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Update index.md
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Denny Crane
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@ -70,13 +70,29 @@ https://dev.mysql.com/doc/refman/8.0/en/window-functions-frames.html
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```text
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aggregate_function (column_name)
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OVER ([PARTITION BY groupping_column] [ORDER BY sorting_column]
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[ROWS or RANGE expression_to_bounds_of_frame])
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OVER ([[PARTITION BY groupping_column] [ORDER BY sorting_column]
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[ROWS or RANGE expression_to_bound_rows_withing_the_group]] | [window_name])
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FROM table_name
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WINDOW window_name as ([[PARTITION BY groupping_column] [ORDER BY sorting_column])
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```
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- `PARTITION BY` - defines how to break a resultset into groups.
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- `ORDER BY` - defines how to order rows inside the group during calculation aggregate_function.
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- `ROWS or RANGE` - defines bounds of a frame, aggregate_function is calculated within a frame.
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- `WINDOW` - allows to reuse a window definition with multiple exressions.
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### Functions
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These functions can be used only as a window function.
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`row_number()` - Number the current row within its partition starting from 1.
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`first_value(x)` - Return a value evaluated against the first row within its ordered frame.
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`last_value(x)` - Return a value evaluated against the last row within its ordered frame.
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`nth_value(x, offset)` - Return a value evaluated against the nth row (offset) in an ordered frame.
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`rank()` - Rank the current row within its partition with gaps.
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`dense_rank()` - Rank the current row within its partition without gaps.
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`lagInFrame(x)` - Return a value evaluated at the row that is at a specified physical offset row before the current row within the ordered frame.
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`leadInFrame(x)` - Return a value evaluated at the row that is offset rows after the current row within the ordered frame.
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```text
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PARTITION
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@ -101,7 +117,8 @@ aggregate_function (column_name)
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CREATE TABLE wf_partition
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(
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`part_key` UInt64,
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`value` UInt64
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`value` UInt64,
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`order` UInt64
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)
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ENGINE = Memory;
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@ -271,6 +288,89 @@ ORDER BY
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│ 1 │ 4 │ 4 │ [3,4,5] │
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│ 1 │ 5 │ 5 │ [4,5] │
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└──────────┴───────┴───────┴──────────────┘
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-- row_number does not respect the frame, so rn_1 = rn_2 = rn_3 != rn_4
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SELECT
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part_key,
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value,
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order,
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groupArray(value) OVER w1 AS frame_values,
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row_number() OVER w1 AS rn_1,
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sum(1) OVER w1 AS rn_2,
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row_number() OVER w2 AS rn_3,
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sum(1) OVER w2 AS rn_4
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FROM wf_frame
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WINDOW
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w1 AS (PARTITION BY part_key ORDER BY order DESC),
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w2 AS (PARTITION BY part_key ORDER BY order DESC
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Rows BETWEEN 1 PRECEDING AND CURRENT ROW)
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ORDER BY
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part_key ASC,
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value ASC;
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┌─part_key─┬─value─┬─order─┬─frame_values─┬─rn_1─┬─rn_2─┬─rn_3─┬─rn_4─┐
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│ 1 │ 1 │ 1 │ [5,4,3,2,1] │ 5 │ 5 │ 5 │ 2 │
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│ 1 │ 2 │ 2 │ [5,4,3,2] │ 4 │ 4 │ 4 │ 2 │
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│ 1 │ 3 │ 3 │ [5,4,3] │ 3 │ 3 │ 3 │ 2 │
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│ 1 │ 4 │ 4 │ [5,4] │ 2 │ 2 │ 2 │ 2 │
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│ 1 │ 5 │ 5 │ [5] │ 1 │ 1 │ 1 │ 1 │
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└──────────┴───────┴───────┴──────────────┴──────┴──────┴──────┴──────┘
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-- first_value and last_value respect the frame
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SELECT
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groupArray(value) OVER w1 AS frame_values_1,
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first_value(value) OVER w1 AS first_value_1,
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last_value(value) OVER w1 AS last_value_1,
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groupArray(value) OVER w2 AS frame_values_2,
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first_value(value) OVER w2 AS first_value_2,
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last_value(value) OVER w2 AS last_value_2
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FROM wf_frame
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WINDOW
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w1 AS (PARTITION BY part_key ORDER BY order ASC),
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w2 AS (PARTITION BY part_key ORDER BY order ASC Rows BETWEEN 1 PRECEDING AND CURRENT ROW)
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ORDER BY
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part_key ASC,
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value ASC;
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┌─frame_values_1─┬─first_value_1─┬─last_value_1─┬─frame_values_2─┬─first_value_2─┬─last_value_2─┐
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│ [1] │ 1 │ 1 │ [1] │ 1 │ 1 │
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│ [1,2] │ 1 │ 2 │ [1,2] │ 1 │ 2 │
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│ [1,2,3] │ 1 │ 3 │ [2,3] │ 2 │ 3 │
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│ [1,2,3,4] │ 1 │ 4 │ [3,4] │ 3 │ 4 │
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│ [1,2,3,4,5] │ 1 │ 5 │ [4,5] │ 4 │ 5 │
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└────────────────┴───────────────┴──────────────┴────────────────┴───────────────┴──────────────┘
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-- second value within the frame
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SELECT
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groupArray(value) OVER w1 AS frame_values_1,
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nth_value(value, 2) OVER w1 AS second_value
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FROM wf_frame
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WINDOW w1 AS (PARTITION BY part_key ORDER BY order ASC Rows BETWEEN 3 PRECEDING AND CURRENT ROW)
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ORDER BY
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part_key ASC,
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value ASC
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┌─frame_values_1─┬─second_value─┐
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│ [1] │ 0 │
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│ [1,2] │ 2 │
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│ [1,2,3] │ 2 │
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│ [1,2,3,4] │ 2 │
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│ [2,3,4,5] │ 3 │
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└────────────────┴──────────────┘
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-- second value within the frame + Null for missing values
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SELECT
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groupArray(value) OVER w1 AS frame_values_1,
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nth_value(toNullable(value), 2) OVER w1 AS second_value
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FROM wf_frame
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WINDOW w1 AS (PARTITION BY part_key ORDER BY order ASC Rows BETWEEN 3 PRECEDING AND CURRENT ROW)
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ORDER BY
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part_key ASC,
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value ASC
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┌─frame_values_1─┬─second_value─┐
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│ [1] │ ᴺᵁᴸᴸ │
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│ [1,2] │ 2 │
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│ [1,2,3] │ 2 │
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│ [1,2,3,4] │ 2 │
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│ [2,3,4,5] │ 3 │
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└────────────────┴──────────────┘
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```
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## Real world examples
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