yq/pkg/yqlib/doc/operators/group-by.md

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# Group By
This is used to group items in an array by an expression.
## Group by field
Given a sample.yml file of:
```yaml
[{foo: 1, bar: 10}, {foo: 3, bar: 100}, {foo: 1, bar: 1}]
```
then
```bash
yq 'group_by(.foo)' sample.yml
```
will output
```yaml
- - [{foo: 1, bar: 10}, {foo: 3, bar: 100}, {foo: 1, bar: 1}]
- [{foo: 1, bar: 10}, {foo: 3, bar: 100}, {foo: 1, bar: 1}]
- - [{foo: 1, bar: 10}, {foo: 3, bar: 100}, {foo: 1, bar: 1}]
```
## Group by field, with nuls
Given a sample.yml file of:
```yaml
[{cat: dog}, {foo: 1, bar: 10}, {foo: 3, bar: 100}, {no: foo for you}, {foo: 1, bar: 1}]
```
then
```bash
yq 'group_by(.foo)' sample.yml
```
will output
```yaml
- - [{cat: dog}, {foo: 1, bar: 10}, {foo: 3, bar: 100}, {no: foo for you}, {foo: 1, bar: 1}]
- [{cat: dog}, {foo: 1, bar: 10}, {foo: 3, bar: 100}, {no: foo for you}, {foo: 1, bar: 1}]
- - [{cat: dog}, {foo: 1, bar: 10}, {foo: 3, bar: 100}, {no: foo for you}, {foo: 1, bar: 1}]
- [{cat: dog}, {foo: 1, bar: 10}, {foo: 3, bar: 100}, {no: foo for you}, {foo: 1, bar: 1}]
- - [{cat: dog}, {foo: 1, bar: 10}, {foo: 3, bar: 100}, {no: foo for you}, {foo: 1, bar: 1}]
```