--- title: Hash join id: 5956795bc9e2c415eb244de1 challengeType: 5 forumTopicId: 302284 --- ## 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. The "hash join" algorithm consists of two steps:
  1. Hash phase: Create a multimap from one of the two tables, mapping from each join column value to all the rows that contain it.
  2. Join phase: Scan the other table, and find matching rows by looking in the multimap created before.
In pseudo-code, the algorithm could be expressed as follows: 
let A = the first input table (or ideally, the larger one)
let B = the second input table (or ideally, the smaller one)
let jA = the join column ID of table A
let jB = the join column ID of table B
let MB = a multimap for mapping from single values to multiple rows of table B (starts out empty)
let C = the output table (starts out empty)
for each row b in table B:
  place b in multimap MB under key b(jB)
for each row a in table A:
  for each row b in multimap MB under key a(jA):
    let c = the concatenation of row a and row b
    place row c in table C
## Instructions
Implement the "hash join" algorithm as a function and demonstrate that it passes the test-case listed below. The function should accept two arrays of objects and return an array of combined objects.

Input

A =
Age Name
27 Jonah
18 Alan
28 Glory
18 Popeye
28 Alan
B =
Character Nemesis
Jonah Whales
Jonah Spiders
Alan Ghosts
Alan Zombies
Glory Buffy
jA = Name (i.e. column 1) jB = Character (i.e. column 0)

Output

|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| The order of the rows in the output table is not significant.
## Tests
```yml tests: - text: hashJoin should be a function. testString: assert(typeof hashJoin === 'function'); - text: '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" }]) should return [{"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"}]' testString: assert.deepEqual(hashJoin(hash1, hash2), res); ```
## Challenge Seed
```js function hashJoin(hash1, hash2) { return []; } ```
### After Test
```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 }]; ```
## Solution
```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'); } ```