docs: add ModifyColumn on nightly (#935)

Co-authored-by: Yingwen <[email protected]> Co-authored-by: Yiran <[email protected]>
GreptimeTeam · May 7, 2024 · e974911 · e974911
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diff --git a/docs/nightly/en/reference/sql/alter.md b/docs/nightly/en/reference/sql/alter.md
@@ -2,7 +2,7 @@
 
 `ALERT` can be used to modify any table settings or data within the table:
 
-* Add/Drop a column
+* Add/Drop/Modify a column
 * Rename a table
 
 ## Syntax
@@ -11,6 +11,7 @@
 ALTER TABLE [db.]table
    [ADD COLUMN name type [options]
     | DROP COLUMN name
+    | MODIFY COLUMN name type
     | RENAME name
    ]
 ```
@@ -54,6 +55,16 @@ ALTER TABLE monitor DROP COLUMN load_15;
 
 The removed column can't be retrieved immediately by all subsequent queries.
 
+### Modify column type
+
+Modify the date type of a column
+
+```sql
+ALTER TABLE monitor MODIFY COLUMN load_15 STRING;
+```
+
+The modified column cannot be a tag (primary key) or time index, and it must be nullable to ensure that the data can be safely converted (returns `NULL` on cast failures).
+
 ### Rename table
 
 Renames the table:

diff --git a/docs/nightly/zh/reference/sql/alter.md b/docs/nightly/zh/reference/sql/alter.md
@@ -2,7 +2,7 @@
 
 `ALERT` 可以用来修改表的设置或者表中的数据：
 
-* 添加/删除列
+* 添加/删除/修改列
 * 重命名表
 
 ## Syntax
@@ -11,6 +11,7 @@
 ALTER TABLE [db.]table
    [ADD COLUMN name type [options] 
     | DROP COLUMN name
+    | MODIFY COLUMN name type
     | RENAME name
    ]
 ```
@@ -55,6 +56,16 @@ ALTER TABLE monitor DROP COLUMN load_15;
 
 后续的所有查询立刻不能获取到被移除的列。
 
+### 修改列类型
+
+修改列的数据类型
+
+```sql
+ALTER TABLE monitor MODIFY COLUMN load_15 STRING;
+```
+
+被修改的的列不能是 tag 列（primary key）或 time index 列，同时该列必须允许空值 `NULL` 存在来保证数据能够安全地进行转换（转换失败时返回 `NULL`）。
+
 ### 重命名表
 
 ```sql