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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

Key options: --fields, --filter / --filter-expr, --output, --format-out, --engine auto|duckdb|iterable, --zipfile, --duckdb-threads / --duckdb-memory / --duckdb-temp-dir. Also --table, --flatten-nested, --on-error, --error-log, --quotechar, --trust (shared options).