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# How to make a difference of two arrays in ClickHouse?

One of the great things about ClickHouse is its support for arrays, which can be used to store and manipulate large sets of data.

The process of making a difference of two arrays in ClickHouse is relatively straightforward. The basic idea is to use the `arrayFilter` function to filter out any elements that are present in both arrays. To do this, we will use a simple query that looks like this:

```sql
select arrayFilter(x -> has(firstArray, x) != true, secondArray) diff;
```

This query takes two arrays, `firstArray` and `secondArray`, and filters out any elements that are present in both arrays. The result of this query is a new array, `diff`, which contains only the elements that are unique to the `secondArray`.

The key to this query is the `arrayFilter` function, which takes two arguments: a lambda function, and an array. The lambda function is used to filter out elements that are not unique to the `secondArray`, while the array is the one we want to filter. The `has` function used in the lambda function is a built-in ClickHouse function that returns `true` if the element passed to it is present in the specified array.