Simple Join

Datalog allows you to express joins clearly and simply. You specify a join by referencing the same variable in multiple places in a query. Let's look at the hello world join as an example.

Live Editor

// Already in the browser's scope
// import * as datalog from '@datalogui/datalog'
// import { useQuery } from '@datalogui/react'

const Greetings = datalog.intoTable([
  { language: "en", greeting: "Hello" },
  { language: "es", greeting: "Hola" }
  // ...
])
const Nouns = datalog.intoTable([
  { language: "en", noun: "world" },
  { language: "es", noun: "todos" }
  // ...
])

const Greeting = () => {
  const results = useQuery(({ greeting, noun }) => {
    Greetings({ language: "en", greeting })
    Nouns({ language: "en", noun })
  })
  return results.map(({greeting, noun}) => `${greeting} ${noun}`)
}

render(<Greeting />)

// Already in the browser's scope
// import * as datalog from '@datalogui/datalog'
// import { useQuery } from '@datalogui/react'

const Greetings = datalog.intoTable([
  { language: "en", greeting: "Hello" },
  { language: "es", greeting: "Hola" }
  // ...
])
const Nouns = datalog.intoTable([
  { language: "en", noun: "world" },
  { language: "es", noun: "todos" }
  // ...
])

const Greeting = () => {
  const results = useQuery(({ greeting, noun }) => {
    Greetings({ language: "en", greeting })
    Nouns({ language: "en", noun })
  })
  return results.map(({greeting, noun}) => `${greeting} ${noun}`)
}

render(<Greeting />)

Result

note

The examples here use React, but they would work equally well with most other UI frameworks.

A full working example above is nice, but let's zoom into the actual query part.

datalog.query(({ greeting, noun }) => {
  Greetings({ language: "en", greeting })
  Nouns({ language: "en", noun })
})

In this query we are joining the datums from Greetings and Nouns when their language are both "en". In case you're more familiar with SQL, this is analogous to joining with a where clause. For reference, here's The equivalent SQL query.

We can take this a step forward and have it return every greeting-noun pair in the same language just by removing the given constrain of "en".

const Query = datalog.query(({ language, greeting, noun }) => {
  Greetings({ language, greeting })
  Nouns({ language, noun })
})

Query.view().readAllData()
// =>
/*
[
  {
    "language": "en",
    "greeting": "Hello",
    "noun": "world"
  },
  {
    "language": "es",
    "greeting": "Hola",
    "noun": "todos"
  }
]
*/

Notice we had to introduce a new language variable into the query function. This is how Datalog knows which variable is a free variable whose value will be determined by the query and which one is already known.

That's all there is to joins! Pretty straightforward if I do say so myself.