Add SQL-style operators (#1322)

src/builtin.jq

@@ -287,3 +287,20 @@ def walk(f):
   elif type == "array" then map( walk(f) ) | f
   else f
   end;
+
+# SQL-ish operators here:
+def INDEX(stream; idx_expr):
+  reduce stream as $row ({};
+    .[$row|idx_expr|
+      if type != "string" then tojson
+      else .
+      end] |= $row);
+def INDEX(idx_expr): INDEX(.[]; idx_expr);
+def JOIN($idx; idx_expr):
+  [.[] | [., $idx[idx_expr]]];
+def JOIN($idx; stream; idx_expr):
+  stream | [., $idx[idx_expr]];
+def JOIN($idx; stream; idx_expr; join_expr):
+  stream | [., $idx[idx_expr]] | join_expr;
+def IN(s): reduce (first(select(. == s)) | true) as $v (false; if . or $v then true else false end);
+def IN(src; s): reduce (src|IN(s)) as $v (false; if . or $v then true else false end);