bring up JOIN USING and make README query work (#5477)

This commit gets the using condition for joins working and now allows
the query that is now in the top-level README to work.  We unified
some of the join logic between SQL and SPQ and relaxed some of the
checking of group-by keys with selection, which had been too aggressive.
We also added a test that emulates the README query with smaller data
in local files rather than APIs and we fixed up the README query to add 
the repos field from the RHS of the join  to make it more interesting.

Author: mccanne

compiler/ast/ast.go

@@ -535,8 +535,7 @@ type (
 		Kind       string      `json:"kind" unpack:""`
 		Style      string      `json:"style"`
 		RightInput Seq         `json:"right_input"`
-		LeftKey    Expr        `json:"left_key"`
-		RightKey   Expr        `json:"right_key"`
+		Cond       JoinExpr    `json:"cond"`
 		Args       Assignments `json:"args"`
 		Loc        `json:"loc"`
 	}

compiler/ast/sql.go

@@ -87,24 +87,24 @@ type (
 
 type JoinExpr interface {
 	Node
-	joinOp()
+	joinExpr()
 }
 
-type JoinOn struct {
+type JoinOnExpr struct {
 	Kind string `json:"kind" unpack:""`
 	Expr Expr   `json:"expr"`
 	Loc  `json:"loc"`
 }
 
-func (*JoinOn) joinOp() {}
+func (*JoinOnExpr) joinExpr() {}
 
-type JoinUsing struct {
+type JoinUsingExpr struct {
 	Kind   string `json:"kind" unpack:""`
 	Fields []Expr `json:"fields"`
 	Loc    `json:"loc"`
 }
 
-func (*JoinUsing) joinOp() {}
+func (*JoinUsingExpr) joinExpr() {}
 
 func (*SQLPipe) OpAST()   {}
 func (*Select) OpAST()    {}

compiler/ast/unpack.go

@@ -109,8 +109,8 @@ var unpacker = unpack.New(
 	OrderBy{},
 	Limit{},
 	With{},
-	JoinOn{},
-	JoinUsing{},
+	JoinOnExpr{},
+	JoinUsingExpr{},
 )
 
 // UnmarshalOp transforms a JSON representation of an operator into an Op.