`select`

Selects and reorders columns from files. Supports filtering, nested dot-notation fields, and engine selection. When the DuckDB engine is used, filter expressions are pushed to SQL when possible and results can be written directly via COPY for CSV, JSON, and Parquet output.

--filter is the documented flag (--filter-expr is an alias). Syntax: Basic usage.

undatum select --fields name,email,status data.jsonl
undatum select --fields name,email --filter '`status` == "active"' data.jsonl
undatum select --fields user.name,user.email --engine duckdb data.jsonl
undatum select --fields name,email --engine duckdb --output subset.csv data.jsonl
undatum select --fields name --table Sheet2 workbook.xlsx
undatum select --fields name,capital_city.lat --flatten-nested nested.jsonl

# SQL condition and a computed column
undatum select data.csv --where "amount > 100 AND city = 'Berlin'" --fields name,amount
undatum select data.csv --add "total = price * quantity" --add "big = amount > 200"

--where takes a SQL condition (DuckDB syntax) on typed values — text columns get the number, date or boolean type most of their values have — while the output keeps the original text; unknown columns are reported with suggestions. See SQL expressions. --add name = expression is repeatable; an existing name is replaced in place.

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Reads: any readable format · Writes: any writable format; CSV/TSV/JSON/JSON Lines on stdout · Memory: streaming · Engines: auto, duckdb, python

undatum select [OPTIONS] INPUT_FILE

Argument	Description
`INPUT_FILE`	Path to input file. (required)

Option	Description	Default
`-o`, `--output` TEXT	Optional output file path. If not specified, prints to stdout.
`-f`, `--fields` TEXT	Comma-separated list of field names to select and reorder.
`-d`, `--delimiter` TEXT	CSV delimiter character (auto-detected when omitted).
`--quotechar` TEXT	CSV quote character (iterabledata default '"' when omitted).
`--encoding` TEXT	File encoding (e.g., 'utf8', 'latin1').
`--verbose` / `--no-verbose`	Enable verbose logging output.	`--no-verbose`
`-F`, `--format-in` TEXT	Override input file format detection (e.g., 'csv', 'jsonl', 'xlsx').
`-O`, `--format-out` TEXT	Override output format (e.g., 'csv', 'jsonl').
`--zipfile` / `--no-zipfile`	Treat input file as a ZIP archive.	`--no-zipfile`
`--filter`, `--filter-expr` TEXT	Filter expression to apply (e.g., "`status` == 'active'").
`--start-page` INTEGER	Sheet index (0-based) for Excel files.	`0`
`--table`, `--sheet` TEXT	Table or sheet name for multi-table sources (Excel, SQLite, lakehouse).
`-e`, `--engine` [auto\|duckdb\|python]	Processing engine: auto (default), duckdb, or python.
`--duckdb-threads` INTEGER	Number of threads for DuckDB engine.
`--duckdb-memory` TEXT	Memory limit for DuckDB (e.g., '4GB', '512MB').
`--duckdb-temp-dir` TEXT	Temporary directory for DuckDB.
`--trust`	Acknowledge pickle deserialization risk when reading pickle sources.
`--on-error` TEXT	Parse-error policy: raise (default), skip, or warn.
`--error-log` TEXT	Append parse errors as JSONL (use with --on-error skip or warn).
`--flatten-nested`	Unfold nested dict / array-of-dict fields into dotted paths (e.g. city.lat).
`--max-nested-depth` INTEGER	With --flatten-nested, maximum nest depth to unfold (engine default 5).
`--keep-nested-parents` / `--no-keep-nested-parents`	With --flatten-nested, keep parent dict/array fields alongside dotted children.	`--keep-nested-parents`
`--where` TEXT	Keep records where this SQL condition is true (DuckDB syntax), e.g. "amount > 100 AND city = 'Berlin'". Text values are typed automatically.
`--add` TEXT	Add a computed column: 'name = SQL expression', e.g. 'total = price * qty' (repeatable; an existing name is replaced in place).

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