andygrove opened a new issue, #41:
URL: https://github.com/apache/datafusion-java/issues/41
### Is your feature request related to a problem or challenge?
The DataFrame API gained `dropColumns` and `withColumnRenamed` in #30
but the most common column-shaping primitive — adding or replacing a
column with an expression — is still missing. `unnestColumns` for
struct / list flattening is in the same family.
### Describe the solution you'd like
- `DataFrame.withColumn(String name, String expr)`. The expression is
a SQL fragment parsed via DataFusion's `parse_sql_expr`, matching the
convention already used by `DataFrame.filter(String)`. Behaviour
mirrors `DataFusion::DataFrame::with_column` — replaces a column of
the same name in place, otherwise appends.
- `DataFrame.unnestColumns(String... columns)` and an overload
accepting an `UnnestOptions` value (preserve-nulls, recursions). Maps
to `unnest_columns` / `unnest_columns_with_options`.
- Tests in `DataFrameTransformationsTest` (round-trip on a small
fixture, schema check, error on unknown column).
### Describe alternatives you've considered
`SELECT a + b AS c, *` via `ctx.sql(...)`. Works for `withColumn`, but
requires the DataFrame to be registered as a table first; awkward for
intermediate transformations.
### Additional context
Follows the same JNI shape as #19's `filter(String)` — `parse_sql_expr`
on the native side, no Java-side `Expr` model required. Keeps this
issue independent of #N (joins issue) where an `Expr` builder may
eventually be needed.
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