Move exericse to better place

Fixes hadley#792

relational-data.Rmd

@@ -95,11 +95,6 @@ For nycflights13:
 3.  `weather` only contains information for the origin (NYC) airports.
     If it contained weather records for all airports in the USA, what additional relation would it define with `flights`?
 
-4.  We know that some days of the year are "special", and fewer people than usual fly on them.
-    How might you represent that data as a data frame?
-    What would be the primary keys of that table?
-    How would it connect to the existing tables?
-
 ## Keys
 
 The variables used to connect each pair of tables are called **keys**.
@@ -165,7 +160,12 @@ For example, in this data there's a many-to-many relationship between airlines a
 
 1.  Add a surrogate key to `flights`.
 
-2.  Identify the keys in the following datasets
+2.  We know that some days of the year are "special", and fewer people than usual fly on them.
+    How might you represent that data as a data frame?
+    What would be the primary keys of that table?
+    How would it connect to the existing tables?
+
+3.  Identify the keys in the following datasets
 
     a.  `Lahman::Batting`,
     b.  `babynames::babynames`
@@ -175,7 +175,7 @@ For example, in this data there's a many-to-many relationship between airlines a
 
     (You might need to install some packages and read some documentation.)
 
-3.  Draw a diagram illustrating the connections between the `Batting`, `People`, and `Salaries` tables in the Lahman package.
+4.  Draw a diagram illustrating the connections between the `Batting`, `People`, and `Salaries` tables in the Lahman package.
     Draw another diagram that shows the relationship between `People`, `Managers`, `AwardsManagers`.
 
     How would you characterise the relationship between the `Batting`, `Pitching`, and `Fielding` tables?