Retrieving Tables and Columns Using EVALUATE and SELECT COLUMNS in DAX

What Is EVALUATE in DAX?

EVALUATE is used in a DAX query to return a table as the result.
A DAX query must return a table, even if that table contains only one column
or one row.

The basic syntax is:

EVALUATE
<Table Expression>

Here, <Table Expression> can be an existing table,
a function that returns a table, or a more complex table expression.

1. Retrieving an Entire Table

Query

EVALUATE
Sales

Step-by-step explanation

Step 1: EVALUATE
EVALUATE tells DAX that we want to return
the result of a table expression.

Step 2: Sales
Sales is the table expression.
Since it is an existing table, DAX returns the table as it is.

What will you get?

The result contains all columns and rows from the
Sales table.

The example above is only for illustration.
Your actual result will contain the data available in your Sales table.

2. Retrieving a Single Column

Query

EVALUATE
SELECTCOLUMNS(
Sales,
"Customer", Sales[CustomerKey]
)

Step-by-step explanation

Step 1: EVALUATE
We want to return the result of a table expression.

Step 2: SELECTCOLUMNS
SELECTCOLUMNS creates a new table containing
only the columns we specify.

Step 3: Sales
This is the source table from which we want to retrieve data.

Step 4: “Customer”
This is the name we want to give to the output column.

Step 5: Sales[CustomerKey]
This is the column whose values will be returned.

What will you get?

The result contains only one column:

The output column is named Customer,
even though the original column is named CustomerKey.

Understanding the SELECTCOLUMNS syntax

The first argument of SELECTCOLUMNS is the source table.
The remaining arguments are written as:

"New Column Name", <Column Expression>

3. Retrieving Multiple Columns

Query

EVALUATE
SELECTCOLUMNS(
Sales,
"Customer", Sales[CustomerKey],
"Qty", Sales[Order Quantity],
"Ship Date", Sales[ShipDateKey]
)

Step-by-step explanation

Step 1: EVALUATE
Returns the result of the table expression.

Step 2: SELECTCOLUMNS
Creates a new table containing only the columns we specify.

Step 3: Sales
Specifies the source table.

Step 4: “Customer”, Sales[CustomerKey]
Creates an output column named Customer
using values from Sales[CustomerKey].

Step 5: “Qty”, Sales[Order Quantity]
Creates an output column named Qty
using values from Sales[Order Quantity].

Step 6: “Ship Date”, Sales[ShipDateKey]
Creates an output column named Ship Date
using values from Sales[ShipDateKey].

What will you get?

The output column names are different from the original column names.

Understanding the Difference

Important Notes

1. EVALUATE returns a table

Even when you retrieve only one column, the result is still a table.

2. SELECTCOLUMNS does not modify the original table

It creates a new table expression for the query result.
The original Sales table remains unchanged.

3. Output column names are customizable

You can use names such as:

"Customer"
"Qty"
"Ship Date"

These names are only for the output of the query.

4. The order of columns follows the query

The columns appear in the same order in which you specify them inside
SELECTCOLUMNS.

EVALUATE
SELECTCOLUMNS(
Sales,
"Customer", Sales[CustomerKey],
"Quantity", Sales[Order Quantity],
"Shipping Date", Sales[ShipDateKey]
)

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