fix aggregate expressions with aliases, fixes #112.

[smola]

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[codecov-io]

Codecov Report

Merging #116 into master will increase coverage by 0.25%.

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
+ Coverage   60.87%   61.12%   +0.25%     
==========================================
  Files          41       41              
  Lines        1756     1770      +14     
==========================================
+ Hits         1069     1082      +13     
- Misses        622      625       +3     
+ Partials       65       63       -2

Impacted Files	Coverage Δ
sql/parse/parse.go	58.85% <100%> (+1.85%)	✅
sql/plan/group_by.go	79.24% <55.55%> (+0.03%)	✅

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