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Swift API

DuckDB offers a Swift API. See the announcement post for details.

## Instantiating DuckDB

DuckDB supports both in-memory and persistent databases. To work with an in-memory datatabase, run:

    let database = try Database(store: .inMemory)

To work with a persistent database, run:

    let database = try Database(store: .file(at: "test.db"))

Queries can be issued through a database connection.

    let connection = try database.connect()

DuckDB supports multiple connections per database.

## Application Example

The rest of the page is based on the example of our announcement post, which uses raw data from NASA's Exoplanet Archive loaded directly into DuckDB.

### Creating an Application-Specific Type

We first create an application-specific type that we'll use to house our database and connection and through which we'll eventually define our app-specific queries.

    import DuckDB

    final class ExoplanetStore {

      let database: Database
      let connection: Connection

      init(database: Database, connection: Connection) {
        self.database = database
        self.connection = connection
      }
    }

### Loading a CSV File

We load the data from NASA's Exoplanet Archive:

    wget https://exoplanetarchive.ipac.caltech.edu/TAP/sync?query=select+pl_name+,+disc_year+from+pscomppars&format=csv -O downloaded_exoplanets.csv

Once we have our CSV downloaded locally, we can use the following SQL command to load it as a new table to DuckDB:

    CREATE TABLE exoplanets AS
        SELECT * FROM read_csv('downloaded_exoplanets.csv');

Let's package this up as a new asynchronous factory method on our `ExoplanetStore` type:

    import DuckDB
    import Foundation

    final class ExoplanetStore {

      // Factory method to create and prepare a new ExoplanetStore
      static func create() async throws -> ExoplanetStore {

      // Create our database and connection as described above
        let database = try Database(store: .inMemory)
        let connection = try database.connect()

      // Download the CSV from the exoplanet archive
      let (csvFileURL, _) = try await URLSession.shared.download(
        from: URL(string: "https://exoplanetarchive.ipac.caltech.edu/TAP/sync?query=select+pl_name+,+disc_year+from+pscomppars&format=csv")!)

      // Issue our first query to DuckDB
      try connection.execute("""
          CREATE TABLE exoplanets AS
              SELECT * FROM read_csv('\(csvFileURL.path)');
      """)

      // Create our pre-populated ExoplanetStore instance
        return ExoplanetStore(
        database: database,
          connection: connection
      )
      }

      // Let's make the initializer we defined previously
      // private. This prevents anyone accidentally instantiating
      // the store without having pre-loaded our Exoplanet CSV
      // into the database
      private init(database: Database, connection: Connection) {
      ...
      }
    }

### Querying the Database

The following example queires DuckDB from within Swift via an async function. This means the callee won't be blocked while the query is executing. We'll then cast the result columns to Swift native types using DuckDB's `ResultSet` `cast(to:)` family of methods, before finally wrapping them up in a `DataFrame` from the TabularData framework.

    ...

    import TabularData

    extension ExoplanetStore {

      // Retrieves the number of exoplanets discovered by year
      func groupedByDiscoveryYear() async throws -> DataFrame {

      // Issue the query we described above
        let result = try connection.query("""
          SELECT disc_year, count(disc_year) AS Count
            FROM exoplanets
            GROUP BY disc_year
            ORDER BY disc_year
          """)

        // Cast our DuckDB columns to their native Swift
        // equivalent types
        let discoveryYearColumn = result[0].cast(to: Int.self)
        let countColumn = result[1].cast(to: Int.self)

        // Use our DuckDB columns to instantiate TabularData
        // columns and populate a TabularData DataFrame
        return DataFrame(columns: [
          TabularData.Column(discoveryYearColumn).eraseToAnyColumn(),
          TabularData.Column(countColumn).eraseToAnyColumn(),
        ])
      }
    }

### Complete Project

For the complete example project, clone the DuckDB Swift repository and open up the runnable app project located in `Examples/SwiftUI/ExoplanetExplorer.xcodeproj`.

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