CREATE TABLE table_23730973_5 (rating VARCHAR, viewers__millions_ VARCHAR)

What was the rating/share for 18-49 for the episode that had 5.90 million viewers?
SELECT rating / SHARE(18 - 49) FROM table_23730973_5 WHERE viewers__millions_ = "5.90"
SELECT "rating" / NULLIF(SHARE(-31), 0) FROM "table_23730973_5" WHERE "5.90" = "viewers__millions_"