# `Selecto.Window`

Window functions provide powerful analytical capabilities for PostgreSQL queries.

Window functions allow you to perform calculations across a set of table rows
that are related to the current row. Unlike aggregate functions, window functions
do not group rows into a single output row — they retain individual row identity.

## Examples

    # Ranking functions
    selecto
    |> Selecto.window_function(:row_number, over: [partition_by: ["category"], order_by: ["sales_date"]])
    |> Selecto.window_function(:rank, over: [partition_by: ["region"], order_by: [{"total_sales", :desc}]])
    
    # Offset functions  
    selecto
    |> Selecto.window_function(:lag, ["sales_amount", 1], over: [partition_by: ["customer_id"], order_by: ["sales_date"]])
    |> Selecto.window_function(:lead, ["sales_amount"], over: [order_by: ["sales_date"]], as: "next_month_sales")
    
    # Aggregate window functions
    selecto  
    |> Selecto.window_function(:sum, ["sales_amount"], over: [partition_by: ["region"], order_by: ["sales_date"]])
    |> Selecto.window_function(:avg, ["sales_amount"], over: [order_by: ["sales_date"], frame: {:rows, :unbounded_preceding, :current_row}])

# `add_window_function`

Add a window function to the Selecto query.

## Parameters

- `selecto` - The Selecto struct
- `function` - Window function type (atom)
- `arguments` - Arguments for the function (optional for ranking functions)
- `options` - Window function options

## Options

- `:over` - Window specification (required)
  - `:partition_by` - Fields to partition by
  - `:order_by` - Fields and directions to order by  
  - `:frame` - Window frame specification
- `:as` - Output column alias

## Examples

    # Row number within each category, ordered by date
    selecto |> Selecto.window_function(:row_number, 
      over: [partition_by: ["category"], order_by: ["created_at"]])
      
    # Running total of sales by customer
    selecto |> Selecto.window_function(:sum, ["amount"], 
      over: [partition_by: ["customer_id"], order_by: ["date"]], 
      as: "running_total")
      
    # Previous month's sales for comparison
    selecto |> Selecto.window_function(:lag, ["amount", 1], 
      over: [partition_by: ["customer_id"], order_by: ["month"]], 
      as: "prev_month")

# `parse_frame_public`

Parse a window frame specification.

Public function for testing frame parsing.

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*Consult [api-reference.md](api-reference.md) for complete listing*