Fix deleting from joined datasets on MySQL

While here, no longer include joins in delete_from_sql by default.
Since all of the adapters that support modifying joins override it,
there's no reason to include it.

Also, remove the order on the joined dataset in the integration
test so that updating works on MySQL (which allows order clauses
in update statements, but not when multiple tables are used).

Note that MySQL allows deleting from multiple tables at once, but
Sequel doesn't support this.  Sequel assumes the first table in
FROM is the one being deleted from, and the rest are just for
selecting rows.  This is for consistency with how Sequel works on
PostgreSQL and MSSQL.

lib/sequel/adapters/shared/mysql.rb

@@ -352,6 +352,17 @@ def _insert_sql
       def delete_clause_methods
         DELETE_CLAUSE_METHODS
       end
+
+      # Consider the first table in the joined dataset is the table to delete
+      # from, but include the others for the purposes of selecting rows.
+      def delete_from_sql(sql)
+        if joined_dataset?
+          sql << " #{source_list(@opts[:from][0..0])} FROM #{source_list(@opts[:from])}"
+          select_join_sql(sql)
+        else
+          super
+        end
+      end
 
       # MySQL supports the IGNORE and ON DUPLICATE KEY UPDATE clauses for INSERT statements
       def insert_clause_methods

lib/sequel/dataset/sql.rb

@@ -679,12 +679,6 @@ def delete_clause_methods
       DELETE_CLAUSE_METHODS
     end
 
-    # Included both from a join tables if modifying joins is allowed
-    def delete_from_sql(sql)
-      select_from_sql(sql)
-      select_join_sql(sql) if supports_modifying_joins?
-    end
-
     # Converts an array of expressions into a comma separated string of
     # expressions.
     def expression_list(columns)
@@ -984,6 +978,7 @@ def select_compounds_sql(sql)
     def select_from_sql(sql)
       sql << " FROM #{source_list(@opts[:from])}" if @opts[:from]
     end
+    alias delete_from_sql select_from_sql
 
     # Modify the sql to add the expressions to GROUP BY
     def select_group_sql(sql)

spec/core/dataset_spec.rb

@@ -3564,7 +3564,7 @@ def @ds.fetch_rows(sql)
 
   specify "should allow deleting from joined datasets" do
     @ds.delete
-    @ds.db.sqls.should == ['DELETE FROM b, c INNER JOIN d USING (id) WHERE (id = 2)']
+    @ds.db.sqls.should == ['DELETE FROM b, c WHERE (id = 2)']
   end
 
   specify "should allow updating joined datasets" do

spec/integration/dataset_test.rb

@@ -878,7 +878,7 @@ def uprev
       @db.create_table!(:a){Integer :a; Integer :d}
       @db.create_table!(:b){Integer :b; Integer :e}
       @db.create_table!(:c){Integer :c; Integer :f}
-      @ds = @db.from(:a, :b).join(:c, :c=>:e.identifier).where(:d=>:b, :f=>6).order(:a)
+      @ds = @db.from(:a, :b).join(:c, :c=>:e.identifier).where(:d=>:b, :f=>6)
       @db[:a].insert(1, 2)
       @db[:a].insert(3, 4)
       @db[:b].insert(2, 5)