ebnf show collation (#17018) (#17038)

sql-statements/sql-statement-show-collation.md

@@ -13,16 +13,24 @@ This statement provides a static list of collations, and is included to provide
 
 ## Synopsis
 
-**ShowCollationStmt:**
+```ebnf+diagram
+ShowCollationStmt ::=
+    "SHOW" "COLLATION" ShowLikeOrWhere?
 
-![ShowCollationStmt](/media/sqlgram/ShowCollationStmt.png)
+ShowLikeOrWhere ::=
+    "LIKE" SimpleExpr
+|   "WHERE" Expression
+```
 
 ## Examples
 
 When new collation framework is disabled, only binary collations are displayed.
 
 ```sql
-mysql> SHOW COLLATION;
+SHOW COLLATION;
+```
+
+```
 +-------------+---------+------+---------+----------+---------+
 | Collation   | Charset | Id   | Default | Compiled | Sortlen |
 +-------------+---------+------+---------+----------+---------+
@@ -38,7 +46,10 @@ mysql> SHOW COLLATION;
 When new collation framework is enabled, `utf8_general_ci` and `utf8mb4_general_ci` are additionally supported.
 
 ```sql
-mysql> SHOW COLLATION;
+SHOW COLLATION;
+```
+
+```
 +--------------------+---------+------+---------+----------+---------+
 | Collation          | Charset | Id   | Default | Compiled | Sortlen |
 +--------------------+---------+------+---------+----------+---------+
@@ -57,6 +68,25 @@ mysql> SHOW COLLATION;
 11 rows in set (0.001 sec)
 ```
 
+To filter on the character set, you can add a `WHERE` clause.
+
+```sql
+SHOW COLLATION WHERE Charset="utf8mb4";
+```
+
+```sql
++--------------------+---------+-----+---------+----------+---------+
+| Collation          | Charset | Id  | Default | Compiled | Sortlen |
++--------------------+---------+-----+---------+----------+---------+
+| utf8mb4_0900_ai_ci | utf8mb4 | 255 |         | Yes      |       1 |
+| utf8mb4_0900_bin   | utf8mb4 | 309 |         | Yes      |       1 |
+| utf8mb4_bin        | utf8mb4 |  46 | Yes     | Yes      |       1 |
+| utf8mb4_general_ci | utf8mb4 |  45 |         | Yes      |       1 |
+| utf8mb4_unicode_ci | utf8mb4 | 224 |         | Yes      |       1 |
++--------------------+---------+-----+---------+----------+---------+
+5 rows in set (0.00 sec)
+```
+
 ## MySQL compatibility
 
 The usage of the `SHOW COLLATION` statement in TiDB is fully compatible with MySQL. However, charsets in TiDB might have different default collations compared with MySQL. For details, refer to [Compatibility with MySQL](/mysql-compatibility.md). If you find any compatibility differences, [report a bug](https://docs.pingcap.com/tidb/stable/support).