Components of a Semantic Model

Fact tables, dimension tables, joins, and hierarchies are a semantic model's key components.

Component	Description
Fact Tables	Fact tables contain numerical data (measures) that are important to a company, for example employee salaries, sales by product, costs, and revenue. Measures are aggregated by summing, averages, counts, or using calculated expressions. Fact tables must use the same level of detail (grain) to enable comparing the measures. Measures aggregated from facts are defined in a fact table. Measures are typically calculated data such as dollar value or quantity sold, and are specified in terms of hierarchies. For example, you might want to determine the sum of dollars for a given product in a given market over a given time period. Each measure has its own aggregation rule such as SUM, AVG, MIN, or MAX. A business might want to compare values of a measure and need a calculation to express the comparison.
Dimension Tables	A business uses facts to measure performance by well-established dimensions, for example, by time, product, and market. Every dimension has a set of descriptive attributes. Dimension table attributes provide context to numeric data (fact tables) by providing descriptions such as type, category, owner, area, account, or priority. Attributes stored in this dimension might include Service Request Owner, Area, Account, or Priority.
Joins	Joins indicate relationships between fact tables and dimension tables in the model. When you create joins, you specify the fact table, dimension table, fact column, and dimension column you want to join. Joins allow queries to return rows where there is at least one match in both tables. Tip: Analysts can use the option Include Null Values when building reports to return rows from one table where there’re no matching rows in another table.
Hierarchies	Hierarchies are sets of top-down relationships between dimension tables' attributes. In hierarchies, levels roll up from lower levels to higher levels. For example, months can roll up into a year. These rollups occur over the hierarchy elements and span natural business relationships.