This article explains how level of detail expressions are computed and how they function in Tableau. For more information about LOD expressions and how they work, see the Understanding Level of Detail (LOD) Expressions(Link opens in a new window) whitepaper on the Tableau website. Show
Row Level Expressions and View Level ExpressionsIn Tableau, expressions referencing unaggregated datasource columns are computed for each row in the underlying table. In this case, the dimensionality of the expression is row level. An example of a row-level expression is: This calculation will be evaluated in each row of the database. For each row, the Sales value in that row will be divided by the Profit value in that row, producing a new column with the result of the multiplication (a profit ratio). If you create a calculation with this definition, save it with the name [ProfitRatio], and then drag it from the Data pane to a shelf, Tableau typically aggregates the calculated field for the view: By contrast, expressions referencing aggregated data source columns are computed at the dimensionality defined by the dimensions in the view. In this case, the dimensionality of the expression is view level. An example of a view-level expression is: If you drag this calculation to a shelf (or type it directly on a shelf as an ad-hoc calculation), Tableau encloses it in an AGG function: AGG(SUM(Sales) / SUM(Profit)) This is what is known as an aggregate calculation. For details, see Aggregate Functions in Tableau(Link opens in a new window). Dimension and set fields placed on any of the locations highlighted in the following image contribute to the view level of detail:
Before level of detail expressions were supported in Tableau, it was not possible to create calculations at a level of detail other than the view level. For example, if you attempt to save the following expression, Tableau displays the error message: “Cannot mix aggregate and non-aggregate arguments with this function”: The user’s intent in this case was to compare store sales for each individual store to the average of sales for all stores. This can now be accomplished with a level of detail expression: This is what is known as a table-scoped level of detail expression. See Table-Scoped Limitations for Level of Detail ExpressionsThe following limitations and constraints apply for level of detail expressions. Also see Data Source Constraints for Level of Detail Expressions.
In addition, some data sources have complexity limits. Tableau will not disable calculations for these databases, but query errors are a possibility if calculations become too complex. Level of Detail Expressions Can Be Dimensions or MeasuresWhen you save a level of detail expression, Tableau adds it to either the Dimensions or the Measures area in the Data pane. FIXED level of detail expressions can result in measures or dimensions, depending on the underlying field in the aggregate expression. So MIN([Date])} will be a dimension because [Date] is a dimension, and {fixed Store : SUM([Sales])} will be a measure because [Sales] is a measure. When a FIXED level of detail expression is saved as a measure you have the option of moving it to dimensions. INCLUDE and EXCLUDE level of detail expressions are always measures. Filters and Level of Detail ExpressionsThere are several different kinds of filters in Tableau and they get executed in the following order from top to bottom.
The text on the right shows where level of detail expressions are evaluated in this sequence. Extract Filters (in orange) are only relevant if you’re creating a Tableau Extract from a data source. Table calculations filters (dark blue) are applied after calculations are executed and therefore hide marks without filtering out the underlying data used in the calculations. If you’re familiar with SQL, you can think of measure filters as equivalent to the HAVING clause in a query, and dimension filters as equivalent to the WHERE clause. FIXED calculations are applied before dimension filters, so unless you promote the fields on your Filter shelf to Use Context Filters, they will be ignored. For example, consider if you have the following calculation on one shelf in a view, along with [State] on a different shelf: This calculation will give you the ratio of a state’s sales to total sales. If you then put [State] on the Filters shelf to hide some of the states, the filter will affect only the numerator in the calculation. Since the denominator is a FIXED level of detail expression, it will still divide the sales for the states still in the view against the total sales for all states—including the ones that have been filtered out of the view. INCLUDE and EXCLUDE level of detail expressions are considered after Dimension filters. So if you want filters to apply to your FIXED level of detail expression but don’t want to use Context Filters, consider rewriting them as INCLUDE or EXCLUDE expressions. Aggregation and Level of Detail ExpressionsThe level of detail of the view determines the number of marks in your view. When you add a level of detail expression to the view, Tableau must reconcile two levels of detail—the one in the view, and the one in your expression. The behavior of a level of detail expression in the view varies depending on whether the expression's level of detail is coarser, finer, or the same as the level of detail in the view. What do we mean by “coarser” or “finer” in this case? Level of Detail Expression is Coarser Than View Level of DetailAn expression has a coarser level of detail than the view when it references a subset of the dimensions in the view. For example, for a view that contained the dimensions [Category] and [Segment], you could create a level of detail expression that uses only one of these dimensions: In this case, the expression has a coarser level of detail than the view. It bases its values on one dimension ([Segment]), whereas the view is basing its view on two dimensions ([Segment] and [Category]). The result is that using the level of detail expression in the view causes certain values to be replicated—that is, to appear multiple times.
Replicated values are useful for comparing specific values against average values within a category. For example the following calculation subtracts average sales for a customer from the average sales overall: When values are being replicated, changing the aggregation for the relevant field in the view (for example, from AVG to SUM) will not change the result of the aggregation. Level of Detail Expression is Finer Than View Level of DetailAn expression has a finer level of detail than the view when it references a superset of the dimensions in the view. When you use such an expression in the view, Tableau will aggregate results up to the view level. For example, the following level of detail expression references two dimensions: When this expression is used in a view that has only [Segment] as its level of detail, the values must be aggregated. Here’s what you would see if you dragged that expression to a shelf: An aggregation—in this case, average—is automatically assigned by Tableau. You can change the aggregation as needed. Adding a Level of Detail Expression to the ViewWhether a level of detail expression is aggregated or replicated in the view is determined by the expression type (FIXED, INCLUDE, or EXCLUDE) and whether the expression’s granularity is coarser or finer than the view’s.
Level of detail expressions are always automatically wrapped in an aggregate when they are added to a shelf in the view unless they’re used as dimensions. So if you double-click on a shelf and type and then press Enter to commit the expression, what you now see on the shelf is But if you double-click into the shelf to edit the expression, what you see in edit mode is the original expression. If you wrap a level of detail expression in an aggregation when you create it, Tableau will use the aggregation you specified rather than assigning one when any calculation including that expression is placed on a shelf. When no aggregation is needed (because the expression’s level of detail is coarser than the view’s), the aggregation you specified is still shown when the expression is on a shelf, but it is ignored. Data Source Constraints for Level of Detail ExpressionsFor some data sources, only more recent versions support level of detail expressions. Some data sources do not support level of detail expressions at all. In addition, some data sources have complexity limits. Tableau will not disable calculations for these databases, but query errors are a possibility if calculations become too complex.
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