docs: add PIVOT/UNPIVOT (#10781)

* docs(pivot): add pivot

* docs: add PIVOT and UNPIVOT

docs/doc/14-sql-commands/20-query-syntax/05-query-pivot.md

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+---
+title: PIVOT
+---
+
+The `PIVOT` operation in Databend allows you to transform a table by rotating it and aggregating results based on specified columns. 
+
+It is a useful operation for summarizing and analyzing large amounts of data in a more readable format. In this document, we will explain the syntax and provide an example of how to use the `PIVOT` operation.
+
+**See also:**
+[UNPIVOT](./05-query-unpivot.md)
+
+
+## Syntax
+
+```sql
+SELECT ...
+FROM ...
+   PIVOT ( <aggregate_function> ( <pivot_column> )
+            FOR <value_column> IN ( <pivot_value_1> [ , <pivot_value_2> ... ] ) )
+
+[ ... ]
+```
+
+Where:
+* `<aggregate_function>`: The aggregate function for combining the grouped values from `pivot_column`.
+* `<pivot_column>`: The column that will be aggregated using the specified `<aggregate_function>`.
+* `<value_column>`: The column whose unique values will become new columns in the pivoted result set.
+* `<pivot_value_N>`: A unique value from the `<value_column>` that will become a new column in the pivoted result set.
+
+
+## Examples
+
+Let's say we have a table called monthly_sales that contains sales data for different employees across different months. We can use the `PIVOT` operation to summarize the data and calculate the total sales for each employee in each month.
+
+### Creating and Inserting Data
+
+
+```sql
+-- Create the monthly_sales table
+CREATE TABLE monthly_sales(
+  empid INT, 
+  amount INT, 
+  month VARCHAR
+);
+
+-- Insert sales data
+INSERT INTO monthly_sales VALUES
+  (1, 10000, 'JAN'),
+  (1, 400, 'JAN'),
+  (2, 4500, 'JAN'),
+  (2, 35000, 'JAN'),
+  (1, 5000, 'FEB'),
+  (1, 3000, 'FEB'),
+  (2, 200, 'FEB'),
+  (2, 90500, 'FEB'),
+  (1, 6000, 'MAR'),
+  (1, 5000, 'MAR'),
+  (2, 2500, 'MAR'),
+  (2, 9500, 'MAR'),
+  (1, 8000, 'APR'),
+  (1, 10000, 'APR'),
+  (2, 800, 'APR'),
+  (2, 4500, 'APR');
+```
+
+### Using PIVOT
+
+Now, we can use the `PIVOT` operation to calculate the total sales for each employee in each month. We will use the `SUM` aggregate function to calculate the total sales, and the MONTH column will be pivoted to create new columns for each month.
+
+```sql
+SELECT * 
+FROM monthly_sales
+PIVOT(SUM(amount) FOR MONTH IN ('JAN', 'FEB', 'MAR', 'APR'))
+ORDER BY EMPID;
+```
+
+Output:
+```sql
++-------+-------+-------+-------+-------+
+| empid | jan   | feb   | mar   | apr   |
++-------+-------+-------+-------+-------+
+|     1 | 10400 |  8000 | 11000 | 18000 |
+|     2 | 39500 | 90700 | 12000 |  5300 |
++-------+-------+-------+-------+-------+
+```

docs/doc/14-sql-commands/20-query-syntax/05-query-unpivot.md

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+---
+title: UNPIVOT
+---
+
+The `UNPIVOT` operation rotates a table by transforming columns into rows. 
+
+It is a relational operator that accepts two columns (from a table or subquery), along with a list of columns, and generates a row for each column specified in the list. In a query, it is specified in the FROM clause after the table name or subquery.
+
+**See also:**
+[PIVOT](./05-query-pivot.md)
+
+
+## Syntax
+
+```sql
+SELECT ...
+FROM ...
+    UNPIVOT ( <value_column>
+    FOR <name_column> IN ( <column_list> ) )
+
+[ ... ]
+```
+
+Where:
+* `<value_column>`: The column that will store the values extracted from the columns listed in `<column_list>`.
+* `<name_column>`: The column that will store the names of the columns from which the values were extracted.
+* `<column_list>`: The list of columns to be unpivoted, separated by commas.
+
+
+## Examples
+
+Let's unpivot the individual month columns to return a single sales value by month for each employee:
+
+### Creating and Inserting Data
+
+
+```sql
+-- Create the unpivoted_monthly_sales table
+CREATE TABLE unpivoted_monthly_sales(
+  empid INT, 
+  jan INT,
+  feb INT,
+  mar INT,
+  apr INT
+);
+
+-- Insert sales data
+INSERT INTO unpivoted_monthly_sales VALUES
+  (1, 10400,  8000, 11000, 18000),
+  (2, 39500, 90700, 12000,  5300);
+```
+
+### Using UNPIVOT
+
+
+```sql
+SELECT *
+FROM unpivoted_monthly_sales
+    UNPIVOT (amount
+    FOR month IN (jan, feb, mar, apr));
+```
+
+Output:
+```sql
++-------+-------+--------+
+| empid | month | amount |
++-------+-------+--------+
+|     1 | jan   |  10400 |
+|     1 | feb   |   8000 |
+|     1 | mar   |  11000 |
+|     1 | apr   |  18000 |
+|     2 | jan   |  39500 |
+|     2 | feb   |  90700 |
+|     2 | mar   |  12000 |
+|     2 | apr   |   5300 |
++-------+-------+--------+
+```