JavaScript In Practice: ES6 And Beyond/

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SQL and Map-Reduce-Filter

SQL and Map-Reduce-Filter

We'll see how the map, reduce and filter methods prove useful in SQL queries.

We'll cover the following...

Exercise:

Suppose the following tables are given in the form of arrays of objects:

Node.js
var inventory = [
    {
        id: 1,
        owner: 1,
        name: 'Sword',
        weight: 10,
        value: 100
    },
    {
        id: 2,
        owner: 1,
        name: 'Shield',
        weight: 20,
        value: 50
    },
    {
        id: 3,
        owner: 2,
        name: 'Sword',
        weight: 9,
        value: 150
    }
];
var characters = [
    {
        id: 1,
        name: 'Zsolt',
        points: 500,
        level: 5
    },
    {
        id: 2,
        name: 'Ron',
        points: 200,
        level: 2
    },
    {
        id: 3,
        name: 'Jack',
        points: 0,
        level: 1
    }
]
console.log(characters.map( c => c.name ));

Translate the following SQL query using map, reduce, and filter:

MySQL
SELECT characters.name, SUM( inventory.value ) AS totalValue
FROM characters, inventory
WHERE characters.id = inventory.owner
GROUP BY characters.name

You are not allowed to use loops, if statements, logical operators, or the ternary operator.

Solution:

We need to list all character names and the sum of the value of their items.

As characters and inventory are arrays, we can use the map, reduce, and filter. ...