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# Method name: filter(filter: WixDataFilter)

# Method package: wixData

# Method menu location: wixData --> WixDataAggregate --> filter

# Method Link: https://dev.wix.com/docs/velo/apis/wix-data/wix-data-aggregate/filter.md

# Method Description:

Filters out items from being used in an aggregation.

The `filter()` function refines a `WixDataAggregate` so that it only
 includes items from the aggregate's collection which match the specified
 filter criteria.

 To create a filter, use the wix-data [`filter()`](wix-data.html#filter) function.

 Filtering using `filter()` takes place before grouping is performed on the
 aggregation. To filter grouped results, use the [`having()`](#having) function.
 > **Note:** Aggregations can only be used on collections you have created. They cannot be
 used on [Wix App Collections](https://support.wix.com/en/article/cms-formerly-content-manager-working-with-wix-app-collections).



# Method Code Examples:

*** Note: do not assume any prop names or enum values other than the ones in the example.

## Filter out items in an aggregation

```javascript
let filter = wixData.filter().eq("year", 2010);

let newAggregate = aggregate.filter(filter);
```

## Filter out items in an aggregation and run the aggregation

```javascript
import wixData from 'wix-data';

// ...

let filter = wixData.filter().eq("year", 2010);

wixData.aggregate("PopulationData")
  .filter(filter)
  .max("population", "maxPopulation")
  .run()
  .then((results) => {
    if (results.items.length > 0) {
      let items = results.items;        // see below
      let numItems = results.length;    // 1
      let hasNext = results.hasNext();  // false
    } else {
      // handle case where no matching items found
    }
  })
  .catch((error) => {
    let errorMsg = error.message;
    let code = error.code;
  });

// items is: [{"_id":"0","maxPopulation":8192000}]
```

## Create an aggregation with filtering and grouping and run it

```javascript
import wixData from 'wix-data';

// ...

const filter = wixData.filter().eq("year", 2010);
const having = wixData.filter().gt("maxPopulation", 1000000);

wixData.aggregate("PopulationData")
  .filter(filter)
  .group("state")
  .max("population", "maxPopulation")
  .having(having)
  .descending("maxPopulation")
  .skip(5)
  .limit(3)
  .run()
  .then((results) => {
    if (results.items.length > 0) {
      let items = results.items;
      let numItems = results.length;
      let hasNext = results.hasNext();
    } else {
      // handle case where no matching items found
    }
  })
  .catch((error) => {
    let errorMsg = error.message;
    let code = error.code;
  });
```

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