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    Categories: Window

    LAG

    Returns the row before the current one in a partition based on the ORDER BY clause without the need for a self-join. If there are no rows, this function returns NULL.

    Syntax

    LAG(expression, [offset]) OVER ([PARTITION BY partition_expression] [ORDER BY order_expression]) → same as input type

    • expression: An expression that is returned.
    • offset: An optional parameter. The number of rows before the current row from which to obtain a value. Supports only a value of 1.
    • partition_expression: An optional expression that groups rows into partitions.
    • order_expression: An optional expression that specifies the order of the rows within each partition.

    Examples

    SELECT "Category", 
      "Descript", 
      "DayOfWeek",
      LAG(DayOfWeek, 1) 
        OVER (
          PARTITION BY "Category" 
          ORDER BY "DayOfWeek")
    FROM Samples."samples.dremio.com"."SF_incidents2016.json"
    
    -- Category, Descript, DayOfWeek, EXPR$3
    -- ARSON, ARSON, Friday, null 
    -- ARSON, ARSON, Friday, Friday
    
    

    Usage Notes

    The example uses Dremio’s Samples Data Lake, which includes sample datasets that you can practice with. You must manually add this data lake. To add in Dremio Cloud, see Add Dremio’s Samples Data Lake to Your Project. Guidance to add in Dremio Software is in development.