Added numerous operations like concat(...), coalesce(...), abs(...), etc

[liuzicheng1987]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @liuzicheng1987, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the sqlgen library by integrating a wide range of SQL operations directly into its type-safe C++ query building interface. The core change involves a fundamental refactoring of how SQL expressions are represented internally, moving to a more generalized 'Operation' concept. This allows for much more complex and expressive queries to be constructed in C++, with compile-time guarantees for type correctness and nullability, and is supported by new documentation and comprehensive tests for both PostgreSQL and SQLite.

Highlights

• Expanded SQL Functionality: I've introduced a comprehensive set of SQL operations directly into the C++ API, including mathematical functions (abs, ceil, floor, exp, ln, log2, sqrt, sin, cos, tan, round), string manipulation (length, lower, upper, ltrim, rtrim, trim, replace, concat), type conversion (cast), and null handling (coalesce). This significantly broadens the types of queries that can be constructed with sqlgen.
• Unified Expression Representation: A major refactoring was performed to introduce a new Operation type in the transpilation and dynamic layers. This unified representation allows for arbitrary nesting and chaining of expressions, making arithmetic, string, and other operations first-class citizens in the query builder. This also enables operations to be used seamlessly within WHERE clauses and SELECT statements.
• Enhanced Type Safety and Nullability: Robust compile-time type checking and nullability propagation rules have been implemented for all new operations. This ensures that the C++ types inferred for query results accurately reflect the potential nullability of the corresponding SQL expressions, preventing common runtime errors and improving code reliability.
• Comprehensive Documentation and Testing: A new, detailed documentation page (docs/other_operations.md) has been added to explain the usage and semantics of these new operations, including their nullability behavior. Extensive new test cases have also been added for both PostgreSQL and SQLite to validate the functionality across various scenarios, including operations on nullable columns.

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[gemini-code-assist]

Code Review

This pull request introduces several new SQL operations and functions, including concat, coalesce, abs, and others, enhancing the sqlgen library's capabilities. It also includes updates to documentation and internal structures to support these new features. The code introduces new operations and functions, and also updates the documentation.