id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
656089264,MDU6SXNzdWU2NTYwODkyNjQ=,4220,combine_first of Datasets changes dtype of variable present only in one Dataset,601177,open,0,,,2,2020-07-13T19:36:41Z,2023-05-12T08:55:10Z,,NONE,,,,"<!-- Please include a self-contained copy-pastable example that generates the issue if possible.

Please be concise with code posted. See guidelines below on how to provide a good bug report:

- Craft Minimal Bug Reports: http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports
- Minimal Complete Verifiable Examples: https://stackoverflow.com/help/mcve

Bug reports that follow these guidelines are easier to diagnose, and so are often handled much more quickly.
-->

**What happened**: I was combining two `Datasets` using `combine_first` and to my surprise the `dtype` of one of the `DataArrays` in the merged `Dataset` was changed (from `bool` to `float64`).

**What you expected to happen**: No change in `dtype`.

**Minimal Complete Verifiable Example**:

```python
>>> import xarray as xr
>>> ds = xr.Dataset(coords={'abc': list('abc')})
>>> ds['x'] = ('abc', [1., 2., 3.])
>>> ds['y'] = ('abc', [-1., -2., -3.])
>>> ds['t'] = ('abc', [True, False, True])
>>> ds
<xarray.Dataset>
Dimensions:  (abc: 3)
Coordinates:
  * abc      (abc) <U1 'a' 'b' 'c'
Data variables:
    x        (abc) float64 1.0 2.0 3.0
    y        (abc) float64 -1.0 -2.0 -3.0
    t        (abc) bool True False True
>>> xy4b = ds[['x', 'y']].sel(abc=~ds.t) * 10
>>> xy4b.combine_first(ds)
Out[14]: 
<xarray.Dataset>
Dimensions:  (abc: 3)
Coordinates:
  * abc      (abc) object 'a' 'b' 'c'
Data variables:
    x        (abc) float64 1.0 20.0 3.0
    y        (abc) float64 -1.0 -20.0 -3.0
    t        (abc) float64 1.0 0.0 1.0
```

**Anything else we need to know?**: No.

**Environment**:

<details><summary>Output of <tt>xr.show_versions()</tt></summary>

commit: None
python: 3.7.8 (default, Jul  5 2020, 21:51:42) 
[GCC 9.3.0]
python-bits: 64
OS: Linux
OS-release: 5.4.48-gentoo
machine: x86_64
processor: Intel(R) Core(TM) i7-2620M CPU @ 2.70GHz
byteorder: little
LC_ALL: None
LANG: nl_BE.UTF-8
LOCALE: nl_BE.UTF-8
libhdf5: 1.10.5
libnetcdf: 4.6.1

xarray: 0.12.1
pandas: 1.0.4
numpy: 1.18.5
scipy: 1.4.1
netCDF4: 1.5.3
pydap: None
h5netcdf: None
h5py: 2.10.0
Nio: None
zarr: None
cftime: 1.1.3
nc_time_axis: None
PseudonetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.3.2
dask: 1.2.0
distributed: None
matplotlib: 3.2.1
cartopy: None
seaborn: None
setuptools: 46.4.0
pip: 20.0.2
conda: None
pytest: None
IPython: 7.16.1
sphinx: 3.0.4
</details>
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/4220/reactions"", ""total_count"": 2, ""+1"": 2, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue
294380515,MDU6SXNzdWUyOTQzODA1MTU=,1888,Getting DataArrays from netCDF4 files correctly and without hassle,601177,open,0,,,4,2018-02-05T12:45:42Z,2020-12-06T18:07:47Z,,NONE,,,,"#### Context
Consider a netCDF4 file with a group structure. For example, the following toy:
```python
import netCDF4 as nc
# netCDF4 file
f = nc.Dataset('simple_hierarchy.nc', 'w')
# coordinates in root
f.createDimension('x', 3)
f.createVariable('x', 'f4', ('x',), fill_value=False)
f['x'][:] = [1.1, 2.2, 3.3]
f.createDimension('y', 2)
f.createVariable('y', 'f4', ('y',), fill_value=False)
f['y'][:] = [-0.9, -1.8]
# variables in root
f.createVariable('u', 'i1', (), fill_value=False)
f.createVariable('v', 'u1', ('x','y'), fill_value=False)
# group
f.createGroup('g')
g = f['g']
# new/modified coordinates in g
g.createDimension('y', 3)
g.createVariable('y', 'f4', ('y',), fill_value=False)
g['y'][:] = [-0.9, -1.8, -2.7]
# variable in g
g.createVariable('w', 'u1', ('x', 'y'), fill_value=False)
f.close()
```

#### Current behavior

1. It is currently a hassle to get a DataArray from variable in a group with multiple non-coordinate variables:
    ```python
    >>> xr.open_dataarray('simple_hierarchy.nc')
    …
    ValueError: Given file dataset contains more than one data variable. 
    Please read with xarray.open_dataset and then select the variable you want.
    >>> xr.open_dataarray('simple_hierarchy.nc', group='v')
    xr.open_dataarray('simple_hierarchy.nc', group='v')
    …
    OSError: [Errno group not found: v] 'v'
    >>> xr.open_dataarray('simple_hierarchy.nc', drop_variables='u')
    <xarray.DataArray 'v' (x: 3, y: 2)>
    array([[120, 219],
           [178, 172],
           [  9, 127]], dtype=uint8)
    Coordinates:
      * x        (x) float32 1.1 2.2 3.3
      * y        (y) float32 -0.9 -1.8
    ```

2. Also, coordinates defined at a group level closer tot the root are not taken into account:
    ```python
    >>> xr.open_dataarray('simple_hierarchy.nc', group='g')
    <xarray.DataArray 'w' (x: 3, y: 3)>
    array([[216, 219, 178],
           [172,   9, 127],
           [  0,   0,  64]], dtype=uint8)
    Coordinates:
      * y        (y) float32 -0.9 -1.8 -2.7
    Dimensions without coordinates: x
    ```
    So the DataArray is not loaded correctly, as part of its defining coordinates are missing.

#### Suggested behavior

1. Add a `variable` kwarg in the `open_dataarray` method:
    ```python
    >>> xr.open_dataarray('simple_hierarchy.nc', variable='v')
    <xarray.DataArray 'v' (x: 3, y: 2)>
    array([[120, 219],
           [178, 172],
           [  9, 127]], dtype=uint8)
    Coordinates:
      * x        (x) float32 1.1 2.2 3.3
      * y        (y) float32 -0.9 -1.8
    ```

2. Have the function that loads variables go up the group hierarchy to see if some coordinate arrays can be found for dimensions lacking them within this group:
    ```python
    >>> xr.open_dataarray('simple_hierarchy.nc', group='g')
    <xarray.DataArray 'w' (x: 3, y: 3)>
    array([[216, 219, 178],
           [172,   9, 127],
           [  0,   0,  64]], dtype=uint8)
    Coordinates:
      * x        (x) float32 1.1 2.2 3.3
      * y        (y) float32 -0.9 -1.8 -2.7
    ```
    I guess care needs to be taken as well upon writing to netCDF, to make sure no spurious dimension/coordinate definitions are added.

#### Version

xarray 0.9.6","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/1888/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue
498297720,MDU6SXNzdWU0OTgyOTc3MjA=,3343,Interpolation using non-dimension coordinates,601177,open,0,,,4,2019-09-25T13:45:38Z,2019-09-30T12:53:35Z,,NONE,,,,"#### MCVE Code Sample

```python
>>> da = xr.DataArray([1., 2.], coords=[('dim', [.5, 1.])])
>>> da.coords['nondim'] = ('dim', [0., 1.])
>>> da
<xarray.DataArray (dim: 2)>
array([1., 2.])
Coordinates:
  * dim      (dim) float64 0.5 1.0
    nondim   (dim) float64 0.0 1.0
>>> da.interp(dim=.75)
<xarray.DataArray ()>
array(1.5)
Coordinates:
    nondim   float64 0.5
    dim      float64 0.75
>>> da.interp(nondim=.5)
Traceback (most recent call last):

  File ""<ipython-input-192-e3df34cff90f>"", line 1, in <module>
    da.interp(nondim=.5)

  File ""/usr/lib64/python3.6/site-packages/xarray/core/dataarray.py"", line 951, in interp
    **coords_kwargs)

  File ""/usr/lib64/python3.6/site-packages/xarray/core/dataset.py"", line 1860, in interp
    indexers = OrderedDict(self._validate_indexers(coords))

  File ""/usr/lib64/python3.6/site-packages/xarray/core/dataset.py"", line 1316, in _validate_indexers
    raise ValueError(""dimensions %r do not exist"" % invalid)

ValueError: dimensions ['nondim'] do not exist
```

#### Expected Output
The same interpolation as for `'dim'`.

#### Problem Description
Apparently, xarray currently cannot interpolate using non-dimension coordinates.

#### Output of ``xr.show_versions()``
<details>
commit: None

python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.19.72-gentoo
machine: x86_64
processor: Intel(R) Core(TM) i7-2620M CPU @ 2.70GHz
byteorder: little
LC_ALL: None
LANG: nl_BE.UTF-8
LOCALE: nl_BE.UTF-8

xarray: 0.10.8
pandas: 0.24.2
numpy: 1.14.5
scipy: 1.1.0
netCDF4: 1.5.1.2
h5netcdf: None
h5py: 2.9.0
Nio: None
zarr: None
bottleneck: 1.2.1
cyordereddict: None
dask: 1.2.0
distributed: None
matplotlib: 2.2.2
cartopy: None
seaborn: None
setuptools: 40.6.3
pip: 19.1
conda: None
pytest: 3.10.1
IPython: 5.4.1
sphinx: 1.7.5
</details>
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3343/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue