docs: Update documentation for SelectBuilder and Dialect trait to clarify projection behavior and database support

Author: kosiew

datafusion/sql/src/unparser/ast.rs

@@ -144,6 +144,7 @@ pub struct SelectBuilder {
     /// This field uses `Option` to distinguish between three distinct states:
     /// - `None`: No projection has been set (not yet initialized)
     /// - `Some(vec![])`: Empty projection explicitly set (generates `SELECT FROM ...` or `SELECT 1 FROM ...`)
+    /// - `Some(vec![SelectItem::Wildcard(...)])`: Wildcard projection (generates `SELECT * FROM ...`)
     /// - `Some(vec![...])`: Non-empty projection with specific columns/expressions
     ///
     /// Use `projection()` to set this field and `already_projected()` to check if it has been set.

datafusion/sql/src/unparser/dialect.rs

@@ -226,32 +226,23 @@ pub trait Dialect: Send + Sync {
     /// - Testing row existence without retrieving column data
     /// - Performance optimization when only row counts or existence checks are needed
     ///
-    /// # Supported Databases
-    ///
-    /// - **PostgreSQL**: Fully supported. Returns rows with zero columns.
-    ///
-    /// # Unsupported Databases
-    ///
-    /// Most other SQL databases require at least one column in the SELECT list:
-    /// - MySQL
-    /// - SQLite
-    /// - SQL Server
-    /// - Oracle
-    /// - DuckDB
+    /// # Default
     ///
-    /// For these databases, the unparser falls back to `SELECT 1 FROM table`.
+    /// Returns `false` for maximum compatibility across SQL dialects. When `false`,
+    /// the unparser falls back to `SELECT 1 FROM table`.
     ///
-    /// # Default
+    /// # Implementation Note
     ///
-    /// Returns `false` for maximum compatibility across SQL dialects.
+    /// Specific dialects should override this method to return `true` if they support
+    /// the empty select list syntax (e.g., PostgreSQL).
     ///
     /// # Example SQL Output
     ///
     /// ```sql
-    /// -- When supported (PostgreSQL/DataFusion):
+    /// -- When supported:
     /// SELECT FROM users WHERE active = true;
     ///
-    /// -- Fallback for other databases:
+    /// -- Fallback when unsupported:
     /// SELECT 1 FROM users WHERE active = true;
     /// ```
     fn supports_empty_select_list(&self) -> bool {