub></i> = the join column ID of table <i>B</i>
<strong>let</strong> <i>M<sub>B</sub></i> = a multimap for mapping from single values to multiple rows of table <i>B</i> (starts out empty)
<strong>let</strong> <i>C</i> = the output table (starts out empty)
<strong>for each</strong> row <i>b</i> in table <i>B</i>:
  <strong>place</strong> <i>b</i> in multimap <i>M<sub>B</sub></i> under key <i>b(j<sub>B</sub>)</i>
<strong>for each</strong> row <i>a</i> in table <i>A</i>:
  <strong>for each</strong> row <i>b</i> in multimap <i>M<sub>B</sub></i> under key <i>a(j<sub>A</sub>)</i>:
    <strong>let</strong> <i>c</i> = the concatenation of row <i>a</i> and row <i>b</i>
    <strong>place</strong> row <i>c</i> in table <i>C</i>
</pre>

# --instructions--

「ハッシュ結合」アルゴリズムを関数として作成し、以下のテストケースで実行してみてください。 この関数は、オブジェクトの2つの配列を受け取り、結合されたオブジェクトの配列を返します。

**入力**

<table>
  <tr>
    <td style="padding: 4px; margin: 5px;">
      <table style="border:none; border-collapse:collapse;">
        <tr>
          <td style="border:none"><i>A =</i></td>
          <td style="border:none">
            <table>
              <tr>
                <th style="padding: 4px; margin: 5px;">Age</th>
                <th style="padding: 4px; margin: 5px;">Name</th>
              </tr>
              <tr>
                <td style="padding: 4px; margin: 5px;">27</td>
                <td style="padding: 4px; margin: 5px;">Jonah</td>
              </tr>
              <tr>
                <td style="padding: 4px; margin: 5px;">18</td>
                <td style="padding: 4px; margin: 5px;">Alan</td>
              </tr>
              <tr>
                <td style="padding: 4px; margin: 5px;">28</td>
                <td style="padding: 4px; margin: 5px;">Glory</td>
              </tr>
              <tr>
                <td style="padding: 4px; margin: 5px;">18</td>
                <td style="padding: 4px; margin: 5px;">Popeye</td>
              </tr>
              <tr>
                <td style="padding: 4px; margin: 5px;">28</td>
                <td style="padding: 4px; margin: 5px;">Alan</td>
              </tr>
            </table>
          </td>
          <td style="border:none; padding-left:1.5em;" rowspan="2"></td>
          <td style="border:none"><i>B =</i></td>
          <td style="border:none">
            <table>
              <tr>
                <th style="padding: 4px; margin: 5px;">Character</th>
                <th style="padding: 4px; margin: 5px;">Nemesis</th>
              </tr>
              <tr>
                <td style="padding: 4px; margin: 5px;">Jonah</td>
                <td style="padding: 4px; margin: 5px;">Whales</td>
              </tr>
              <tr>
                <td style="padding: 4px; margin: 5px;">Jonah</td>
                <td style="padding: 4px; margin: 5px;">Spiders</td>
              </tr>
              <tr>
                <td style="padding: 4px; margin: 5px;">Alan</td>
                <td style="padding: 4px; margin: 5px;">Ghosts</td>
              </tr>
              <tr>
                <td style="padding: 4px; margin: 5px;">Alan</td>
                <td style="padding: 4px; margin: 5px;">Zombies</td>
              </tr>
              <tr>
                <td style="padding: 4px; margin: 5px;">Glory</td>
                <td style="padding: 4px; margin: 5px;">Buffy</td>
              </tr>
            </table>
          </td>
        </tr>
        <tr>
          <td style="border:none">
            <i>j<sub>A</sub> =</i>
          </td>
          <td style="border:none">
            <i><code>Name</code> (例：列 1)</i>
          </td>
          <td style="border:none">
            <i>j<sub>B</sub> =</i>
          </td>
          <td style="border:none">
            <i><code>Character</code> (例：列 0)</i>
          </td>
        </tr>
      </table>
    </td>
  </tr>
</table>

**出力**

| A_age | A_name | B_character | B_nemesis |
| ----- | ------ | ----------- | --------- |
| 27    | Jonah  | Jonah       | Whales    |
| 27    | Jonah  | Jonah       | Spiders   |
| 18    | Alan   | Alan        | Ghosts    |
| 18    | Alan   | Alan        | Zombies   |
| 28    | Glory  | Glory       | Buffy     |
| 28    | Alan   | Alan        | Ghosts    |
| 28    | Alan   | Alan        | Zombies   |

出力テーブルの行の順序は重要ではありません。

# --hints--

`hashJoin` は関数とします。

```js
assert(typeof hashJoin === 'function');
```

`hashJoin([{ age: 27, name: "Jonah" }, { age: 18, name: "Alan" }, { age: 28, name: "Glory" }, { age: 18, name: "Popeye" }, { age: 28, name: "Alan" }], [{ character: "Jonah", nemesis: "Whales" }, { character: "Jonah", nemesis: "Spiders" }, { character: "Alan", nemesis: "Ghosts" }, { character:"Alan", nemesis: "Zombies" }, { character: "Glory", nemesis: "Buffy" }, { character: "Bob", nemesis: "foo" }])` は、`[{"A_age": 27,"A_name": "Jonah", "B_character": "Jonah", "B_nemesis": "Whales"}, {"A_age": 27,"A_name": "Jonah", "B_character": "Jonah", "B_nemesis": "Spiders"}, {"A_age": 18,"A_name": "Alan", "B_character": "Alan", "B_nemesis": "Ghosts"}, {"A_age": 18,"A_name": "Alan", "B_character": "Alan", "B_nemesis": "Zombies"}, {"A_age": 28,"A_name": "Glory", "B_character": "Glory", "B_nemesis": "Buffy"}, {"A_age": 28,"A_name": "Alan", "B_character": "Alan", "B_nemesis": "Ghosts"}, {"A_age": 28,"A_name": "Alan", "B_character": "Alan", "B_nemesis": "Zombies"}]`を返す必要があります。

```js
assert.deepEqual(hashJoin(hash1, hash2), res);
```

# --seed--

## --after-user-code--

```js
const hash1 = [
    { age: 27, name: 'Jonah' },
    { age: 18, name: 'Alan' },
    { age: 28, name: 'Glory' },
    { age: 18, name: 'Popeye' },
    { age: 28, name: 'Alan' }
];

const hash2 = [
    { character: 'Jonah', nemesis: 'Whales' },
    { character: 'Jonah', nemesis: 'Spiders' },
    { character: 'Alan', nemesis: 'Ghosts' },
    { character: 'Alan', nemesis: 'Zombies' },
    { character: 'Glory', nemesis: 'Buffy' },
    { character: 'Bob', nemesis: 'foo' }
];

const res = [
    { A_age: 27, A_name: 'Jonah', B_character: 'Jonah', B_nemesis: 'Whales' },
    { A_age: 27, A_name: 'Jonah', B_character: 'Jonah', B_nemesis: 'Spiders' },
    { A_age: 18, A_name: 'Alan', B_character: 'Alan', B_nemesis: 'Ghosts' },
    { A_age: 18, A_name: 'Alan', B_character: 'Alan', B_nemesis: 'Zombies' },
    { A_age: 28, A_name: 'Glory', B_character: 'Glory', B_nemesis: 'Buffy' },
    { A_age: 28, A_name: 'Alan', B_character: 'Alan', B_nemesis: 'Ghosts' },
    { A_age: 28, A_name: 'Alan', B_character: 'Alan', B_nemesis: 'Zombies' }
];

const bench1 = [{ name: 'u2v7v', num: 1 }, { name: 'n53c8', num: 10 }, { name: 'oysce', num: 9 }, { name: '0mto2s', num: 1 }, { name: 'vkh5id', num: 4 }, { name: '5od0cf', num: 8 }, { name: 'uuulue', num: 10 }, { name: '3rgsbi', num: 9 }, { name: 'kccv35r', num: 4 }, { name: '80un74', num: 9 }, { name: 'h4pp3', num: 6 }, { name: '51bit', num: 7 }, { name: 'j9ndf', num: 8 }, { name: 'vf3u1', num: 10 }, { name: 'g0bw0om', num: 10 }, { name: 'j031x', num: 7 }, { name: 'ij3asc', num: 9 }, { name: 'byv83y', num: 8 }, { name: 'bjzp4k', num: 4 }, { name: 'f3kbnm', num: 10 }];
const bench2 = [{ friend: 'o8b', num: 8 }, { friend: 'ye', num: 2 }, { friend: '32i', num: 5 }, { friend: 'uz', num: 3 }, { friend: 'a5k', num: 4 }, { friend: 'uad', num: 7 }, { friend: '3w5', num: 10 }, { friend: 'vw', num: 10 }, { friend: 'ah', num: 4 }, { friend: 'qv', num: 7 }, { friend: 'ozv', num: 2 }, { friend: '9ri', num: 10 }, { friend: '7nu', num: 4 }, { friend: 'w3', num: 9 }, { friend: 'tgp', num: 8 }, { friend: 'ibs', num: 1 }, { friend: 'ss7', num: 6 }, { friend: 'g44', num: 9 }, { friend: 'tab', num: 9 }, { friend: 'zem', num: 10 }];
```

## --seed-contents--

```js
function hashJoin(hash1, hash2) {

  return [];
}
```

# --solutions--

```js
function hashJoin(hash1, hash2) {
  const hJoin = (tblA, tblB, strJoin) => {
    const [jA, jB] = strJoin.split('=');
    const M = tblB.reduce((a, x) => {
      const id = x[jB];
      return (
        a[id] ? a[id].push(x) : (a[id] = [x]),
        a
      );
    }, {});

    return tblA.reduce((a, x) => {
      const match = M[x[jA]];
      return match ? (
                a.concat(match.map(row => dictConcat(x, row)))
            ) : a;
    }, []);
  };

  const dictConcat = (dctA, dctB) => {
    const ok = Object.keys;
    return ok(dctB).reduce(
            (a, k) => (a[`B_${k}`] = dctB[k]) && a,
            ok(dctA).reduce(
                (a, k) => (a[`A_${k}`] = dctA[k]) && a, {}
            )
        );
  };

  return hJoin(hash1, hash2, 'name=character');
}
```
