Adding COUNT and COUNT(DISTINCT) aggregation support

[vim89]

Summary

• Implements COUNT and COUNT(DISTINCT) aggregations for ScalaSQL addressing issue -->
  Can we add COUNT and COUNT(DISTINCT) aggregations? #95
• Adds comprehensive API with proper SQL semantics and NULL handling
• Provides full cross-dialect compatibility and extensive test coverage
• Includes complete documentation following existing patterns

Changes Made

Core Operations

• .countBy(_.column) - Generates COUNT(column) SQL, ignores NULL values
• .countDistinctBy(_.column) - Generates COUNT(DISTINCT column) SQL for unique non-NULL counts
• .count on mapped expressions - query.map(_.expr).count
• .countDistinct on mapped expressions - query.map(_.expr).countDistinct

Advanced Usage

// Basic counting
Purchase.select.countBy(_.productId)                    // COUNT(product_id)
Purchase.select.countDistinctBy(_.productId)            // COUNT(DISTINCT product_id)

// Expression counting
Purchase.select.map(_.total * 2).count                  // COUNT(total * 2)
Purchase.select.map(p => p.productId % 2).countDistinct // COUNT(DISTINCT productId % 2)

// With filters and group by
Purchase.select.filter(_.total > 100).countBy(_.productId)
Purchase.select.groupBy(_.shippingInfoId)(agg => agg.countDistinctBy(_.productId))

// Multiple aggregates
Purchase.select.aggregate(agg => (
  agg.countBy(_.productId),
  agg.countDistinctBy(_.productId),
  agg.sumBy(_.total)
))

Files Added/Modified

• AggAnyOps.scala - New operations for Aggregatable[Expr[T]]
• AggOps.scala - Enhanced with countBy/countDistinctBy methods
• DbCountOpsTests.scala - Basic COUNT operation tests
• DbCountOpsOptionTests.scala - NULL handling and Option type tests
• DbCountOpsAdvancedTests.scala - Complex expressions and edge cases

Database compatibility

• PostgreSQL - MOD() function
• MySQL - MOD() function
• SQLite - % operator
• H2 - MOD() function
• MS SQL Server - % operator

Test Coverage

• DbCountOpsTests.scala (new): Basic COUNT operations, GROUP BY, filters, joins
• DbCountOpsAdvancedTests.scala (new): Complex types, edge cases, Option handling
• Cross-dialect testing for PostgreSQL, MySQL, SQLite, H2, MS SQL Server
• Window function and CTE integration tests
• NULL value handling with Option types
• Arithmetic expressions (addition, multiplication, modulo)
• String operations and concatenation
• Complex filtering and grouping scenarios
• Multi-dialect SQL generation
• Cross-database compatibility verification

Documentation

• tutorial.md: Added COUNT examples after existing .size documentation
• cheatsheet.md: Updated aggregate functions table and quick reference
• Window functions: Added COUNT support documentation

Test Plan

• Compiles successfully for Scala 2.13.12 and 3.6.2
• All existing tests continue to pass
• Window functions integrate correctly with existing patterns
• Documentation examples validated against actual API
• New COUNT operations work across all supported database dialects
• All newly created tests pass
• Proper NULL handling verified with Option types

#95

[vim89]

Working & Debugging the issues.

[vim89]

Ready for review. Addressing issue #95

[lihaoyi]

Looks fine to me, seems some docs nee to be updated

[vim89]

Looks fine to me, seems some docs nee to be updated

Thanks, updated the documentation. We are good now :-)

[lihaoyi]

tagged 0.2.2 with this PR