docs(api): Update README to reflect SQL support in UnifiedQueryPlanner

Signed-off-by: Chen Dai <daichen@amazon.com>

Author: dai-chen

api/README.md

@@ -8,7 +8,7 @@ This module provides components organized into two main areas aligned with the [
 
 ### Unified Language Specification
 
-- **`UnifiedQueryPlanner`**: Accepts PPL (Piped Processing Language) queries and returns Calcite `RelNode` logical plans as intermediate representation.
+- **`UnifiedQueryPlanner`**: Accepts PPL (Piped Processing Language) or ANSI SQL queries and returns Calcite `RelNode` logical plans as intermediate representation. PPL uses the ANTLR-based parser while SQL uses Calcite's native `SqlParser` → `SqlValidator` → `SqlToRelConverter` pipeline.
 - **`UnifiedQueryTranspiler`**: Converts Calcite logical plans (`RelNode`) into SQL strings for various target databases using different SQL dialects.
 
 ### Unified Execution Runtime
@@ -17,7 +17,7 @@ This module provides components organized into two main areas aligned with the [
 - **`UnifiedFunction`**: Engine-agnostic function interface that enables functions to be evaluated across different execution engines without engine-specific code duplication.
 - **`UnifiedFunctionRepository`**: Repository for discovering and loading functions as `UnifiedFunction` instances, providing a bridge between function definitions and external execution engines.
 
-Together, these components enable complete workflows: parse PPL queries into logical plans, transpile those plans into target database SQL, compile and execute queries directly, or export PPL functions for use in external execution engines.
+Together, these components enable complete workflows: parse PPL or SQL queries into logical plans, transpile those plans into target database SQL, compile and execute queries directly, or export PPL functions for use in external execution engines.
 
 ### Experimental API Design
 
@@ -44,17 +44,30 @@ UnifiedQueryContext context = UnifiedQueryContext.builder()
 
 ### UnifiedQueryPlanner
 
-Use `UnifiedQueryPlanner` to parse and analyze PPL queries into Calcite logical plans. The planner accepts a `UnifiedQueryContext` and can be reused for multiple queries.
+Use `UnifiedQueryPlanner` to parse and analyze PPL or SQL queries into Calcite logical plans. The planner accepts a `UnifiedQueryContext` and can be reused for multiple queries.
 
 ```java
-// Create planner with context
+// Create planner with context (PPL)
 UnifiedQueryPlanner planner = new UnifiedQueryPlanner(context);
 
 // Plan multiple queries (context is reused)
 RelNode plan1 = planner.plan("source = logs | where status = 200");
 RelNode plan2 = planner.plan("source = metrics | stats avg(cpu)");
 ```
 
+For SQL queries, create a context with `QueryType.SQL`:
+
+```java
+UnifiedQueryContext sqlContext = UnifiedQueryContext.builder()
+    .language(QueryType.SQL)
+    .catalog("opensearch", opensearchSchema)
+    .defaultNamespace("opensearch")
+    .build();
+UnifiedQueryPlanner sqlPlanner = new UnifiedQueryPlanner(sqlContext);
+
+RelNode plan = sqlPlanner.plan("SELECT * FROM employees WHERE age > 30");
+```
+
 ### UnifiedQueryTranspiler
 
 Use `UnifiedQueryTranspiler` to convert Calcite logical plans into SQL strings for target databases. The transpiler supports various SQL dialects through Calcite's `SqlDialect` interface.
@@ -226,5 +239,4 @@ public class MySchema extends AbstractSchema {
 
 ## Future Work
 
-- Expand support to SQL language.
 - Extend planner to generate optimized physical plans using Calcite's optimization frameworks.