Added support for joins

[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 introduces comprehensive support for SQL JOIN operations within the sqlgen C++ library. It enables users to construct type-safe and composable queries involving INNER JOIN, LEFT JOIN, RIGHT JOIN, and FULL JOIN, including the ability to chain multiple joins and perform joins on subqueries.

Highlights

• Core Join Functionality: Added inner_join, left_join, right_join, and full_join operations to the sqlgen API, allowing users to express various SQL join types directly in C++.
• Subquery Joins: Implemented the capability to join tables with subqueries, significantly enhancing the flexibility and power of query construction.
• Aliasing System Enhancement: Extended the column and table aliasing mechanism to properly handle disambiguation in multi-table and subquery join scenarios, including new _t1 to _t99 literal operators for convenience.
• Internal Query Representation: Updated the internal dynamic query representation (dynamic::SelectFrom) to include join clauses and their associated metadata, such as join type, source table/subquery, and ON conditions.
• SQL Generation: Modified the SQL generation logic for both PostgreSQL and SQLite to correctly translate the new internal join structures into valid SQL JOIN clauses, including proper aliasing.
• Documentation: Added a dedicated joins.md documentation file with detailed explanations and examples for using the new join features.
• Test Coverage: Introduced a suite of new integration tests for both PostgreSQL and SQLite, covering basic joins, chained joins, nested joins, and grouped joins, ensuring the new functionality works as expected.

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[gemini-code-assist]

Code Review

This pull request introduces comprehensive support for SQL joins, a significant and well-implemented feature. The changes span documentation, the core API, SQL generation for both PostgreSQL and SQLite, and a solid suite of tests. The implementation of aliased columns and joins on subqueries is particularly well done.

My review focuses on a few areas to enhance maintainability and clarity:

• Documentation: The new joins.md file has some redundant sections that could be consolidated.
• Maintainability: There's a large amount of boilerplate code for table alias literals in col.hpp that could be a maintenance burden.
• Correctness/Redundancy: I've identified a couple of minor issues, including a redundant alias assignment and a potentially unreachable code branch in the SQL generation logic.

Overall, this is a great addition to the library. Addressing these points will further improve the quality of the codebase.