Check for a Column with all null values

You are correct that the `Check` variable in your code is a single value, and you cannot add a single value to a column. Instead, you can use the `Check` variable as an input to the `ADDCOLUMNS` function to create a new column with the desired values. Here's an example of how you can modify your code to achieve this:

```
VAR TABLE1 = CALCULATETABLE( SUMMARIZE( TAB[Col1], TAB[Col2], TAB[Col3] ), KEEPFILTERS(FILTERTABLE) )
VAR NumberofRows = COUNTROWS(TABLE1)
VAR NumberofNullRows = COUNTBLANK(TABLE1[Col3])
VAR Check = IF(NumberofRows = NumberofNullRows, "YES ALL NULL", "NO NOT NULL")
VAR TABLE2 = ADDCOLUMNS(TABLE1, "Col4", Check)
EVALUATE TABLE2
```

In this modified version of your code, the `Check` variable is used as an input to the `ADDCOLUMNS` function, which creates a new column `Col4` in `TABLE2` with the value of `Check` for each row. This should give you the desired result of a new column with the value `"YES ALL NULL"` if all values in `Col3` are null, and `"NO NOT NULL"` otherwise.

I hope this helps! Let me know if you have any further questions. 😊