Consolidate sql operations examples (apache#18142) (apache#18743)

## Which issue does this PR close?

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- part of #apache#18142.

## Rationale for this change
This PR is for consolidating all the `sql_ops` examples (analysis,
dialect, frontend, query) into a single example binary. We are agreed on
the pattern and we can apply it to the remaining examples
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---------

Co-authored-by: Sergey Zhukov <szhukov@aligntech.com>

Author: cj-zhukov

datafusion-examples/README.md

@@ -90,10 +90,10 @@ cargo run --example dataframe
 - [`examples/udf/simple_udf.rs`](examples/udf/simple_udf.rs): Define and invoke a User Defined Scalar Function (UDF)
 - [`examples/udf/simple_udtf.rs`](examples/udf/simple_udtf.rs): Define and invoke a User Defined Table Function (UDTF)
 - [`examples/udf/simple_udfw.rs`](examples/udf/simple_udwf.rs): Define and invoke a User Defined Window Function (UDWF)
-- [`sql_analysis.rs`](examples/sql_analysis.rs): Analyse SQL queries with DataFusion structures
-- [`sql_frontend.rs`](examples/sql_frontend.rs): Create LogicalPlans (only) from sql strings
-- [`sql_dialect.rs`](examples/sql_dialect.rs): Example of implementing a custom SQL dialect on top of `DFParser`
-- [`sql_query.rs`](examples/sql_query.rs): Query data using SQL (in memory `RecordBatches`, local Parquet files)
+- [`examples/sql_ops/analysis.rs`](examples/sql_ops/analysis.rs): Analyse SQL queries with DataFusion structures
+- [`examples/sql_ops/frontend.rs`](examples/sql_ops/frontend.rs): Create LogicalPlans (only) from sql strings
+- [`examples/sql_ops/dialect.rs`](examples/sql_ops/dialect.rs): Example of implementing a custom SQL dialect on top of `DFParser`
+- [`examples/sql_ops/query.rs`](examples/sql_ops/query.rs): Query data using SQL (in memory `RecordBatches`, local Parquet files)
 
 ## Distributed
 

datafusion-examples/examples/builtin_functions/date_time.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! See `main.rs` for how to run it.
+
 use std::sync::Arc;
 
 use arrow::array::{Date32Array, Int32Array};

datafusion-examples/examples/builtin_functions/function_factory.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! See `main.rs` for how to run it.
+
 use arrow::datatypes::DataType;
 use datafusion::common::tree_node::{Transformed, TreeNode};
 use datafusion::common::{exec_datafusion_err, exec_err, internal_err, DataFusionError};

datafusion-examples/examples/builtin_functions/regexp.rs

@@ -16,6 +16,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! See `main.rs` for how to run it.
+
 use datafusion::common::{assert_batches_eq, assert_contains};
 use datafusion::error::Result;
 use datafusion::prelude::*;

datafusion-examples/examples/custom_data_source/csv_json_opener.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! See `main.rs` for how to run it.
+
 use std::sync::Arc;
 
 use arrow::datatypes::{DataType, Field, Schema};

datafusion-examples/examples/custom_data_source/csv_sql_streaming.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! See `main.rs` for how to run it.
+
 use datafusion::common::test_util::datafusion_test_data;
 use datafusion::error::Result;
 use datafusion::prelude::*;

datafusion-examples/examples/custom_data_source/custom_datasource.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! See `main.rs` for how to run it.
+
 use std::any::Any;
 use std::collections::{BTreeMap, HashMap};
 use std::fmt::{self, Debug, Formatter};

datafusion-examples/examples/custom_data_source/custom_file_casts.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! See `main.rs` for how to run it.
+
 use std::sync::Arc;
 
 use arrow::array::{record_batch, RecordBatch};

datafusion-examples/examples/custom_data_source/custom_file_format.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! See `main.rs` for how to run it.
+
 use std::{any::Any, sync::Arc};
 
 use arrow::{

datafusion-examples/examples/custom_data_source/file_stream_provider.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! See `main.rs` for how to run it.
+
 /// Demonstrates how to use [`FileStreamProvider`] and [`StreamTable`] to stream data
 /// from a file-like source (FIFO) into DataFusion for continuous querying.
 ///