SQLite Forum

Your syntax is incorrect.

UPDATE activity_overview 
   SET C3 = 1 
  FROM query_projects 
 WHERE activity_overview."Project ID" = query_projects."Project ID"
   AND query_projects."Planned Start" > activity_overview."Early Start"


 - double-quote are used to quote ill-conceived identifiers, not square brackets.  Although SQLite3 will recognize square brackets, that is NOT the standard.  You do not need to use identifier-quotes except where the identifier is ill-conceived although you are free to do so (just as you are free to write SQL while taking a bath or while standing on your head) even though doing so (excessive useless identifier-quoting and/or using ill-conceived identifiers) makes the statement more difficult for humans to parse.  Some things (like anything from Microsoft) loves to sprinkle turds of extraneous parenthesis and quoting where it is not required just because spreading turds is **The Microsoft Way**.

 - for each activity_overview there must only be one applicable query projects record (the WHERE clause must specify a 1:1 relationship).  If this is not the case then this update is ill-conceived and may not work as you expect.  If in fact there may be more than one query_projects record per activity overview then you need to fix the query so that it is not nonsense, perhaps by converting the FROM and WHERE constraints into an EXISTS correlate to avoid that issue.  (In this particular case you might not have this problem although the query itself may be ill-conceived and ill-designed).