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# Modifying JSON with YQL
In memory, YQL operates on immutable values. Thus, when a query needs to change something inside a JSON value, the mindset should be about constructing a new value from pieces of the old one.
This example query takes an input JSON named `$fields`, parses it, substitutes key `a` with 0, drops key `d`, and adds a key `c` with value 3:
```yql
$fields = '{"a": 1, "b": 2, "d": 4}'j;
$pairs = DictItems(Yson::ConvertToInt64Dict($fields));
$result_pairs = ListExtend(ListNotNull(ListMap($pairs, ($item) -> {
$item = if ($item.0 == "a", ("a", 0), $item);
return if ($item.0 == "d", null, $item);
})), [("c", 3)]);
$result_dict = ToDict($result_pairs);
SELECT Yson::SerializeJson(Yson::From($result_dict));
```
## See also
- [{#T}](../udf/list/yson.md)
- [{#T}](../builtins/list.md)
- [{#T}](../builtins/dict.md)
- [{#T}](accessing-json.md)
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