Add advanced join support (RIGHT, FULL, CROSS) to SelectQuery

[cungminh2710]

This PR adds support for advanced SQL join operations to Rain ORM's SelectQuery builder, bringing it closer to feature parity with Drizzle ORM.

Implemented Features:

• Added RightJoin, FullJoin, and CrossJoin methods to SelectQuery.
• Added RightJoinSubquery, FullJoinSubquery, and CrossJoinSubquery methods to SelectQuery.
• Specialized SQL rendering logic to omit the ON clause specifically for CROSS JOIN.
• Updated both writeSQL (standard SELECT) and compileAggregate (aggregate helpers) to handle these joins correctly.

Mapping to Drizzle ORM:

Drizzle ORM supports these join types across dialects that provide them. Rain now matches this behavior by providing a type-safe API for these operations.

Verification:

• Added TestSelectJoinsToSQL in pkg/rain/query_select_test.go covering all new join types and dialects.
• Ran make test and verified all tests passed.
• Code review received a #Correct# rating.

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PR created automatically by Jules for task 3563641484943122923 started by @cungminh2710

[google-labs-jules]

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[greptile-apps]

Greptile Summary

This PR adds RightJoin, FullJoin, and CrossJoin (plus their *Subquery variants) to SelectQuery, and consolidates the previously duplicated join-rendering logic in both writeSQL and compileAggregate into a single writeJoins helper that correctly omits the ON clause for CROSS JOIN and returns early with a clear error when the clause is missing or unexpected.

• writeJoins replaces identical inline loops in writeSQL and compileAggregate, removing duplication and centralising the new ON-clause validation.
• Six public builder methods are added (RightJoin, FullJoin, CrossJoin and three *Subquery counterparts); CrossJoin/CrossJoinSubquery take no on parameter, enforcing the no-ON-clause constraint at the call site.
• TestSelectJoinsToSQL covers all six methods across PostgreSQL, MySQL, and SQLite happy paths; error paths for the new guards are wired up in the test struct but not yet exercised by any case.

Confidence Score: 5/5

The change is safe to merge; the core join-rendering refactor is correct and well-scoped.

The writeJoins refactor is straightforward and the new methods follow the exact same pattern as the existing Join/LeftJoin pair. The new ON-clause guards are a net improvement. The only gap is that those guards are not exercised by any test case, but this does not affect production correctness.

pkg/rain/query_select_test.go — the wantErr infrastructure should be backed by at least two error-path cases to give the new guards coverage.

Important Files Changed

Filename	Overview
pkg/rain/query_select.go	Adds RightJoin, FullJoin, CrossJoin and their *Subquery variants; refactors duplicate inline join-rendering in writeSQL and compileAggregate into a shared writeJoins helper that correctly omits ON for CROSS JOIN and returns an error when the ON clause is absent or unexpected.
pkg/rain/query_select_test.go	Adds TestSelectJoinsToSQL covering the happy paths for all six new methods across postgres/mysql/sqlite dialects; the wantErr field in the case struct is declared and wired up but no case exercises either of the two new error paths in writeJoins.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    A[SelectQuery.writeSQL / compileAggregate] --> B[writeJoins]
    B --> C{for each join}
    C --> D[write kind + table]
    D --> E{kind == CROSS JOIN?}
    E -- No --> F{on == nil?}
    F -- Yes --> G[error: ON clause required]
    F -- No --> H[write ON clause]
    E -- Yes --> I{on != nil?}
    I -- Yes --> J[error: CROSS JOIN has no ON]
    I -- No --> K[no ON clause written]
    H --> C
    K --> C
    C -- done --> L[return nil]

Loading

Prompt To Fix All With AI

Fix the following 1 code review issue. Work through them one at a time, proposing concise fixes.

---

### Issue 1 of 1
pkg/rain/query_select_test.go:57
**Error-path validation is never exercised**

`wantErr` is declared in the test struct and the harness correctly handles it, but no test case in the table ever sets it. The two new guards introduced in `writeJoins` — `return fmt.Errorf("rain: %s requires an ON clause", join.kind)` and `return errors.New("rain: CROSS JOIN does not support an ON clause")` — are therefore completely uncovered. Adding at least two cases (e.g., a `RightJoin` call where `on` is constructed as `nil`, and a CROSS JOIN that mistakenly has an `on` predicate) would confirm the guards actually fire correctly.

_{Reviews (2): Last reviewed commit: "feat(rain): support advanced joins and f..." | Re-trigger Greptile}

[greptile-apps]

FullJoin / FullJoinSubquery silently produce invalid SQL on MySQL

FullJoin emits FULL JOIN which MySQL does not support at all (it has no FULL OUTER JOIN syntax). A caller using a MySQL-dialect DB will receive a driver-level syntax error with no early indication from the ORM. The HasFeature / dialect.Feature mechanism already exists in this file (see the FeatureSelectDistinctOn guard at line 439), so the same pattern could be applied here. Without a guard, every FullJoin call on a MySQL connection silently hands an invalid query to the database.

Prompt To Fix With AI

This is a comment left during a code review.
Path: pkg/rain/query_select.go
Line: 478-483

Comment:
**`FullJoin` / `FullJoinSubquery` silently produce invalid SQL on MySQL**

`FullJoin` emits `FULL JOIN` which MySQL does not support at all (it has no `FULL OUTER JOIN` syntax). A caller using a MySQL-dialect `DB` will receive a driver-level syntax error with no early indication from the ORM. The `HasFeature` / `dialect.Feature` mechanism already exists in this file (see the `FeatureSelectDistinctOn` guard at line 439), so the same pattern could be applied here. Without a guard, every `FullJoin` call on a MySQL connection silently hands an invalid query to the database.

How can I resolve this? If you propose a fix, please make it concise.

[cungminh2710]

@jules fix

[google-labs-jules]

I've refactored the join rendering logic into a deduplicated writeJoins method and added explicit validation for ON clauses (ensuring they are required for non-CROSS joins and prohibited for CROSS JOIN).