### TO\_JSON 

#### Description

The `TO_JSON` function is used to convert a specified expression (expr) into JSON formatted text. This function can handle various data types and map them to the corresponding JSON types. It is very useful for processing and outputting JSON data in SQL queries.

#### Type Mapping Relationships

The following are the correspondences between LakeHouse data types and JSON types:

* `struct` and `map`: Converted to JSON object
* `array`: Converted to JSON array
* `tinyint`, `smallint`, `int`, `bigint`, `float`, `double`: Converted to JSON numeric
* `boolean`: Converted to JSON boolean
* `string`: Converted to JSON string
* `date`, `datetime`: Converted to JSON string, formatted as date or datetime
* `null`: Converted to JSON null value

It should be noted that for the `decimal` type, when converted to JSON, it will be output as `double` type, which may result in a loss of precision. Additionally, for the `map<K, V>` type, if K is not of `string`, `char`, or `varchar` type, the system will first convert it to a string form before outputting. This is because the keys of JSON objects must be of string type.

#### Function Syntax
```
TO_JSON(expr)
```
#### Parameter Description

* `expr`: An expression of any type.

#### Return Result

* Return type: string

#### Usage Example

The following example demonstrates how to use the `TO_JSON` function to handle different types of data:

1. Map an array to a JSON object:
```sql
SELECT TO_JSON(MAP(ARRAY(1, 2), 2));
-- Output: {"[1,2]":2}
```
2. Directly output the JSON string:
```sql
SELECT TO_JSON(JSON '{"a": 1}');
-- Output: {"a": 1}
```
3. Convert the structure to a JSON object: 
```sql
SELECT TO_JSON(NAMED_STRUCT( 'id',1, 'name','John Doe' ));
-- Output: {"id":1,"name":"John Doe"}
```
4. Convert `null` values to JSON null:
```sql
SELECT TO_JSON(NULL);
--Output empty string
+---------------+
| TO_JSON(NULL) |
+---------------+
|               |
+---------------+
```
By the above example, you can see the flexibility and practicality of the `TO_JSON` function when dealing with different types of data. This makes handling JSON data in SQL queries simpler and more efficient.