Yes, it is possible to create these variables within a MySQL data flow. The syntax however would be slightly different. You would want to use @bob:=Bob and then when referencing it within your select statement use @bob. If you experience any problems implementing this in a data flow, please feel free to email Domo Support at support@domo.com.

@ptran,

In my experience, you can't set user defined variables like a typical MySQL script with a SET statement upfront, but you can initialize them inline in a subquery and then reference in an outer select statement. For example, the statement below will initialize @row = 0  and increment +1 for each row in the select statement.

SELECT mydata.*,@row := @row + 1 as `RowNumber`
FROM mydata, (SELECT @row := 0) r;

Technically, you could declare a variable such as @Bob in your statement and then filter on that value, like this:

SELECT name
FROM sample_table, (select @Bob := 'Bob') r on name = @Bob

But that would defeat the purpose of declaring a variable and referencing it in many statements, like you indicated with your UNION approach, so I would suggest another approach:

Step 1: Create a lookup table to store your values. This could be a webform, or a select statement to generate the columns and values in the first transform step. 

Step 2: Join your transaction tables to the lookup table to filter on the specified values

Hope this helps!