docs: add LNNVL function documentation

CN/modules/ROOT/nav.adoc

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 **** xref:master/oracle_builtin_functions/stragg.adoc[stragg]
 **** xref:master/oracle_builtin_functions/dbtimezone_impl.adoc[dbtimezone]
 **** xref:master/oracle_builtin_functions/vsize.adoc[vsize]
+**** xref:master/oracle_builtin_functions/lnnvl.adoc[lnnvl]
 *** xref:master/gb18030.adoc[国标GB18030]
 * 参考指南
 ** xref:master/tools_reference.adoc[工具参考]

CN/modules/ROOT/pages/master/oracle_builtin_functions/lnnvl.adoc

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+:sectnums:
+:sectnumlevels: 5
+
+= LNNVL
+
+== 功能概述
+
+IvorySQL 提供兼容 Oracle 的内置函数 `LNNVL(condition)`。当条件结果为 `TRUE` 时返回
+`FALSE`；当条件结果为 `FALSE` 或 `UNKNOWN`（NULL）时返回 `TRUE`。
+
+该行为等价于 SQL 谓词 `condition IS NOT TRUE`。与 `NOT condition` 不同：条件为 NULL
+时，`NOT` 的结果为 `UNKNOWN`，在 `WHERE` 子句中会丢失这些行；而 `LNNVL` 将未知条件
+视为满足条件。因此，`LNNVL` 适合用于筛选“不满足条件”的数据，同时保留条件结果为
+NULL 的行。
+
+== 语法
+
+```
+LNNVL(condition)
+```
+
+[cols="2,6"]
+|====
+|*参数* |*说明*
+|condition |需要按 Oracle 兼容的空值语义取反的布尔表达式。
+|====
+
+返回类型：`boolean`。
+
+== 真值表
+
+[cols="2,2,3"]
+|====
+|*条件结果* |*LNNVL 结果* |*WHERE 子句是否返回该行*
+|TRUE |FALSE |否
+|FALSE |TRUE |是
+|UNKNOWN |TRUE |是
+|====
+
+`LNNVL(NULL)` 同样返回 `TRUE`，因为 `NULL IS NOT TRUE` 的结果为 `TRUE`。
+
+== 示例
+
+下面的查询展示了条件结果为 TRUE、FALSE 和 UNKNOWN 时的返回值：
+
+```
+SELECT LNNVL(1 = 1) AS true_condition,
+       LNNVL(1 = 2) AS false_condition,
+       LNNVL(NULL::boolean) AS unknown_condition;
+```
+
+```
+ true_condition | false_condition | unknown_condition
+----------------+-----------------+-------------------
+ f              | t               | t
+(1 row)
+```
+
+假设需要查询所有不满足 `amount >= 60` 的数据。普通 `NOT` 会丢弃 `amount` 为 NULL
+的行，而 `LNNVL` 会保留这些行：
+
+```
+CREATE TEMP TABLE lnnvl_orders (id int, amount numeric);
+INSERT INTO lnnvl_orders VALUES (1, 100), (2, NULL), (3, 50);
+
+SELECT id FROM lnnvl_orders
+WHERE NOT (amount >= 60)
+ORDER BY id;
+
+ id
+----
+  3
+(1 row)
+
+SELECT id FROM lnnvl_orders
+WHERE LNNVL(amount >= 60)
+ORDER BY id;
+
+ id
+----
+  2
+  3
+(2 rows)
+
+DROP TABLE lnnvl_orders;
+```
+
+`LNNVL` 也可用于 `LIKE`、`BETWEEN`、`IN`、`EXISTS` 等条件：
+
+```
+SELECT LNNVL('ab' LIKE 'a%') AS like_result,
+       LNNVL(1 BETWEEN 0 AND 5) AS between_result,
+       LNNVL(EXISTS (SELECT 1 FROM dual)) AS exists_result;
+```
+
+```
+ like_result | between_result | exists_result
+-------------+----------------+---------------
+ f           | f              | f
+(1 row)
+```
+
+== 兼容性说明
+
+* 参数必须是布尔表达式。`LNNVL(1)` 会报错，因为该函数没有整数重载，也不支持从整数
+  到布尔值的隐式转换。
+* 在 Oracle 兼容模式下，可以直接调用 `LNNVL(condition)`；在 PostgreSQL 模式下，
+  需要使用带模式限定的形式 `sys.lnnvl(condition)`。
+* IvorySQL 接受任意布尔表达式作为参数，而 Oracle 只接受单个简单条件。若 SQL 还需要
+  在 Oracle 上运行，应将复合条件拆分为多个 `LNNVL` 调用，并使用 `AND` 或 `OR` 组合：
+
+```
+SELECT LNNVL(a > 1) OR LNNVL(b < 2);
+```
+
+== 实现原理
+
+函数在 `contrib/ivorysql_ora/src/builtin_functions/builtin_functions--1.0.sql` 中注册：
+
+```sql
+CREATE FUNCTION sys.lnnvl(pg_catalog.bool)
+RETURNS pg_catalog.bool
+AS $$SELECT $1 IS NOT TRUE$$
+LANGUAGE sql
+CALLED ON NULL INPUT
+PARALLEL SAFE
+IMMUTABLE;
+```
+
+直接使用 `IS NOT TRUE` 即可得到与 Oracle 一致的真值表，包括 NULL 情况。

EN/modules/ROOT/nav.adoc

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 *** xref:master/oracle_builtin_functions/stragg.adoc[stragg]
 *** xref:master/oracle_builtin_functions/dbtimezone_impl_en.adoc[dbtimezone]
 *** xref:master/oracle_builtin_functions/vsize_en.adoc[vsize]
+*** xref:master/oracle_builtin_functions/lnnvl.adoc[lnnvl]
 ** xref:master/gb18030.adoc[GB18030 Character Set]
 * Reference
 ** xref:master/tools_reference.adoc[Tool Reference]

EN/modules/ROOT/pages/master/oracle_builtin_functions/lnnvl.adoc

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+:sectnums:
+:sectnumlevels: 5
+
+= LNNVL
+
+== Overview
+
+IvorySQL provides the Oracle-compatible built-in function `LNNVL(condition)`.
+It returns `FALSE` when the condition evaluates to `TRUE`, and returns `TRUE`
+when the condition evaluates to `FALSE` or `UNKNOWN` (NULL).
+
+This behavior is equivalent to the SQL predicate `condition IS NOT TRUE`.
+Unlike `NOT condition`, which evaluates to `UNKNOWN` when the condition is
+NULL, `LNNVL` treats an unknown condition as a matching condition. This makes
+it useful in `WHERE` clauses when rows that do not satisfy a condition must be
+returned without losing the rows whose condition is NULL.
+
+== Syntax
+
+```
+LNNVL(condition)
+```
+
+[cols="2,6"]
+|====
+|*Parameter* |*Description*
+|condition |A boolean expression to negate with Oracle-compatible NULL semantics.
+|====
+
+Return type: `boolean`.
+
+== Truth table
+
+[cols="2,2,3"]
+|====
+|*Condition result* |*LNNVL result* |*Returned by a WHERE clause*
+|TRUE |FALSE |No
+|FALSE |TRUE |Yes
+|UNKNOWN |TRUE |Yes
+|====
+
+`LNNVL(NULL)` also returns `TRUE`, because `NULL IS NOT TRUE` evaluates to
+`TRUE`.
+
+== Examples
+
+The following query shows the three possible condition results:
+
+```
+SELECT LNNVL(1 = 1) AS true_condition,
+       LNNVL(1 = 2) AS false_condition,
+       LNNVL(NULL::boolean) AS unknown_condition;
+```
+
+```
+ true_condition | false_condition | unknown_condition
+----------------+-----------------+-------------------
+ f              | t               | t
+(1 row)
+```
+
+Suppose a query must return all rows that do not satisfy `amount >= 60`.
+A plain `NOT` drops rows where `amount` is NULL, while `LNNVL` keeps them:
+
+```
+CREATE TEMP TABLE lnnvl_orders (id int, amount numeric);
+INSERT INTO lnnvl_orders VALUES (1, 100), (2, NULL), (3, 50);
+
+SELECT id FROM lnnvl_orders
+WHERE NOT (amount >= 60)
+ORDER BY id;
+
+ id
+----
+  3
+(1 row)
+
+SELECT id FROM lnnvl_orders
+WHERE LNNVL(amount >= 60)
+ORDER BY id;
+
+ id
+----
+  2
+  3
+(2 rows)
+
+DROP TABLE lnnvl_orders;
+```
+
+`LNNVL` can be used with conditions such as `LIKE`, `BETWEEN`, `IN`, and
+`EXISTS`:
+
+```
+SELECT LNNVL('ab' LIKE 'a%') AS like_result,
+       LNNVL(1 BETWEEN 0 AND 5) AS between_result,
+       LNNVL(EXISTS (SELECT 1 FROM dual)) AS exists_result;
+```
+
+```
+ like_result | between_result | exists_result
+-------------+----------------+---------------
+ f           | f              | f
+(1 row)
+```
+
+== Compatibility notes
+
+* The condition must be a boolean expression. `LNNVL(1)` raises an error
+  because there is no integer overload or implicit cast from an integer.
+* In Oracle-compatible mode, call the function as `LNNVL(condition)`.
+  In PostgreSQL mode, use the schema-qualified form `sys.lnnvl(condition)`.
+* IvorySQL accepts any boolean expression as the argument. Oracle accepts a
+  single simple condition. For SQL that must also run on Oracle, write a
+  compound condition as separate `LNNVL` calls combined with `AND` or `OR`:
+
+```
+SELECT LNNVL(a > 1) OR LNNVL(b < 2);
+```
+
+== Implementation
+
+The function is registered in `contrib/ivorysql_ora/src/builtin_functions/builtin_functions--1.0.sql`:
+
+```sql
+CREATE FUNCTION sys.lnnvl(pg_catalog.bool)
+RETURNS pg_catalog.bool
+AS $$SELECT $1 IS NOT TRUE$$
+LANGUAGE sql
+CALLED ON NULL INPUT
+PARALLEL SAFE
+IMMUTABLE;
+```
+
+Using `IS NOT TRUE` directly gives the Oracle-compatible truth table, including
+the NULL case.