When you save a mapping, the Designer checks it for errors, such as targets that do not receive data from any sources. While you can save an invalid mapping, you cannot run a session using it.

The Designer performs validation when you connect ports. You cannot connect an input port to an input port, or two ports with incompatible datatypes. Check the error message displayed when the Designer prevented you from making the connection.

This is not allowed. These are possible workarounds:

Change the mode in which you are working by clicking Layout > Link Columns. Now, when you drag between columns, the Designer tries to link them instead of copying the selected port.

Make sure that you have the Output window open when you validate the mapping. Click View > Output Window to see the results of the validation.

Be sure to test this setting for the source qualifier before you run the workflow. Return to the source qualifier and open the dialog box in which you entered the custom query. You can connect to a database and click the Validate button to test the SQL. The Designer shows you any errors exist that in the SQL you entered. Review the session logs if you need more information.