add desc for flink sql (#100)

README.md

@@ -31,7 +31,7 @@ Add the dependency to your pom.xml.
 
 ## Example
 
-To write data into Nebula Graph using Flink.
+To write data into NebulaGraph using Flink.
 ```
 StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
 NebulaClientOptions nebulaClientOptions = new NebulaClientOptions.NebulaClientOptionsBuilder()
@@ -63,6 +63,142 @@ DataStream<Row> dataStream = playerSource.map(row -> {
 dataStream.addSink(nebulaSinkFunction);
 env.execute("write nebula")
 ```
+
+To read data from NebulaGraph using Flink.
+```
+        NebulaClientOptions nebulaClientOptions = new NebulaClientOptions.NebulaClientOptionsBuilder()
+                .setMetaAddress("127.0.0.1:9559")
+                .build();
+        storageConnectionProvider = new NebulaStorageConnectionProvider(nebulaClientOptions);
+        StreamExecutionEnvironment env = StreamExecutionEnvironment.getExecutionEnvironment();
+        env.setParallelism(1);
+
+        NebulaSourceFunction sourceFunction = new NebulaSourceFunction(storageConnectionProvider)
+                .setExecutionOptions(vertexExecutionOptions);
+        DataStreamSource<BaseTableRow> dataStreamSource = env.addSource(sourceFunction);
+        dataStreamSource.map(row -> {
+            List<ValueWrapper> values = row.getValues();
+            Row record = new Row(15);
+            record.setField(0, values.get(0).asLong());
+            record.setField(1, values.get(1).asString());
+            record.setField(2, values.get(2).asString());
+            record.setField(3, values.get(3).asLong());
+            record.setField(4, values.get(4).asLong());
+            record.setField(5, values.get(5).asLong());
+            record.setField(6, values.get(6).asLong());
+            record.setField(7, values.get(7).asDate());
+            record.setField(8, values.get(8).asDateTime().getUTCDateTimeStr());
+            record.setField(9, values.get(9).asLong());
+            record.setField(10, values.get(10).asBoolean());
+            record.setField(11, values.get(11).asDouble());
+            record.setField(12, values.get(12).asDouble());
+            record.setField(13, values.get(13).asTime().getUTCTimeStr());
+            record.setField(14, values.get(14).asGeography());
+            return record;
+        }).print();
+        env.execute("NebulaStreamSource");
+```
+
+To operate Schema and data using Flink SQL.
+
+1. create graph space
+```
+        NebulaCatalog nebulaCatalog = NebulaCatalogUtils.createNebulaCatalog(
+                "NebulaCatalog",
+                "default",
+                "root",
+                "nebula",
+                "127.0.0.1:9559",
+                "127.0.0.1:9669");
+
+        EnvironmentSettings settings = EnvironmentSettings.newInstance()
+                .inStreamingMode()
+                .build();
+        TableEnvironment tableEnv = TableEnvironment.create(settings);
+
+        tableEnv.registerCatalog(CATALOG_NAME, nebulaCatalog);
+        tableEnv.useCatalog(CATALOG_NAME);
+
+        String createDataBase = "CREATE DATABASE IF NOT EXISTS `db1`"
+                + " COMMENT 'space 1'"
+                + " WITH ("
+                + " 'partition_num' = '100',"
+                + " 'replica_factor' = '3',"
+                + " 'vid_type' = 'FIXED_STRING(10)'"
+                + ")";
+        tableEnv.executeSql(createDataBase);
+```
+2. create tag
+```
+        tableEnvironment.executeSql("CREATE TABLE `person` ("
+                + " vid BIGINT,"
+                + " col1 STRING,"
+                + " col2 STRING,"
+                + " col3 BIGINT,"
+                + " col4 BIGINT,"
+                + " col5 BIGINT,"
+                + " col6 BIGINT,"
+                + " col7 DATE,"
+                + " col8 TIMESTAMP,"
+                + " col9 BIGINT,"
+                + " col10 BOOLEAN,"
+                + " col11 DOUBLE,"
+                + " col12 DOUBLE,"
+                + " col13 TIME,"
+                + " col14 STRING"
+                + ") WITH ("
+                + " 'connector' = 'nebula',"
+                + " 'meta-address' = '127.0.0.1:9559',"
+                + " 'graph-address' = '127.0.0.1:9669',"
+                + " 'username' = 'root',"
+                + " 'password' = 'nebula',"
+                + " 'data-type' = 'vertex',"
+                + " 'graph-space' = 'flink_test',"
+                + " 'label-name' = 'person'"
+                + ")"
+        );
+```
+3. create edge
+```
+        tableEnvironment.executeSql("CREATE TABLE `friend` ("
+                + " sid BIGINT,"
+                + " did BIGINT,"
+                + " rid BIGINT,"
+                + " col1 STRING,"
+                + " col2 STRING,"
+                + " col3 BIGINT,"
+                + " col4 BIGINT,"
+                + " col5 BIGINT,"
+                + " col6 BIGINT,"
+                + " col7 DATE,"
+                + " col8 TIMESTAMP,"
+                + " col9 BIGINT,"
+                + " col10 BOOLEAN,"
+                + " col11 DOUBLE,"
+                + " col12 DOUBLE,"
+                + " col13 TIME,"
+                + " col14 STRING"
+                + ") WITH ("
+                + " 'connector' = 'nebula',"
+                + " 'meta-address' = '127.0.0.1:9559',"
+                + " 'graph-address' = '127.0.0.1:9669',"
+                + " 'username' = 'root',"
+                + " 'password' = 'nebula',"
+                + " 'graph-space' = 'flink_test',"
+                + " 'label-name' = 'friend',"
+                + " 'data-type'='edge',"
+                + " 'src-id-index'='0',"
+                + " 'dst-id-index'='1',"
+                + " 'rank-id-index'='2'"
+                + ")"
+        );
+```
+4. query edge data and insert into another edge type
+```
+        Table table = tableEnvironment.sqlQuery("SELECT * FROM `friend`");
+        table.executeInsert("`friend_sink`").await();
+```
+
 ## Version match
 
 There are the version correspondence between Nebula Flink Connector and Nebula: