sql: alias selections before calling count
sql: Numeric -> float

This used to be marked as int, which is very wrong.
Marking this as float is only somewhat wrong, definitely less wrong.
sql: nelements diverts to count
sql: resolve ambiguity around nelements, Select
Merge branch 'postgres-doesnt-fail-if-different-credentials' into sql-aliased-views
sql: memoize metadata reflection
expr: avoid repring expressions in _str
sql: allow by(x, a=x.sum())
Merge pull request #861 from mrocklin/sql-aliased-views

More use of SQL sub-queries