Converting NL queries to RDBMS statements

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Converting NL queries to RDBMS statements

Existing solutions of NLQ2SQL

Soumendra kumar sahoo
·Oct 20, 2018·

4 min read

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Problem statement

While developing an enterprise search engine, we came into multiple use cases where multiple clients have a large amount of structured data (mainly RDBMS), where they want their non-technical employees to search for expected answers through those databases. For e.g., if there is a table like below:

The user can ask “let me know about the department number of Nirun” to get the response “20-B”, instead of writing the following SQL query


Summarizing these are the requirements break-down

  • Users will ask in a Natural language

  • The query needs to be classified whether to prepare SQL query to hit on DB or to search on free-flowing text documents

  • Users can ask in multiple languages

  • Users should not be expected to remember the exact table/column name of the entire DB

  • User’s query can expect a response from multiple tables or DBs

  • Entire table content may not be available for ingesting; on larger DBs, only the DB schema and sample few rows will be shared by the client for processing

  • For security reasons only the query will be sent as an API to client DB and more than a certain amount of columns and rows cannot be extracted at a time

Solution Approach

We have decided to divide the solution into two separate phases.

  1. Analysis phase

  2. Incremental release phase

In this first part, only the analysis phase is mentioned.

1. Analysis Phase

In this phase, we have gone through multiple tools and concepts available over the internet. The following libraries and concepts we have analyzed.

  • Deep learning based solutions

  • Statistical based solutions

Deep learning based solutions

There are many deep learning based solutions available, a consolidated list you can find here:-

These models are tested on top of a certain type of data; WikiSQL. WikiSQL are annotated NL query to table data like there is SQuAD data by Stanford for Q&A system. Although some model claim to go beyond 85% accuracy, these are data hungry i.e. some models need to ingest the entire table content data while training, on the other hand, other models need labeled data as per the schema of the table. This approach may work for a PoC, research work and few tables, however, are not scalable and hard to get annotated data.

Statistical based solutions

There are few libraries which drew our attention:


This is the best library we found. It has some cool features.


  • GPL 3.0 license

  • Take DB dump as input .sql file

  • Only the DB dump is enough, no need for direct access to the whole DB

  • GUI available for ingesting

  • NLP input and SQL query like JSON in output

  • Multi-language supported

  • Customized thesaurus supported i.e. on a way are synonyms

  • Customized stop word supported

  • Most of the nodes are supported

Unavailable features

  • It uses TreeTagger, which later says to contact the developers for commercial use.

  • Python 3 only

  • Rule-based query template generation

  • Only working on MySQL DB

  • Entity extraction and schema mapping with the elements

  • Pseudo-column support not there i.e. SYSADMIN, SYSTIMESTAMP etc.

  • Only single table support


This is the second best library.


  • License seems like BSD

  • Uses NLTK Tagger which does Tokenizing, PoS Tagging and Lemmatizing

  • A customized RegEx support called, refo

Unavailable features

  • Only supports Sparql and MQL, no support for SQL

  • It has not been maintained over a year

  • The customized regex is needed many efforts to understand and customize on need-basis



  • Apache 2.0, easy license to use

  • Parse tree looks good

Unavailable features

  • Written in Java, More efforts to customize in Python

  • No good documentation

  • It has not been maintained over a year

Possible solutions already built by Non-open source companies

Other Good resources

The final conclusion of the analysis phase

  • ln2sql can be leveraged for taking DB dump as input for the initial release.

  • ln2sql’s customized thesaurus and stop word support are also good

  • SQLizer has mentioned a few good practices, those will be checked.

  • For scalability and customization point of view rest, all need to be built as an in-house product.

Thanks for your time.

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