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# Using the SQL API

DataFusion has a full SQL API that allows you to interact with DataFusion using
SQL query strings. The simplest way to use the SQL API is to use the
[`SessionContext`] struct which provides the highest-level API for executing SQL
queries.

To use SQL, you first register your data as a table and then run queries
using the [`SessionContext::sql`] method. For lower level control such as
preventing DDL, you can use [`SessionContext::sql_with_options`] or the
[`SessionState`] APIs

## Registering Data Sources using `SessionContext::register*`

The `SessionContext::register*` methods tell DataFusion the name of
the source and how to read data. Once registered, you can execute SQL queries
using the [`SessionContext::sql`] method referring to your data source as a table.

The [`SessionContext::sql`] method returns a `DataFrame` for ease of
use. See the ["Using the DataFrame API"] section for more information on how to
work with DataFrames.

### Read a CSV File

```rust
use datafusion::error::Result;
use datafusion::prelude::*;
use arrow::record_batch::RecordBatch;

#[tokio::main]
async fn main() -> Result<()> {
    let ctx = SessionContext::new();
    // register the "example" table
    ctx.register_csv("example", "tests/data/example.csv", CsvReadOptions::new()).await?;
    // create a plan to run a SQL query
    let df = ctx.sql("SELECT a, min(b) FROM example WHERE a <= b GROUP BY a LIMIT 100").await?;
    // execute the plan and collect the results as Vec<RecordBatch>
    let results: Vec<RecordBatch> = df.collect().await?;
    // Use the assert_batches_eq macro to compare the results with expected output
    datafusion::assert_batches_eq!(vec![
        "+---+----------------+",
        "| a | min(example.b) |",
        "+---+----------------+",
        "| 1 | 2              |",
        "+---+----------------+",
        ],
        &results
    );
  Ok(())
}
```

### Read an Apache Parquet file

Similarly to CSV, you can register a Parquet file as a table using the `register_parquet` method.

```rust
use datafusion::error::Result;
use datafusion::prelude::*;
#[tokio::main]
async fn main() -> Result<()> {
    // create local session context
    let ctx = SessionContext::new();
    let testdata = datafusion::test_util::parquet_test_data();

    // register parquet file with the execution context
    ctx.register_parquet(
        "alltypes_plain",
        &format!("{testdata}/alltypes_plain.parquet"),
        ParquetReadOptions::default(),
    )
    .await?;

    // execute the query
    let df = ctx.sql(
        "SELECT int_col, double_col, CAST(date_string_col as VARCHAR) \
         FROM alltypes_plain \
         WHERE id > 1 AND tinyint_col < double_col",
    ).await?;

    // execute the plan, and compare to the expected results
    let results = df.collect().await?;
    datafusion::assert_batches_eq!(vec![
        "+---------+------------+--------------------------------+",
        "| int_col | double_col | alltypes_plain.date_string_col |",
        "+---------+------------+--------------------------------+",
        "| 1       | 10.1       | 03/01/09                       |",
        "| 1       | 10.1       | 04/01/09                       |",
        "| 1       | 10.1       | 02/01/09                       |",
        "+---------+------------+--------------------------------+",
        ],
        &results
    );
    Ok(())
}
```

### Read an Apache Avro file

DataFusion can also read Avro files using the `register_avro` method.

```rust
use datafusion::arrow::util::pretty;
use datafusion::error::Result;
use datafusion::prelude::*;

#[tokio::main]
async fn main() -> Result<()> {
    let ctx = SessionContext::new();
    // find the path to the avro test files
    let testdata = datafusion::test_util::arrow_test_data();
    // register avro file with the execution context
    let avro_file = &format!("{testdata}/avro/alltypes_plain.avro");
    ctx.register_avro("alltypes_plain", avro_file, AvroReadOptions::default()).await?;

    // execute the query
    let df = ctx.sql(
        "SELECT int_col, double_col, CAST(date_string_col as VARCHAR) \
         FROM alltypes_plain \
         WHERE id > 1 AND tinyint_col < double_col"
      ).await?;

    // execute the plan, and compare to the expected results
    let results = df.collect().await?;
    datafusion::assert_batches_eq!(vec![
        "+---------+------------+--------------------------------+",
        "| int_col | double_col | alltypes_plain.date_string_col |",
        "+---------+------------+--------------------------------+",
        "| 1       | 10.1       | 03/01/09                       |",
        "| 1       | 10.1       | 04/01/09                       |",
        "| 1       | 10.1       | 02/01/09                       |",
        "+---------+------------+--------------------------------+",
        ],
        &results
    );
    Ok(())
}
```

## Reading Multiple Files as a table

It is also possible to read multiple files as a single table. This is done
with the ListingTableProvider which takes a list of file paths and reads them
as a single table, matching schemas as appropriate

Coming Soon

```rust

```

## Using `CREATE EXTERNAL TABLE` to register data sources via SQL

You can also register files using SQL using the [`CREATE EXTERNAL TABLE`]
statement.

[`create external table`]: ../user-guide/sql/ddl.md#create-external-table

```rust
use datafusion::error::Result;
use datafusion::prelude::*;
#[tokio::main]
async fn main() -> Result<()> {
    // create local session context
    let ctx = SessionContext::new();
    let testdata = datafusion::test_util::parquet_test_data();

    // register parquet file using SQL
    let ddl = format!(
        "CREATE EXTERNAL TABLE alltypes_plain \
        STORED AS PARQUET LOCATION '{testdata}/alltypes_plain.parquet'"
    );
    ctx.sql(&ddl).await?;

    // execute the query referring to the alltypes_plain table we just registered
    let df = ctx.sql(
        "SELECT int_col, double_col, CAST(date_string_col as VARCHAR) \
         FROM alltypes_plain \
         WHERE id > 1 AND tinyint_col < double_col",
    ).await?;

    // execute the plan, and compare to the expected results
    let results = df.collect().await?;
    datafusion::assert_batches_eq!(vec![
        "+---------+------------+--------------------------------+",
        "| int_col | double_col | alltypes_plain.date_string_col |",
        "+---------+------------+--------------------------------+",
        "| 1       | 10.1       | 03/01/09                       |",
        "| 1       | 10.1       | 04/01/09                       |",
        "| 1       | 10.1       | 02/01/09                       |",
        "+---------+------------+--------------------------------+",
        ],
        &results
    );
    Ok(())
}
```

[`sessioncontext`]: https://docs.rs/datafusion/latest/datafusion/execution/context/struct.SessionContext.html
[`sessioncontext::sql`]: https://docs.rs/datafusion/latest/datafusion/execution/context/struct.SessionContext.html#method.sql
[`sessioncontext::sql_with_options`]: https://docs.rs/datafusion/latest/datafusion/execution/context/struct.SessionContext.html#method.sql_with_options
[`sessionstate`]: https://docs.rs/datafusion/latest/datafusion/execution/session_state/struct.SessionState.html
["using the dataframe api"]: ../library-user-guide/using-the-dataframe-api.md
