xql - Initial Commit (#427)

* Initial Commit

* Lint Fixes

* README file changes

* Code Optimization

* Order by support added,

* Added Road-map in README file

---------

Co-authored-by: Darshan Prajapati <[email protected]>
Co-authored-by: dabhicusp <[email protected]>

xql/README.md

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+# `xql` - Querying Xarray Datasets with SQL
+
+Running SQL like queries on Xarray Datasets. Consider dataset as a table and data variable as a column.
+> Note: For now, we support only zarr datasets.
+
+# Supported Features
+
+* **`Select` Variables** - From a large dataset having hundreds of variables select only needed variables.
+* **Apply `where` clause** - A general where condition like SQL. Applicable for queries which includes data for specific time range or only for specific regions. 
+* **`group by` Functions** - This is supported on the coordinates only. e.g. time, latitude, longitude, etc.
+* **`aggregate` Functions** - Aggregate functions `AVG()`, `MIN()`, `MAX()`, etc. Only supported on data variables.
+* For more checkout the [road-map](https://github.com/google/weather-tools/tree/xql-init/xql#roadmap).
+> Note: For now, we support `where` conditions on coordinates only.
+
+# Quickstart
+
+## Prerequisites
+
+Get an access to the dataset you want to query. Here as an example we're going to use the analysis ready era5 public dataset. [full_37-1h-0p25deg-chunk-1.zarr-v3](https://pantheon.corp.google.com/storage/browser/gcp-public-data-arco-era5/ar/full_37-1h-0p25deg-chunk-1.zarr-v3?project=gcp-public-data-signals).
+
+For this `gcloud` must be configured in your local environment. Refer [Initializing the gcloud CLI](https://cloud.google.com/sdk/docs/initializing) for configuring the `gcloud` locally.
+
+## Usage
+
+```
+# Install required packages
+pip install -r xql/requirements.txt
+
+# Jump into xql
+python xql/main.py
+```
+---
+### Supported meta commands
+`.help`: For usage info.
+
+`.exit`: To exit from the xql interpreter.
+
+`.set`: To set the dataset uri as a shortened key.
+```
+.set era5 gs://gcp-public-data-arco-era5/ar/full_37-1h-0p25deg-chunk-1.zarr-v3
+```
+
+`.show`: To list down dataset shortened key. Eg. `.show` or `.show [key]`
+
+```
+.show era5
+```
+
+`[query]`  =>  Any valid sql like query.
+
+---
+### Example Queries
+
+1. Apply a conditions. Query to get temperature of arctic region in January 2022:
+    ```
+    SELECT 
+        temperature 
+    FROM 'gs://gcp-public-data-arco-era5/ar/full_37-1h-0p25deg-chunk-1.zarr-v3' 
+    WHERE
+        time >= '2022-01-01' AND 
+        time < '2022-02-01' AND 
+        latitude >= 66.5
+    ```
+    > Note: Multiline queries are not yet supported. Convert copied queries into single line before execution.
+
+2. Aggregating results using Group By and Aggregate function. Daily average of temperature of arctic region in January 2022.
+    Setting the table name as shortened key.
+
+    ```
+    .set era5 gs://gcp-public-data-arco-era5/ar/full_37-1h-0p25deg-chunk-1.zarr-v3
+    ```
+    ```
+    SELECT 
+        AVG(temperature) 
+    FROM era5
+    WHERE 
+        time >= '2022-01-01' AND 
+        time < '2022-02-01' AND 
+        latitude >= 66.5
+    GROUP BY time_day
+    ```
+    Replace `time_day` to `time_month` or `time_year` if monthly or yearly average is needed. Also use `MIN()` and `MAX()` functions same way as `AVG()`.
+
+3. `caveat`: Above queries run on the client's local machine and it generates a large two dimensional array so querying for very large amount of data will fall into out of memory erros.
+
+    e.g. Query like below will give OOM errors if the client machine don't have the enough RAM.
+
+    ```
+    SELECT 
+        evaporation,
+        geopotential_at_surface,
+        temperature 
+    FROM era5
+    ```
+
+# Roadmap
+
+_Updated on 2024-01-08_
+
+1. [x] **Select Variables**
+    1. [ ] On Coordinates
+    2. [x] On Variables 
+2. [x] **Where Clause**: `=`, `>`, `>=`, `<`, `<=`, etc.
+    1. [x] On Coordinates
+    2. [ ] On Variables 
+3. [x] **Aggregate Functions**: Only `AVG()`, `MIN()`, `MAX()`, `SUM()` are supported.
+   1. [x] With Group By
+   2. [ ] Without Group By
+   3. [ ] Multiple Aggregate function in a single query
+4. [ ] **Order By**: Only suppoted for coordinates.
+5. [ ] **Limit**: Limiting the result to display.
+6. [ ] **Mathematical Operators** `(+, - , *, / )`: Add support to use mathematical operators in the query.
+7. [ ] **Aliases**: Add support to alias while querying.
+8. [ ] **Join Operations**: Support joining tables and apply query.
+9. [ ] **Nested Queries**: Add support to write nested queries.
+10. [ ] **Custom Aggregate Functions**: Support custom aggregate functions