Merge pull request #347 from ashwinkumar12345/sql_window_functions

SQL window functions

docs/sql/delete.md

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docs/sql/endpoints.md

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docs/sql/limitation.md

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 # Limitations

docs/sql/monitoring.md

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 # Monitoring

docs/sql/protocol.md

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 # Protocol

docs/sql/settings.md

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 # Settings

docs/sql/troubleshoot.md

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 # Troubleshooting

docs/sql/window.md

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+---
+layout: default
+title: Window Functions
+parent: SQL
+nav_order: 11
+---
+
+# Window Functions
+
+A window function performs a calculation across a frame of data rows around the current row and finds a result for each row.
+
+`PARTITION BY` and `ORDER BY` define the frame of data over which the calculation is made.
+
+You can use window functions in the following three categories:
+
+1. Aggregate Functions: `COUNT()`, `MIN()`, `MAX()`, `AVG()`, and `SUM()`.
+2. Ranking Functions: `ROW_NUMBER()`, `RANK()`, `DENSE_RANK()`, `PERCENT_RANK()`, and `NTILE()`.
+3. Analytic Functions: `CUME_DIST()`, `LAG()`, and `LEAD()`.
+
+The syntax of a window function is as follows:
+
+```sql
+function_name (expression [, expression...])
+  OVER (
+    PARTITION BY expression [, expression...]
+    ORDER BY expression [ASC | DESC] [, ...]
+    )
+```
+
+The `PARTITION BY` and `ORDER BY` clauses are optional.
+{: .note }
+
+To use window functions, enable the new SQL engine:
+
+```json
+PUT _cluster/settings
+{
+ "persistent": {
+    "opendistro.sql.engine.new.enabled" : "true"
+  }
+}
+```
+
+## Ranking functions
+
+Ranking functions assign an incremental ranking value to each row in the frame.
+
+The increase in the ranking value depends on how the ranking function is implemented. The ranking value is mostly determined by the field values in the `ORDER BY` clause. If the `PARTITION BY` clause is also present, the ranking function resets its state, while maintaining the incremental ranking value.
+
+If you use the ranking function without the `ORDER BY` clause, the result is undetermined. Without the `ORDER BY` clause, `ROW_NUMBER` assigns a random number to each data row while `RANK` and `DENSE_RANK` assign a ranking value of 1 to each data row.
+
+### RANK
+
+The `RANK` function assigns a ranking value to each row of a result set.
+It assigns the same ranking value for the same field values specified in the `ORDER BY` list.
+
+```sql
+SELECT gender, RANK()
+OVER (
+  ORDER BY gender DESC
+  )
+AS rnk FROM accounts;
+```
+
+| gender | rank
+:--- | :---
+| M        | 1     
+| M        | 1     
+| M        | 1     
+| F        | 4  
+
+In this case, the next few ranks are skipped depending on the number of ties that occur.   
+
+### ROW_NUMBER
+
+`ROW_NUMBER` assigns a number to each data row of the result set sequentially. The row number increases by 1 regardless of the fields specified in the `ORDER BY` list.
+
+```sql
+SELECT gender, balance, ROW_NUMBER()
+OVER (
+  PARTITION BY gender ORDER BY balance
+   )
+AS num FROM accounts;
+```
+
+| gender | balance | num
+:--- | :---
+| F        | 32838     | 1     
+| M        | 4180      | 1     
+| M        | 5686      | 2     
+| M        | 39225     | 3     
+
+
+### DENSE_RANK
+
+Similar to the `RANK` function, `DENSE_RANK` also assigns a ranking value to each row but without any gaps between the ranking values.
+
+```sql
+SELECT gender, DENSE_RANK()
+OVER (
+  ORDER BY gender DESC
+  )
+AS rnk FROM accounts;
+```
+
+| gender | rank
+:--- | :---
+| M        | 1     
+| M        | 1     
+| M        | 1     
+| F        | 2