Implement COUNT and COUNT(DISTINCT) aggregation operations #5
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Add comprehensive COUNT aggregation support addressing issue com-lihaoyi#95: - Add AggAnyOps.scala with count/countDistinct for Expr[T] types - Add countBy/countDistinctBy methods to AggOps.scala - Full cross-database compatibility (PostgreSQL, MySQL, SQLite, H2, MS SQL) - Proper NULL handling (COUNT ignores NULL values, COUNT(*) counts all) - Support for expressions: map(...).count, map(...).countDistinct - Multi-dialect SQL generation with proper operator precedence - Comprehensive test coverage including edge cases and complex expressions Features: - .countBy(_.column) generates COUNT(column) SQL - .countDistinctBy(_.column) generates COUNT(DISTINCT column) SQL - .count/.countDistinct on mapped expressions for advanced scenarios - Works with arithmetic expressions, filters, group by, and joins - Consistent API design matching existing aggregation functions
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Add comprehensive COUNT aggregation support addressing issue com-lihaoyi#95:
Features: