huaxingao commented on a change in pull request #27779:
[SPARK-30962][DOC]Documentation for Alter table command phase 2
URL: https://github.com/apache/spark/pull/27779#discussion_r388008600
##########
File path: docs/sql-ref-syntax-ddl-alter-table.md
##########
@@ -83,6 +82,108 @@ ALTER TABLE table_identifier ADD COLUMNS ( col_spec [ ,
col_spec ... ] )
</dl>
+### ALTER OR CHANGE COLUMN
+`ALTER TABLE ALTER COLUMN` or `ALTER TABLE CHANGE COLUMN` statement changes
column's comment.
+
+#### Syntax
+{% highlight sql %}
+ALTER TABLE table_identifier { ALTER | CHANGE } COLUMN col_spec
alterColumnAction
+{% endhighlight %}
+
+#### Parameters
+<dl>
+ <dt><code><em>table_identifier</em></code></dt>
+ <dd>
+ Specifies a table name, which may be optionally qualified with a database
name.<br><br>
+ <b>Syntax:</b>
+ <code>
+ [ database_name. ] table_name
+ </code>
+ </dd>
+</dl>
+
+<dl>
+ <dt><code><em>COLUMN col_spec</em></code></dt>
+ <dd>Specifies the columns to be altered to be changed.</dd>
+</dl>
+
+<dl>
+ <dt><code><em>alterColumnAction</em></code></dt>
+ <dd>
+ Change the comment string.<br><br>
Review comment:
I saw the following in ```SqlBase.g4```. Can ```alterColumnAction```
change type, colPosition, setOrDrop as well?
```
alterColumnAction
: TYPE dataType
| commentSpec
| colPosition
| setOrDrop=(SET | DROP) NOT NULL
;
```
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