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# Method name: group(propertyName: Array<string>)

# Method package: wixData

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

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

# Method Description:

Groups items together in an aggregation.

The `group()` function refines a `WixDataAggregate` so that its items are
 grouped by the specified property or properties.

 You can perform aggregations on the groups using the following functions:

 + [`avg()`](#avg)
 + [`count()`](#count)
 + [`max()`](#max)
 + [`min()`](#min)
 + [`sum()`](#sum)


 To filter grouped results, use the [`having()`](#having) function.
 > **Notes:** 
 > + Aggregations can only be used on collections you have created. They cannot be used on [Wix App Collections](https://dev.wix.com/docs/develop-websites/articles/databases/wix-data/collections/working-with-wix-app-collections-and-code.md).
 > + You can only call the `group()` function once per aggregate query.



# Method Code Examples:

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

## Group items in an aggregation

```javascript
let newAggregate = aggregate.group("state");
```

## Group items by multiple fields in an aggregation

```javascript
let newAggregate = aggregate.group("state", "year");
```

## Group items in an aggregation and run the aggregation

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

// ...

wixData.aggregate("PopulationData")
  .group("state")
  .max("population")
  .run()
  .then((results) => {
    if (results.items.length > 0) {
      let items = results.items;        // see below
      let numItems = results.length;    // 3
      let hasNext = results.hasNext();  // false
    } else {
      // handle case where no matching items found
    }
  })
  .catch((error) => {
    let errorMsg = error.message;
    let code = error.code;
  });

/* Given the sample data above, items is:
 * [
 *   {"_id": "FL", "populationMax": 401000},
 *   {"_id": "CA", "populationMax": 3796000},
 *   {"_id": "NY", "populationMax": 8192000}
 * ]
 */
```

## Group items in an aggregation and run the aggregation

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

// ...

wixData.aggregate("PopulationData")
  .group("state", "year")
  .max("population")
  .run()
  .then((results) => {
    if (results.items.length > 0) {
      let items = results.items;        // see below
      let numItems = results.length;    // 6
      let hasNext = results.hasNext();  // false
    } else {
      // handle case where no matching items found
    }
  })
  .catch((error) => {
    let errorMsg = error.message;
    let code = error.code;
  });

/* Given the sample data above, items is:
 * [
 *   {
 *     "_id": {"state": "NY", "year": 2000},
 *     "populationMax": 8015000,
 *     "state": "NY",
 *     "year": 2000
 *   },
 *   {
 *     "_id": {"state": "FL", "year": 2000},
 *     "populationMax": 362000,
 *     "state": "FL",
 *     "year": 2000
 *   },
 *   {
 *     "_id": {"state": "NY", "year": 2010},
 *     "populationMax": 8192000,
 *     "state": "NY",
 *     "year": 2010
 *   },
 *   {
 *     "_id": {"state": "CA", "year": 2010},
 *     "populationMax": 3796000,
 *     "state": "CA",
 *     "year": 2010
 *   },
 *   {
 *     "_id": {"state": "CA", "year": 2000},
 *     "populationMax": 3703000,
 *     "state": "CA",
 *     "year": 2000
 *   },
 *   {
 *     "_id": {"state": "FL", "year": 2010},
 *     "populationMax": 401000,
 *     "state": "FL",
 *     "year": 2010
 *   }
 * ]
 */
```

## 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;
  });
```

---