First few tests added.
Added more tests. Step commit.
More tests!
Making progress, saving progress.
Saving working commit.
sql: join on SQL objects
Change to table, more tests.
WIP - clean join
Back to working commit. Refactoring to follow later.
table: add frequencies attribute to tableexprs
wip: clean sql join
replace frequencies with count_values
Test build on latest refactoring.
Tests, changes
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Final commit. Ready for review.
Merge pull request #660 from mrocklin/frequencies

Add frequencies attribute to tableexprs
complex joins significantly cleaner
projection of join test
sql: lower_column test