improve NDJSON querying documentation with context and explanations

- Add comprehensive introduction explaining what NDJSON is and its advantages
- Provide concrete NDJSON data examples showing realistic book catalog data
- Add detailed explanations for all existing code examples including syntax breakdown
- Explain metadata columns usage and practical use cases
- Add related documentation links for better navigation
- Enhance tutorial flow with clear step-by-step context

Author: drmingdrmer

docs/en/guides/40-load-data/04-transform/03-querying-ndjson.md

@@ -3,6 +3,24 @@ title: Querying NDJSON Files in Stage
 sidebar_label: NDJSON
 ---
 
+In Databend, you can directly query NDJSON files stored in stages without first loading the data into tables. This approach is particularly useful for data exploration, ETL processing, and ad-hoc analysis scenarios.
+
+## What is NDJSON?
+
+NDJSON (Newline Delimited JSON) is a JSON-based file format where each line contains a complete and valid JSON object. This format is especially well-suited for streaming data processing and big data analytics.
+
+**Example NDJSON file content:**
+```json
+{"id": 1, "title": "Database Fundamentals", "author": "John Doe", "price": 45.50, "category": "Technology"}
+{"id": 2, "title": "Machine Learning in Practice", "author": "Jane Smith", "price": 68.00, "category": "AI"}
+{"id": 3, "title": "Web Development Guide", "author": "Mike Johnson", "price": 52.30, "category": "Frontend"}
+```
+
+**Advantages of NDJSON:**
+- **Stream-friendly**: Can be parsed line by line without loading entire file into memory
+- **Big data compatible**: Widely used in log files, data exports, and ETL pipelines
+- **Easy to process**: Each line is an independent JSON object, enabling parallel processing
+
 ## Query NDJSON Files in Stage
 
 Syntax:
@@ -57,6 +75,8 @@ CREATE FILE FORMAT ndjson_query_format
 
 ### Step 3. Query NDJSON Files
 
+Now you can query the NDJSON files directly from the stage. This example extracts the `title` and `author` fields from each JSON object:
+
 ```sql
 SELECT $1:title, $1:author
 FROM @ndjson_query_stage
@@ -66,7 +86,15 @@ FROM @ndjson_query_stage
 );
 ```
 
-If the NDJSON files are compressed with gzip, we can use the following query:
+**Explanation:**
+- `$1:title` and `$1:author`: Extract specific fields from the JSON object. The `$1` represents the entire JSON object as a variant, and `:field_name` accesses individual fields
+- `@ndjson_query_stage`: References the external stage created in Step 1
+- `FILE_FORMAT => 'ndjson_query_format'`: Uses the custom file format defined in Step 2
+- `PATTERN => '.*[.]ndjson'`: Regex pattern that matches all files ending with `.ndjson`
+
+### Querying Compressed Files
+
+If the NDJSON files are compressed with gzip, modify the pattern to match compressed files:
 
 ```sql
 SELECT $1:title, $1:author
@@ -76,9 +104,11 @@ FROM @ndjson_query_stage
     PATTERN => '.*[.]ndjson[.]gz'
 );
 ```
+
+**Key difference:** The pattern `.*[.]ndjson[.]gz` matches files ending with `.ndjson.gz`. Databend automatically decompresses gzip files during query execution thanks to the `COMPRESSION = AUTO` setting in the file format.
 ### Query with Metadata
 
-Query NDJSON files directly from a stage, including metadata columns like `METADATA$FILENAME` and `METADATA$FILE_ROW_NUMBER`:
+You can also include file metadata in your queries, which is useful for tracking data lineage and debugging:
 
 ```sql
 SELECT
@@ -90,4 +120,20 @@ FROM @ndjson_query_stage
     FILE_FORMAT => 'ndjson_query_format',
     PATTERN => '.*[.]ndjson'
 );
-```
+```
+
+**Metadata columns explained:**
+- `METADATA$FILENAME`: Shows which file each row came from - helpful when querying multiple files
+- `METADATA$FILE_ROW_NUMBER`: Shows the line number within the source file - useful for tracking specific records
+
+**Use cases:**
+- **Data lineage**: Track which source file contributed each record
+- **Debugging**: Identify problematic records by file and line number
+- **Incremental processing**: Process only specific files or ranges within files
+
+## Related Documentation
+
+- [Loading NDJSON Files](../03-load-semistructured/03-load-ndjson.md) - How to load NDJSON data into tables
+- [NDJSON File Format Options](/sql/sql-reference/file-format-options#ndjson-options) - Complete NDJSON format configuration
+- [CREATE STAGE](/sql/sql-commands/ddl/stage/ddl-create-stage) - Managing external and internal stages
+- [Querying Metadata](./04-querying-metadata.md) - More details about metadata columns