Improve documentation for DEFAULT column values

Author: ngocnguyen1802

docs/src/main/sphinx/connector/memory.md

@@ -25,6 +25,19 @@ CREATE TABLE example.default.nation AS
 SELECT * from tpch.tiny.nation;
 ```
 
+Create the table orders if it does not already exist, adding a table comment, adding a column comment and a column with default value:
+
+```
+CREATE TABLE IF NOT EXISTS orders (
+  orderkey bigint,
+  orderstatus varchar,
+  totalprice double COMMENT 'Price in cents.',
+  orderdate date,
+  status varchar default 'created'
+)
+COMMENT 'A table to keep track of orders.'
+```
+
 Insert data into a table in the Memory connector:
 
 ```

docs/src/main/sphinx/sql/alter-table.md

@@ -92,6 +92,12 @@ Add column `zip` to the `users` table:
 ALTER TABLE users ADD COLUMN zip varchar;
 ```
 
+Add colum `zip` to the `users` table with default value as `90210`:
+
+```
+ALTER TABLE users ADD COLUMN zip varchar DEFAULT '90210';
+```
+
 Add column `zip` to the `users` table if table `users` exists and column `zip`
 not already exists:
 

docs/src/main/sphinx/sql/create-table.md

@@ -71,14 +71,15 @@ WITH (format = 'ORC')
 ```
 
 Create the table `orders` if it does not already exist, adding a table comment
-and a column comment:
+and a column comment and a column with default value:
 
 ```
 CREATE TABLE IF NOT EXISTS orders (
   orderkey bigint,
   orderstatus varchar,
   totalprice double COMMENT 'Price in cents.',
-  orderdate date
+  orderdate date,
+  status varchar default 'created'
 )
 COMMENT 'A table to keep track of orders.'
 ```