Update sequence functions docs

functions-and-operators/sequence-functions.md

@@ -7,11 +7,161 @@ summary: This document introduces sequence functions supported in TiDB.
 
 Sequence functions in TiDB are used to return or set values of sequence objects created using the [`CREATE SEQUENCE`](/sql-statements/sql-statement-create-sequence.md) statement.
 
-| Function name | Feature description |
+| Function name | Description |
 | :-------------- | :------------------------------------- |
-| `NEXTVAL()` or `NEXT VALUE FOR` | Returns the next value of a sequence |
-| `SETVAL()` | Sets the current value of a sequence |
-| `LASTVAL()` | Returns the last used value of a sequence |
+| [`NEXTVAL()`](#nextval) | Returns the next value of a sequence. |
+| [`NEXT VALUE FOR`](#next-value-for) | Returns the next value of a sequence (alias for `NEXTVAL()`). |
+| [`SETVAL()`](#setval) | Sets the current value of a sequence. |
+| [`LASTVAL()`](#lastval) | Returns the last value generated by a sequence in the current session. |
+
+## `NEXTVAL()`
+
+The `NEXTVAL()` function returns the next value of a sequence.
+
+Example:
+
+Create a sequence named `s1`:
+
+```sql
+CREATE SEQUENCE s1;
+```
+
+Get the next value from `s1`:
+
+```sql
+SELECT NEXTVAL(s1);
+```
+
+The output is as follows:
+
+```
++-------------+
+| NEXTVAL(s1) |
++-------------+
+|           1 |
++-------------+
+1 row in set (0.00 sec)
+```
+
+## `NEXT VALUE FOR`
+
+The `NEXT VALUE FOR` function is an alias for [`NEXTVAL()`](#nextval).
+
+Example:
+
+Get the next value from `s1` using `NEXTVAL()`:
+
+```sql
+SELECT NEXTVAL(s1);
+```
+
+The output is as follows:
+
+```
++-------------+
+| NEXTVAL(s1) |
++-------------+
+|           2 |
++-------------+
+1 row in set (0.00 sec)
+```
+
+Get the next value from `s1` using `NEXT VALUE FOR`:
+
+```sql
+SELECT NEXT VALUE FOR s1;
+```
+
+The output is as follows:
+
+```
++-------------------+
+| NEXT VALUE FOR s1 |
++-------------------+
+|                 3 |
++-------------------+
+1 row in set (0.00 sec)
+```
+
+## `SETVAL()`
+
+The `SETVAL(n)` function sets the current value of a sequence.
+
+Example:
+
+Get the next value from `s1`:
+
+```sql
+SELECT NEXTVAL(s1);
+```
+
+The output is as follows:
+
+```
++-------------+
+| NEXTVAL(s1) |
++-------------+
+|           4 |
++-------------+
+1 row in set (0.00 sec)
+```
+
+Set the current value of `s1` to `10`:
+
+```sql
+SELECT SETVAL(s1, 10);
+```
+
+The output is as follows:
+
+```
++----------------+
+| SETVAL(s1, 10) |
++----------------+
+|             10 |
++----------------+
+1 row in set (0.00 sec)
+```
+
+Verify the next value after setting it to `10`:
+
+```sql
+SELECT NEXTVAL(s1);
+```
+
+The output is as follows:
+
+```
++-------------+
+| NEXTVAL(s1) |
++-------------+
+|          11 |
++-------------+
+1 row in set (0.00 sec)
+```
+
+## `LASTVAL()`
+
+The `LASTVAL()` function returns the last value generated by a sequence **in the current session**.
+
+Example:
+
+Get the last value generated by `s1` in the current session:
+
+```sql
+SELECT LASTVAL(s1);
+```
+
+The output is as follows:
+
+```
++-------------+
+| LASTVAL(s1) |
++-------------+
+|          11 |
++-------------+
+1 row in set (0.00 sec)
+```
 
 ## MySQL compatibility