Merge pull request #208 from HuangSongZ/master

Hierarchical Query identifys the leaves

doc/sql_migration/sql_migration04.md

@@ -870,6 +870,93 @@ The example below shows migration when the root data is displayed.
 </tbody>
 </table>
 
+##### 4.2.3.7 Hierarchical Query identifys the leaves
+
+**Functional differences**
+
+ - **Oracle database**
+     - Specifying CONNECT_BY_ISLEAF in the select list of a hierarchical query can identify the leaf rows. This returns 1 if the current row is a leaf. Otherwise it returns 0.
+ - **PostgreSQL**
+     - CONNECT_BY_ISLEAF cannot be specified.
+
+**Migration procedure**
+
+In a recursive query that uses a WITH clause, the leaf can be checked using a sub-query so that the same result is returned. Use the following procedure to perform migration:
+
+1. Replace the hierarchical query with syntax that uses a recursive query (WITH clause).
+2. Add a sub-query to the column list of the query result of the WITH clause.
+
+The following shows the conversion format containing rootName.
+
+~~~
+ WITH RECURSIVE queryName(
+     columnUsed1, columnUsed2
+) AS
+(   SELECT columnUsed, columnUsed2
+      FROM  targetTableOfHierarchicalQuery
+    UNION ALL
+    SELECT columnUsed(qualified by n), columnUsed2(qualified by n)
+      FROM  targetTableOfHierarchicalQuery n,
+            queryName w
+      WHERE conditionalExprOfConnectByClause 
+)
+SELECT *,
+       CASE WHEN EXISTS(select * from queryName p where p.columnUsed1 = e.columnUsed2)
+            THEN 0 ELSE 1 END 
+        as is_leaf
+ FROM queryName e;
+~~~
+
+For conditionalExprOfConnectByClause, use w to qualify the part qualified by PRIOR.
+
+**Migration example**
+
+The example below shows migration when the leaf data is displayed.
+<table>
+<thead>
+<tr>
+<th align="center">Oracle database</th>
+<th align="center">PostgreSQL</th>
+</tr>
+</thead>
+<tbody>
+<tr>
+<td align="left">
+<pre><code>SELECT staff_id, name, <b>CONNECT_BY_ISLEAF </b>
+  FROM staff_table 
+  START WITH staff_id = '1001' 
+  CONNECT BY PRIOR staff_id = manager_id;
+<br>
+<br>
+<br>
+<br>
+<br>
+<br>
+<br>
+<br>
+ </code></pre>
+</td>
+
+<td align="left">
+<pre><code>WITH RECURSIVE staff_table_w( staff_id, 
+                            name ) AS 
+ (   SELECT staff_id, name
+       FROM staff_table 
+     UNION ALL 
+     SELECT n.staff_id, n.name
+       FROM staff_table n, staff_table_w w 
+       WHERE w.staff_id = n.manager_id
+ )
+ SELECT staff_id, name,
+       <b>CASE WHEN EXISTS(select 1 from staff_table_w p where p.manager_id = e.staff_id)
+            THEN 0 ELSE 1 END 
+        as is_leaf </b>
+ FROM staff_table_w e;</code></pre>
+</td>
+</tr>
+</tbody>
+</table>
+
 #### 4.2.4 MINUS
 
 **Description**