# The Graph — Subgraph Development & GraphQL Queries

> The Graph is a decentralized indexing protocol for querying blockchain data via GraphQL.

## Querying Existing Subgraphs

```javascript
// Query Uniswap V2 — latest 10 swaps
const ENDPOINT = 'https://api.thegraph.com/subgraphs/name/uniswap/uniswap-v2'

const query = `
  {
    swaps(first: 10, orderBy: timestamp, orderDirection: desc) {
      id
      timestamp
      amount0In
      amount0Out
      token0 { symbol }
      token1 { symbol }
    }
  }
`

const response = await fetch(ENDPOINT, {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ query }),
})
const { data } = await response.json()
```

## Building a Subgraph

### 1. subgraph.yaml (manifest)

```yaml
specVersion: 0.0.5
schema:
  file: ./schema.graphql
dataSources:
  - kind: ethereum
    name: MyContract
    network: mainnet
    source:
      address: '0xContractAddress'
      abi: MyContract
      startBlock: 18000000
    mapping:
      kind: ethereum/events
      apiVersion: 0.0.7
      language: wasm/assemblyscript
      entities:
        - Transfer
      eventHandlers:
        - event: Transfer(indexed address,indexed address,uint256)
          handler: handleTransfer
      file: ./src/mapping.ts
```

### 2. schema.graphql

```graphql
type Transfer @entity {
  id: ID!
  from: Bytes!
  to: Bytes!
  value: BigInt!
  blockTimestamp: BigInt!
}
```

### 3. mapping.ts (AssemblyScript handler)

```typescript
import { Transfer as TransferEvent } from '../generated/MyContract/MyContract'
import { Transfer } from '../generated/schema'

export function handleTransfer(event: TransferEvent): void {
  let entity = new Transfer(event.transaction.hash.toHex() + '-' + event.logIndex.toString())
  entity.from = event.params.from
  entity.to = event.params.to
  entity.value = event.params.value
  entity.blockTimestamp = event.block.timestamp
  entity.save()
}
```

## Deployment

```bash
npm install -g @graphprotocol/graph-cli
graph init --studio my-subgraph
graph codegen && graph build
graph auth --studio <DEPLOY_KEY>
graph deploy --studio my-subgraph
```

## Key Concepts

| Term | Description |
|------|-------------|
| Subgraph | Indexing definition: events to watch + how to store data |
| Manifest (subgraph.yaml) | Specifies contract, network, event handlers |
| Schema | GraphQL entity definitions |
| Mapping | AssemblyScript handlers that transform events to entities |
| Indexer | Node running subgraphs and serving queries |
| Subgraph Studio | Hosted deployment for The Graph Network |