add distinct on support

[delta575]

Subject: add distinct on support

Feature or Bugfix

• Feature

Purpose

• Add support for distinct on behavior

Detail

• Use ROW_NUMBER() OVER ( PARTITION BY < field >), then select row_number = 1

Relates

• SELECT DISTINCT ON is not supported #14

[codecov]

Codecov Report

Merging #104 (8d7f03f) into master (57b3d4a) will decrease coverage by 2.56%.
The diff coverage is 57.50%.

❗ Current head 8d7f03f differs from pull request most recent head 35a3861. Consider uploading reports for the commit 35a3861 to get more accurate results

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
- Coverage   64.00%   61.44%   -2.57%     
==========================================
  Files           4        5       +1     
  Lines         439      555     +116     
  Branches      127      164      +37     
==========================================
+ Hits          281      341      +60     
- Misses        116      158      +42     
- Partials       42       56      +14     

Impacted Files	Coverage Δ
django_redshift_backend/compiler.py	52.83% <52.83%> (ø)
django_redshift_backend/base.py	62.67% <92.85%> (-0.56%)	⬇️

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