76 lines
9.3 KiB
Markdown
76 lines
9.3 KiB
Markdown
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---
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title: Hash join
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id: 5956795bc9e2c415eb244de1
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challengeType: 5
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videoUrl: ''
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localeTitle: 哈希加入
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---
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## Description
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<section id="description"><p> <a href="https://en.wikipedia.org/wiki/Join_(SQL)#Inner_join" title="wp：Join_（SQL）#Inner_join">内连接</a>是一种操作，它根据匹配的列值将两个数据表组合到一个表中。实现此操作的最简单方法是<a href="https://en.wikipedia.org/wiki/Nested loop join" title="wp：嵌套循环连接">嵌套循环连接</a>算法，但更可扩展的替代方法是<a href="https://en.wikipedia.org/wiki/hash join" title="wp：哈希联接">散列连接</a>算法。 </p><p>实现“散列连接”算法，并演示它通过下面列出的测试用例。 </p><p>您应该将表表示为在编程语言中感觉自然的数据结构。 </p><p> “散列连接”算法包含两个步骤： </p>哈希阶段：从两个表中的一个表创建一个<a href="https://en.wikipedia.org/wiki/Multimap" title="wp：Multimap">多</a>图，从每个连接列值映射到包含它的所有行。多图必须支持基于散列的查找，它比简单的线性搜索更好地扩展，因为这是该算法的重点。理想情况下，我们应该为较小的表创建多图，从而最小化其创建时间和内存大小。加入阶段：扫描另一个表，通过查看之前创建的多图来查找匹配的行。 <p>在伪代码中，算法可以表示如下： </p><pre>让A =第一个输入表（或理想情况下，更大的输入表）
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设B =第二个输入表（或理想情况下，较小的输入表）
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令j <sub>A</sub> =表A的连接列ID
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令j <sub>B</sub> =表B的连接列ID
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让M <sub>B</sub> =一个多图，用于从单个值映射到表B的多行（从空白开始）
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让C =输出表（从空开始）
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对于表B中的每一行b：
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将b放在密钥b（j <sub>B</sub> ）下的多映射M <sub>B中</sub>
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对于表A中的每一行a：
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对于a（j <sub>A</sub> ）项下多图M <sub>B中的</sub>每一行b：
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设c =第a行和第b行的串联
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将行c放在表C中<p></p>
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</pre>测试用例<p>输入</p><table><tbody><tr><td style="padding: 4px; margin: 5px;"><table style="border:none; border-collapse:collapse;"><tbody><tr><td style="border:none"> <i>A =</i> </td><td style="border:none"><table><tbody><tr><th style="padding: 4px; margin: 5px;">年龄</th><th style="padding: 4px; margin: 5px;">名称</th></tr><tr><td style="padding: 4px; margin: 5px;"> 27 </td><td style="padding: 4px; margin: 5px;">约拿</td></tr><tr><td style="padding: 4px; margin: 5px;"> 18 </td><td style="padding: 4px; margin: 5px;">艾伦</td></tr><tr><td style="padding: 4px; margin: 5px;"> 28 </td><td style="padding: 4px; margin: 5px;">荣耀</td></tr><tr><td style="padding: 4px; margin: 5px;"> 18 </td><td style="padding: 4px; margin: 5px;">大力水手</td></tr><tr><td style="padding: 4px; margin: 5px;"> 28 </td><td style="padding: 4px; margin: 5px;">艾伦</td></tr></tbody></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><tbody><tr><th style="padding: 4px; margin: 5px;">字符</th><th style="padding: 4px; margin: 5px;">复仇者</th></tr><tr><td style="padding: 4px; margin: 5px;">约拿</td><td style="padding: 4px; margin: 5px;">鲸鱼</td></tr><tr><td style="padding: 4px; margin: 5px;">约拿</td><td style="padding: 4px; margin: 5px;">蜘蛛</td></tr><tr><td style="padding: 4px; margin: 5px;">艾伦</td><td style="padding: 4px; margin: 5px;">鬼</td></tr><tr><td style="padding: 4px; margin: 5px;">艾伦</td><td style="padding: 4px; margin: 5px;">植物大战僵尸</td></tr><tr><td style="padding: 4px; margin: 5px;">荣耀</td><td style="padding: 4px; margin: 5px;">巴菲</td></tr></tbody></table></td></tr><tr><td style="border:none"> <i>j <sub>A</sub> =</i> </td><td style="border:none"> <i><code>Name</code> （即第1栏）</i> </td><td style="border:none"> <i>j <sub>B</sub> =</i> </td><td style="border:none"> <i><code>Character</code> （即第0列）</i> </td></tr></tbody></table></td><td style="padding: 4px; margin: 5px;"></td></tr></tbody></table><p>产量</p><table><tbody><tr><th style="padding: 4px; margin: 5px;"> A.Age </th><th style="padding: 4px; margin: 5px;">一个名字</th><th style="padding: 4px; margin: 5px;"> B.Character </th><th style="padding: 4px; margin: 5px;"> B.Nemesis </th></tr><tr><td style="padding: 4px; margin: 5px;"> 27 </td><td style="padding: 4px; margin: 5px;">约拿</td><td style="padding: 4px; margin: 5px;">约拿</td><td style="padding: 4px; margin: 5px;">鲸鱼</td></tr><tr><td style="padding: 4px; margin: 5px;"> 27 </td><td style="padding: 4px; margin: 5px;">约拿</td><td style="padding: 4px; margin: 5px;">约拿</td><td style="padding: 4px; margin: 5px;">蜘蛛</td></tr><tr><td style="padding: 4px; margin: 5px;"> 18 </td><td style="padding: 4px; margin: 5px;">艾伦</td><td style="padding: 4px; margin: 5px;">艾伦</td><td style="padding: 4px; margin: 5px;">鬼</td></tr><tr><td style="padding: 4px; margin: 5px;"> 18 </td><td style="padding: 4px; margin: 5px;">艾伦</td><td style="padding: 4px; margin: 5px;">艾伦</td><td style="padding: 4px; margin: 5px;">植物大战僵尸</td></tr><tr><td style="padding: 4px; margin: 5px;"> 28 </td><td style="padding: 4px; margin: 5px;">荣耀</td><td style="padding: 4px; margin: 5px;">荣耀</td><td style="padding: 4px; margin: 5px;">巴菲</td></tr><tr><td style="padding: 4px; margin: 5px;"> 28 </td><td style="padding: 4px; margin: 5px;">艾伦</td><td style="padding: 4px; margin: 5px;">艾伦</td><td style="padding: 4px; margin: 5px;">鬼</td></tr><tr><td style="padding: 4px; margin: 5px;"> 28 </td><td style="padding: 4px; margin: 5px;">艾伦</td><td style="padding: 4px; margin: 5px;">艾伦</td><td style="padding: 4px; margin: 5px;">植物大战僵尸</td></tr></tbody></table><p></p><p></p><p>输出表中行的顺序并不重要。 </p><p>如果你使用数字索引数组来表示表行（而不是按名称引用列），你可以用<code style="white-space:nowrap">[[27, "Jonah"], ["Jonah", "Whales"]]</code>的
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## Instructions
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<section id="instructions">
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</section>
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## Tests
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<section id='tests'>
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```yml
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tests:
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- text: <code>hashJoin</code>是一个函数。
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testString: 'assert(typeof hashJoin === "function", "<code>hashJoin</code> is a function.");'
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- text: '<code>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" }])</code>应该返回<code>[{"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"}]</code>'
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testString: 'assert.deepEqual(hashJoin(hash1, hash2), res, "<code>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" }])</code> should return <code>[{"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"}]</code>");'
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```
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</section>
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## Challenge Seed
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<section id='challengeSeed'>
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<div id='js-seed'>
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```js
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function hashJoin (hash1, hash2) {
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// Good luck!
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return [];
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}
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```
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</div>
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### After Test
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<div id='js-teardown'>
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```js
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console.info('after the test');
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```
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</div>
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</section>
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## Solution
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<section id='solution'>
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```js
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// solution required
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```
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</section>
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