Qrlew (/ˈkɝlu/) is the open source library that rewrites SQL queries into privacy-preserving variants using Differential Privacy (DP).

Use Qrlew if you want to bring privacy guarantees to your SQL pipelines. It is:

• SQL-to-SQL: Qrlew turns SQL queries into differentially-private SQL queries that can be executed at scale on many SQL datastore, in many SQL dialects.
• Feature-rich: Qrlew covers the broadest range of SQL queries, including JOIN and nested queries
• Privacy-optimized: Qrlew keeps track of tight bounds and ranges throughout each step, minimizing the amount of noise needed to achieve differential privacy.

The rewriting process occurs in three stages: The data practitioners’s query is parsed into a Relation, which is rewritten into a DP equivalent and finally executed by the the data owner which returns the privacy-safe result.

Qrlew assumes the central model of differential privacy, where a trusted central organization: hospital, insurance company, utility provider, called the data owner, collects and stores personal data in a secure database and whishes to let untrusted data practitioners run SQL queries on its data.

At a high level we pursued the following requirements:

• Ease of use for the data practitioners. The data practitioners are assumed to be data experts but no privacy experts. They should be able to express their queries using the most common dialect for data analysis: SQL.
• Ease of integration for the data owner. SQL is a common language to express data analysis tasks supported by most datastores of all scale.
• Simplicity for the data owner to setup privacy protection. Differential privacy is about capping the sensitivity of a result to the addition or removal of an individual that we call privacy unit. Qrlew assumes that the data owner can tell if a table is public and, if it is not, that it can assign exactly one privacy unit to each row of data. In the case there are multiple related tables, Qrlew enables to define easily the privacy units for each table transitively.
• Simple integration with synthetic data when available. Some queries are not very suitable for DP-rewriting (e.g.: SELECT * FROM table), in those cases Qrlew can use synthetic data as a fallback if provided.

These requirements dictated the overall query rewriting architecture and features.

The Qrlew library, solves the problem of running a SQL query with DP guarantees in three steps:

1. the SQL query submitted by the data practitioners is parsed and converted into an intermediate representation called Relation, this Relation is designed to ease the tracking of data types ranges or possible values, to ease the tracking of the privacy unit in the next step.
2. The Relation is rewritten into a DP variant
3. The DP variant of the Relation can be rendered as an SQL query string in any dialect.

At the end of this process, the query string can submitted to the data store of the data owner. The output can be shared with the data practitioner or published without worrying about privacy leakage.

To learn more about Qrlew read the Qrlew white paper.

You can cite us:

@misc{grislain2024qrlew,
  title={Qrlew: Rewriting SQL into Differentially Private SQL}, 
  author={Nicolas Grislain and Paul Roussel and Victoria de Sainte Agathe},
  year={2024},
  eprint={2401.06273},
  archivePrefix={arXiv},
  primaryClass={cs.DB},
  url={https://arxiv.org/pdf/2401.06273.pdf}
}