patito.Relation.case

Relation.case(*, from_column, to_column, mapping, default)

Map values of one column over to a new column.

Parameters

• from_column (str) – Name of column defining the domain of the mapping.

• to_column (str) – Name of column to insert the mapped values into.

• mapping (Dict[Union[str, float, int, None], Union[str, float, int, None]]) – Dictionary defining the mapping. The dictionary keys represent the input values, while the dictionary values represent the output values. Items are inserted into the SQL case statement by their repr() string value.

• default (Union[str, float, int, None]) – Default output value for inputs which have no provided mapping.

Examples

The following case statement…

>>> import patito as pt
>>> db = pt.Database()
>>> relation = db.to_relation("select 1 as a union select 2 as a")
>>> relation.case(
...     from_column="a",
...     to_column="b",
...     mapping={1: "one", 2: "two"},
...     default="three",
... ).order(by="a").to_df()
shape: (2, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ str │
╞═════╪═════╡
│ 1   ┆ one │
├╌╌╌╌╌┼╌╌╌╌╌┤
│ 2   ┆ two │
└─────┴─────┘

… is equivalent with:

>>> case_statement = pt.sql.Case(
...     on_column="a",
...     mapping={1: "one", 2: "two"},
...     default="three",
...     as_column="b",
... )
>>> relation.select(f"*, {case_statement}").order(by="a").to_df()
shape: (2, 2)
┌─────┬─────┐
│ a   ┆ b   │
│ --- ┆ --- │
│ i64 ┆ str │
╞═════╪═════╡
│ 1   ┆ one │
├╌╌╌╌╌┼╌╌╌╌╌┤
│ 2   ┆ two │
└─────┴─────┘

Return type

Relation