This section details on the original issue you should resolve

<issue_title>Set Operations - UNION, INTERSECT, EXCEPT Support</issue_title>
<issue_description>Priority: 2 (High)
Impact: SQL-99 Compliance
Test Files: test/test316.js, test/test322.js, test/test338.js, test/test339.js
Test Count: 21 tests

Problem Description

Multiple test files contain skipped tests for SQL-99 set operations including UNION, UNION ALL, INTERSECT, and EXCEPT. These operations are fundamental to SQL for combining and comparing result sets from multiple queries.

Specific Test Cases

test316.js - Basic UNION ALL (2 tests)

• Lines 9-16: SEARCH DISTINCT operations
• Lines 18-35: Simple UNION ALL with ORDER BY variations

test322.js - UNION Operations (3 tests)

• Lines 14-43: UNION ALL with CORRESPONDING clause
• Lines 45-51: SEARCH UNION operations

test338.js - EXCEPT Operations (3 tests)

• Lines 39-54: Basic EXCEPT operations
• Lines 56-73: EXCEPT with CTE (Common Table Expressions)

test339.js - Complex Set Operations (13 tests)

• Lines 35-45: Basic UNION operations
• Lines 48-59: UNION ALL operations
• Lines 61-72: EXCEPT operations
• Lines 73-97: INTERSECT operations
• Lines 99-202: Complex combinations (UNION UNION, UNION EXCEPT, UNION INTERSECT, INTERSECT UNION)

Expected Behavior

AlaSQL should support SQL-99 standard set operations:

1. UNION: Combines result sets, removes duplicates
2. UNION ALL: Combines result sets, includes duplicates
3. INTERSECT: Returns rows common to both result sets
4. EXCEPT: Returns rows from first result set not in second
5. CORRESPONDING: Match columns by name/position
6. Complex Combinations: Multiple set operations in single query

Current Status

• Test Status: All skipped (it.skip)
• Error: Unknown (tests not executed)
• Root Cause: Set operations not implemented in query engine

Implementation Requirements

1. Parser Support

Add set operation syntax to src/alasqlparser.jison:

SELECT column_list FROM table1
UNION [ALL]
SELECT column_list FROM table2

SELECT column_list FROM table1
INTERSECT
SELECT column_list FROM table2

SELECT column_list FROM table1
EXCEPT
SELECT column_list FROM table2

UNION [ALL] CORRESPONDING

2. Query Execution Engine

• Implement set operation algorithms
• Handle column type compatibility
• Support duplicate elimination (UNION)
• Support result set combination (UNION ALL)
• Implement intersection logic (INTERSECT)
• Implement difference logic (EXCEPT)

3. Column Handling

• Column name resolution and matching
• Data type compatibility checking
• CORRESPONDING clause support
• Implicit column casting when needed

4. Complex Operations

• Multiple set operations in single query
• Operator precedence (INTERSECT before UNION/EXCEPT)
• Parentheses for operation grouping
• ORDER BY with set operations

5. SEARCH Integration

• SEARCH UNION operations for graph queries
• Integration with existing SEARCH framework

SQL-99 Features Involved

• UNION and UNION ALL operations
• INTERSECT operations
• EXCEPT operations (MINUS in some dialects)
• CORRESPONDING clause
• Set operation precedence
• Complex set operation combinations
• ORDER BY with set operations

Dependencies

• AlaSQL parser (src/alasqlparser.jison)
• Query execution engine
• Result set processing
• Column type system
• Sorting and deduplication algorithms

Acceptance Criteria

Basic Set Operations (test316.js, test322.js)

• All 5 tests enabled and passing
• UNION ALL with duplicate preservation
• UNION with duplicate elimination
• CORRESPONDING clause support
• SEARCH UNION operations

EXCEPT Operations (test338.js)

• All 3 tests enabled and passing
• Basic EXCEPT functionality
• EXCEPT with subqueries and CTEs
• Proper result set difference calculation

Complex Set Operations (test339.js)

• All 13 tests enabled and passing
• Basic UNION, INTERSECT, EXCEPT
• Multiple set operations in single query
• Operator precedence handling
• Complex combinations (UNION EXCEPT, etc.)
• ORDER BY with set operations

Implementation Strategy

Phase 1: Basic Operations

1. UNION ALL - Simple result combination
2. UNION - Add duplicate elimination
3. Basic EXCEPT - Simple difference operation

Phase 2: Advanced Operations

1. INTERSECT - Intersection logic
2. CORRESPONDING - Column matching
3. Complex Combinations - Multiple operations

Phase 3: Integration

1. SEARCH Integration - Graph query support
2. Performance Optimization - Efficient algorithms
3. Edge Cases - Type compatibility, error handling

Test Implementation Examples

-- Basic UNION
SELECT col FROM a UNION SELECT col FROM b

-- UNION ALL with duplicates
SELECT col FROM a UNION ALL SELECT col FROM b

-- INTERSECT
SELECT col FROM a INTERSECT SELECT col FROM b

-- EXCEPT
SELECT col FROM a ...

</details>



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- Fixes AlaSQL/alasql#2361

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