feat: Add dialect-aware SQL serialization with ToSql trait for ClickHouse PascalCase types

[callicles]

feat: Add dialect-aware SQL serialization with ToSql trait for ClickHouse PascalCase types

Summary

This PR introduces a new ToSql trait that enables dialect-aware SQL serialization, specifically addressing ClickHouse's requirement for PascalCase type names (e.g., String, Int64, Nullable(String)) instead of the standard uppercase convention (STRING, INT64).

Related Issue: #2153

Problem

ClickHouse is case-sensitive for data type names and requires PascalCase. When using sqlparser-rs to parse and regenerate SQL:

let sql = "CREATE TABLE t (col String)";
let ast = Parser::parse_sql(&dialect, sql)?;
let regenerated = format!("{}", ast[0]);  // Produces "CREATE TABLE t (col STRING)"

The regenerated SQL fails with UNKNOWN_TYPE errors when executed against ClickHouse because STRING is not recognized—only String is valid.

Solution

Introduce a ToSql trait that accepts dialect context:

pub trait ToSql {
    fn to_sql(&self, dialect: &dyn Dialect) -> String;
    fn write_sql(&self, f: &mut dyn Write, dialect: &dyn Dialect) -> fmt::Result;
}

Usage:

use sqlparser::ast::ToSql;

let sql = "CREATE TABLE t (col String)";
let ast = Parser::parse_sql(&ClickHouseDialect {}, sql)?;
let regenerated = ast[0].to_sql(&ClickHouseDialect {});
// Produces: "CREATE TABLE t (col String)" ✓

Design Decisions

1. Coexistence with Display: The existing Display trait continues to work unchanged (uppercase types). ToSql is opt-in for users who need dialect-aware formatting.

2. impl_to_sql_display! macro: Types without DataType fields delegate to their Display implementation via macro, avoiding code duplication.

3. Recursive propagation: Types containing DataType fields implement write_sql explicitly, calling write_sql on nested types to propagate dialect context through the AST.

4. requires_pascalcase_types() dialect method: A new method on the Dialect trait that returns true for ClickHouse.

Changes

File	Description
src/ast/to_sql.rs	New module with ToSql trait, impl_to_sql_display! macro, and helpers
src/ast/data_type.rs	ToSql impl for DataType with PascalCase formatting
src/ast/mod.rs	ToSql impls for Expr, Statement, Function, window types, etc.
src/ast/ddl.rs	ToSql impls for CreateTable, AlterTable, ColumnDef, etc.
src/ast/query.rs	ToSql impls for Query, Select, TableFactor, etc.
src/dialect/mod.rs	Add requires_pascalcase_types() to Dialect trait
src/dialect/clickhouse.rs	Override requires_pascalcase_types() to return true
tests/sqlparser_clickhouse.rs	Comprehensive tests including round-trip verification

Coverage

ToSql is implemented for all major AST types users are likely to serialize:

• Statements: Statement, Query, Select, CreateTable, AlterTable, CreateView, CreateFunction, Declare, Prepare
• Expressions: Expr (all variants including Cast, Case, BinaryOp, Function, etc.)
• Types: DataType, ColumnDef, ViewColumnDef, StructField, UnionField
• Query components: TableFactor, Join, OrderByExpr, WindowSpec, Cte, With
• Functions: Function, FunctionArg, FunctionArguments, window functions

Testing

Added 8 new tests covering:

• Basic DataType PascalCase formatting
• CREATE TABLE with nested types (Nullable, Array, Map)
• ALTER TABLE operations
• SELECT with CAST expressions
• Complex expressions (CASE, nested CAST)
• CREATE VIEW statements
• Round-trip test: parse → to_sql → parse → verify equivalence

test result: ok. 55 passed; 0 failed (ClickHouse tests)

Migration Guide

If you currently use Display and need dialect-aware formatting:

// Before
let sql = format!("{}", statement);

// After
use sqlparser::ast::ToSql;
let sql = statement.to_sql(&dialect);

Breaking Changes

None. This is a purely additive change. Existing code using Display continues to work unchanged.

Disclaimer: This was worked on with AI

[iffyio]

Hi @callicles! I don't think this approach will be feasible unfortunately, since it's essentially duplicating all display logic