feat: query to fetch unique columns

[aaqilniz]

Checklist

• [x] DCO (Developer Certificate of Origin) signed in all commits
• [x] npm test passes on your machine
• [ ] New tests added or existing tests modified to cover all changes
• [x] Code conforms with the style guide
• [x] Commit messages are following our guidelines

[coveralls]

Pull Request Test Coverage Report for Build 8689340015

Details

• 2 of 11 (18.18%) changed or added relevant lines in 1 file are covered.
• 9 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.5%) to 58.772%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
lib/sql.js	2	11	18.18%
<!--	Total:	2	11

Files with Coverage Reduction	New Missed Lines	%
lib/sql.js	9	50.24%
<!--	Total:	9

Totals
Change from base Build 8688690237:	-0.5%
Covered Lines:	715
Relevant Lines:	1146

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💛 - Coveralls

[dhmlau]

Thanks @aaqilniz for the PR. Could you please add some tests for the changes? Thanks.