SQL duplicate fields and TClientDataSet read-only "gotcha"

Today was the first day of a 2.75-day Delphi 2010 training event I'm doing in Stockholm, Sweden for 17 people. One of the topics we covered today was DBX4 and DataSnap. The first demo (always live!) showed some strange behaviour: a TSQLConnection component pointing to a database, with a TSQLDataSet component containing some SQL SELECT query, then a TDataSetProvider and a TClientDataSet, followed by TDataSource and TDBGrid - all should be well, except that the fields in the grid were read-only. There was no way I could edit them. Huh? This never happened to me before. Saving the contents of the TClientDataSet to a briefcase file and loading them into another TClientDataSet also didn't help. I gave up, and rebooted my machine (from XP to Windows 7), where the same demo - rebuild from scratch - worked as a charm this time.

Back in the hotel, I loaded the original project, and it would still fail. I explicitly listed all fields in the TClientDataSet in the Fields Editor, and they would all have their "ReadOnly" property set to True. I could not see why, until I noticed two DEPT_NO fields (DEPT_NO and DEPT_NO2).

It turns out, the SQL SELECT statement I had written for the original project was:

select DEPT_NO, EMP_NO, DEPT_NO, FIRST_NAME, LAST_NAME from EMPLOYEE

The field DEPT_NO accidentally appeared twice in the select list. This actually caused the ClientDataSet to turn all fields "read-only", so I also couldn't edit them in the DBGrid.

I'm sure it's documented somewhere, but in the meantime this left me puzzled (and I'm sure the people in Stockholm tomorrow will get a good laugh when I tell them all about it).