docs: fix SQL examples a deployment rejects

- CREATE INDEX: the example left out USING, which the server requires,
  and the page said SQL cannot choose the algorithm or name the index.
  USING names the algorithm, the page lists them, and a name before ON
  is taken where the deployment's configuration allows it.
- The search join passed fts() a bare string; fts() takes JSON.
- Sampling a subquery's result is refused, so the sampling and subquery
  pages sample the table inside the subquery, and the refusals list it.
- Two unaliased casts in one projection produce the same column name;
  the example aliases them.
- A json_get nested in another json_get is refused. The page shows one
  level of nesting and json_extract with a JSONPath for deeper values,
  and corrects what json_extract and json_extract_with_type return.

docs/web/sql/dql/sampling.mdx

@@ -64,7 +64,7 @@ It attaches to a table in `FROM`, including inside a subquery, a CTE, or either
 side of a join:
 
 ```sql
-SELECT * FROM (SELECT * FROM documents) d TABLESAMPLE (10)
+SELECT * FROM (SELECT * FROM documents TABLESAMPLE (10)) d
 
 SELECT * FROM a TABLESAMPLE (5 ROWS) JOIN b ON a.id = b.id
 
@@ -86,6 +86,7 @@ be honoured exactly is an error:
 | `TABLESAMPLE (101)` | a percentage outside `0..=100` is refused, not clamped |
 | `TABLESAMPLE (1.5 ROWS)` | a row count must be whole |
 | `TABLESAMPLE (BUCKET 1 OUT OF 4)` | bucket sampling is not supported |
+| `(SELECT ...) d TABLESAMPLE (10)` | a subquery's result cannot be sampled; sample the table inside it |
 | the `SAMPLE` spelling | use `TABLESAMPLE` |
 | the `OFFSET` modifier | not supported |
 

docs/web/sql/dql/subqueries.mdx

@@ -19,11 +19,11 @@ SELECT * FROM (
 WHERE n > 100
 ```
 
-A derived table needs an alias. It can be [sampled](/sql/dql/sampling) like a base
-table:
+A derived table needs an alias. Its result cannot be
+[sampled](/sql/dql/sampling); sample the table inside it instead:
 
 ```sql
-SELECT * FROM (SELECT * FROM documents) d TABLESAMPLE (10)
+SELECT * FROM (SELECT * FROM documents TABLESAMPLE (10)) d
 ```
 
 ## As a scalar {#scalar}