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Relational API

The Relational API is an alternative API that can be used to incrementally construct queries. The API is centered around `DuckDBPyRelation` nodes. The relations can be seen as symbolic representations of SQL queries. They do not hold any data – and nothing is executed – until a method that triggers execution is called.

## Constructing Relations

Relations can be created from SQL queries using the `duckdb.sql` method. Alternatively, they can be created from the various data ingestion methods (`read_parquet`, `read_csv`, `read_json`).

For example, here we create a relation from a SQL query:

    import duckdb

    rel = duckdb.sql("SELECT * FROM range(10_000_000_000) tbl(id)")
    rel.show()

    ┌────────────────────────┐
    │           id           │
    │         int64          │
    ├────────────────────────┤
    │                      0 │
    │                      1 │
    │                      2 │
    │                      3 │
    │                      4 │
    │                      5 │
    │                      6 │
    │                      7 │
    │                      8 │
    │                      9 │
    │                      · │
    │                      · │
    │                      · │
    │                   9990 │
    │                   9991 │
    │                   9992 │
    │                   9993 │
    │                   9994 │
    │                   9995 │
    │                   9996 │
    │                   9997 │
    │                   9998 │
    │                   9999 │
    ├────────────────────────┤
    │         ? rows         │
    │ (>9999 rows, 20 shown) │
    └────────────────────────┘

Note how we are constructing a relation that computes an immense amount of data (10B rows or 74 GB of data). The relation is constructed instantly – and we can even print the relation instantly.

When printing a relation using `show` or displaying it in the terminal, the first `10K` rows are fetched. If there are more than `10K` rows, the output window will show `>9999 rows` (as the amount of rows in the relation is unknown).

## Data Ingestion

Outside of SQL queries, the following methods are provided to construct relation objects from external data.

- `from_arrow`
- `from_df`
- `read_csv`
- `read_json`
- `read_parquet`

## SQL Queries

Relation objects can be queried through SQL through replacement scans. If you have a relation object stored in a variable, you can refer to that variable as if it was a SQL table (in the `FROM` clause). This allows you to incrementally build queries using relation objects.

    import duckdb

    rel = duckdb.sql("SELECT * FROM range(1_000_000) tbl(id)")
    duckdb.sql("SELECT sum(id) FROM rel").show()

    ┌──────────────┐
    │   sum(id)    │
    │    int128    │
    ├──────────────┤
    │ 499999500000 │
    └──────────────┘

## Operations

There are a number of operations that can be performed on relations. These are all short-hand for running the SQL queries – and will return relations again themselves.

### `aggregate(expr, groups = {})`

Apply an (optionally grouped) aggregate over the relation. The system will automatically group by any columns that are not aggregates.

    import duckdb

    rel = duckdb.sql("SELECT * FROM range(1_000_000) tbl(id)")
    rel.aggregate("id % 2 AS g, sum(id), min(id), max(id)")

    ┌───────┬──────────────┬─────────┬─────────┐
    │   g   │   sum(id)    │ min(id) │ max(id) │
    │ int64 │    int128    │  int64  │  int64  │
    ├───────┼──────────────┼─────────┼─────────┤
    │     0 │ 249999500000 │       0 │  999998 │
    │     1 │ 250000000000 │       1 │  999999 │
    └───────┴──────────────┴─────────┴─────────┘

### `except_(rel)`

Select all rows in the first relation, that do not occur in the second relation. The relations must have the same number of columns.

    import duckdb

    r1 = duckdb.sql("SELECT * FROM range(10) tbl(id)")
    r2 = duckdb.sql("SELECT * FROM range(5) tbl(id)")
    r1.except_(r2).show()

    ┌───────┐
    │  id   │
    │ int64 │
    ├───────┤
    │     5 │
    │     6 │
    │     7 │
    │     8 │
    │     9 │
    └───────┘

### `filter(condition)`

Apply the given condition to the relation, filtering any rows that do not satisfy the condition.

    import duckdb

    rel = duckdb.sql("SELECT * FROM range(1_000_000) tbl(id)")
    rel.filter("id > 5").limit(3).show()

    ┌───────┐
    │  id   │
    │ int64 │
    ├───────┤
    │     6 │
    │     7 │
    │     8 │
    └───────┘

### `intersect(rel)`

Select the intersection of two relations – returning all rows that occur in both relations. The relations must have the same number of columns.

    import duckdb

    r1 = duckdb.sql("SELECT * FROM range(10) tbl(id)")
    r2 = duckdb.sql("SELECT * FROM range(5) tbl(id)")
    r1.intersect(r2).show()

    ┌───────┐
    │  id   │
    │ int64 │
    ├───────┤
    │     0 │
    │     1 │
    │     2 │
    │     3 │
    │     4 │
    └───────┘

### `join(rel, condition, type = "inner")`

Combine two relations, joining them based on the provided condition.

    import duckdb

    r1 = duckdb.sql("SELECT * FROM range(5) tbl(id)").set_alias("r1")
    r2 = duckdb.sql("SELECT * FROM range(10, 15) tbl(id)").set_alias("r2")
    r1.join(r2, "r1.id + 10 = r2.id").show()

    ┌───────┬───────┐
    │  id   │  id   │
    │ int64 │ int64 │
    ├───────┼───────┤
    │     0 │    10 │
    │     1 │    11 │
    │     2 │    12 │
    │     3 │    13 │
    │     4 │    14 │
    └───────┴───────┘

### `limit(n, offset = 0)`

Select the first n rows, optionally offset by offset.

    import duckdb

    rel = duckdb.sql("SELECT * FROM range(1_000_000) tbl(id)")
    rel.limit(3).show()

    ┌───────┐
    │  id   │
    │ int64 │
    ├───────┤
    │     0 │
    │     1 │
    │     2 │
    └───────┘

### `order(expr)`

Sort the relation by the given set of expressions.

    import duckdb

    rel = duckdb.sql("SELECT * FROM range(1_000_000) tbl(id)")
    rel.order("id DESC").limit(3).show()

    ┌────────┐
    │   id   │
    │ int64  │
    ├────────┤
    │ 999999 │
    │ 999998 │
    │ 999997 │
    └────────┘

### `project(expr)`

Apply the given expression to each row in the relation.

    import duckdb

    rel = duckdb.sql("SELECT * FROM range(1_000_000) tbl(id)")
    rel.project("id + 10 AS id_plus_ten").limit(3).show()

    ┌─────────────┐
    │ id_plus_ten │
    │    int64    │
    ├─────────────┤
    │          10 │
    │          11 │
    │          12 │
    └─────────────┘

### `union(rel)`

Combine two relations, returning all rows in `r1` followed by all rows in `r2`. The relations must have the same number of columns.

    import duckdb

    r1 = duckdb.sql("SELECT * FROM range(5) tbl(id)")
    r2 = duckdb.sql("SELECT * FROM range(10, 15) tbl(id)")
    r1.union(r2).show()

    ┌───────┐
    │  id   │
    │ int64 │
    ├───────┤
    │     0 │
    │     1 │
    │     2 │
    │     3 │
    │     4 │
    │    10 │
    │    11 │
    │    12 │
    │    13 │
    │    14 │
    └───────┘

## Result Output

The result of relations can be converted to various types of Python structures, see the result conversion page for more information.

The result of relations can also be directly written to files using the below methods.

- `write_csv`
- `write_parquet`

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