First create a string list variable.
Next use a ForEach activity to iterate through each query row.
Lastly use a AddToList activity within the foreach. THis will add the column value for each row, to the variable string list.
Depending on your full requirements it could be simpler to use a Decision.
First create your query.
Then create a decision using an expression like this:
If the client name exists then it follows the true path, if not it follows the false path.