add usage document

Author: yuanyl630

CN/modules/ROOT/nav.adoc

@@ -61,6 +61,7 @@
 *** xref:master/7.18.adoc[18、Force View]
 *** xref:master/7.19.adoc[19、嵌套子函数]
 *** xref:master/7.20.adoc[20、sys_guid 函数]
+*** xref:master/7.21.adoc[21、空字符串转null]
 ** IvorySQL贡献指南
 *** xref:master/8.1.adoc[社区贡献指南]
 *** xref:master/8.2.adoc[asciidoc语法快速参考]

CN/modules/ROOT/pages/master/7.20.adoc

@@ -12,7 +12,7 @@ IvorySQL的sys_guid() 是一个强大的随机数产生函数，它产生并返
 == 使用示例
 
 ```
-highgo=# select sys_guid() from dual;
+ivorysql=# select sys_guid() from dual;
               sys_guid              
 ------------------------------------
  \x3ed9426c8a093442a38bea09a74f44a1

CN/modules/ROOT/pages/master/7.21.adoc

@@ -0,0 +1,51 @@
+:sectnums:
+:sectnumlevels: 5
+
+:imagesdir: ./_images
+
+= 兼容Oracle 空字符串转NULL
+
+== 目的
+
+- 在 IvorySQL 的 Oracle 兼容模式下,支持将空字符串转换成NULL进行存储,提供与Oracle数据库一致的行为。
+
+== 功能说明
+
+- Oracle兼容模式下支持将空字符串转换成NULL进行存储。
+- 通过参数 `ivorysql.enable_emptystring_to_null` 控制该特性,默认值为 `on`。
+- 当该参数开启时,插入空字符串会自动转换为NULL值存储。
+- 可以通过 `IS NULL` 条件查询到转换后的NULL值。
+
+== 测试用例
+
+[source,sql]
+----
+-- 创建测试表
+ivorysql=# create table abc (id int);
+CREATE TABLE
+
+-- 查看空字符串转NULL参数状态
+ivorysql=# show ivorysql.enable_emptystring_to_null;
+ ivorysql.enable_emptystring_to_NULL
+-------------------------------------
+ on
+(1 row)
+
+-- 插入空字符串
+ivorysql=# insert into abc values('');
+INSERT 0 1
+
+-- 查询表数据,显示为NULL
+ivorysql=# select * from abc;
+ id
+----
+
+(1 row)
+
+-- 使用IS NULL条件查询
+ivorysql=# select * from abc where id is null;
+ id
+----
+
+(1 row)
+----

EN/modules/ROOT/nav.adoc

@@ -60,6 +60,7 @@
 ** xref:master/7.18.adoc[18、Force View]
 ** xref:master/7.19.adoc[19、Nested Subfunctions]
 ** xref:master/7.20.adoc[20、sys_guid Function]
+** xref:master/7.21.adoc[21、Empty String to NULL]
 * xref:master/8.adoc[Community contribution]
 * xref:master/9.adoc[Tool Reference]
 * xref:master/10.adoc[FAQ]

EN/modules/ROOT/pages/master/7.20.adoc

@@ -12,7 +12,7 @@ IvorySQL's sys_guid() is a powerful random number generation function that gener
 == Usage example
 
 ```
-highgo=# select sys_guid() from dual;
+ivorysql=# select sys_guid() from dual;
               sys_guid              
 ------------------------------------
  \x3ed9426c8a093442a38bea09a74f44a1

EN/modules/ROOT/pages/master/7.21.adoc

@@ -0,0 +1,52 @@
+:sectnums:
+:sectnumlevels: 5
+
+:imagesdir: ./_images
+
+= Oracle Compatible Empty String to NULL
+
+== Purpose
+
+- In IvorySQL's Oracle compatibility mode, support converting empty strings to NULL for storage, providing behavior consistent with Oracle Database.
+
+== Feature Description
+
+- Oracle compatibility mode supports converting empty strings to NULL for storage.
+- The feature is controlled by parameter `ivorysql.enable_emptystring_to_null`, with default value `on`.
+- When this parameter is enabled, inserting an empty string will automatically convert it to NULL value for storage.
+- NULL values after conversion can be queried using the `IS NULL` condition.
+
+== Test Cases
+
+[source,sql]
+----
+-- Create test table
+ivorysql=# create table abc (id int);
+CREATE TABLE
+
+-- Check empty string to NULL parameter status
+ivorysql=# show ivorysql.enable_emptystring_to_null;
+ ivorysql.enable_emptystring_to_NULL
+-------------------------------------
+ on
+(1 row)
+
+-- Insert empty string
+ivorysql=# insert into abc values('');
+INSERT 0 1
+
+-- Query table data, displays as NULL
+ivorysql=# select * from abc;
+ id
+----
+
+(1 row)
+
+-- Query using IS NULL condition
+ivorysql=# select * from abc where id is null;
+ id
+----
+
+(1 row)
+----
+