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With an introduction complete, get ready to dive deep!
One significant difference between DAX and the Excel formula language is that DAX allows you to pass entire tables between expressions, rather than being constrained to a single value. One powerful effect is that DAX allows you to filter tables in its expressions, then work with the filtered set of values.
With DAX, you can create entirely new calculated tables and then treat them like any other table - including creating relationships between them and other tables in your data model.
DAX has a rich set of table functions, including the following:
These functions return a full table rather, rather than a value. Typically you'll use the results of a table function in further analysis as part of a greater expression, rather than using that returned table a final value. It's important to note that When you use a table function, the results inherit the relationships of their columns.
You can mix table functions in your expression, as long as each function uses a table and returns a table. For example, consider the following DAX expression:
FILTER (ALL (Table), Condition)
That expression would put a filter over the entirety of Table, ignoring any current filter content.
The DISTINCT function returns the distinct values of a column that are also visible in the current context. So touse the above DAX expression example, using ALL in that expression ignores filters, while replacing ALL with DISTINCT would observe them.
One common question that Power BI report builders want to answer is the following:
That may be a simple question to answer with a table displayed in front of you, but DAX approaches in a different way in a different way, particularly when there's a relationship between tables.
For example, Power BI and DAX includes values that are not properly cross-indexed. If the incoming relationship is broken, DAX adds a new row to the related table that has blanks in every field, and links that new row to the unindexed row to guarantee referential integrity. If your function includes blank rows, such as is often the case when using ALL, those blank rows will then be included in the number of values returned for that column.
You can also create entire calculated tables using DAX functions. Calculated tables created using DAX require a NAME and a TABLE function. Calculated tables can be used like any other table, including establishing relationships.
Video content courtesy of Alberto Ferrari, SQLBI