Fix enum null.String support, add mysql support

testdata/mysql_test_schema.sql

@@ -1,3 +1,23 @@
+CREATE TABLE event_one (
+  id     serial PRIMARY KEY NOT NULL,
+  name   VARCHAR(255),
+  day    enum('monday','tuesday','wednesday')
+);
+
+CREATE TABLE event_two (
+  id     serial PRIMARY KEY NOT NULL,
+  name   VARCHAR(255),
+  face    enum('happy','sad','bitter')
+);
+
+CREATE TABLE event_three (
+  id     serial PRIMARY KEY NOT NULL,
+  name   VARCHAR(255),
+  face   enum('happy','sad','bitter'),
+  mood   enum('happy','sad','bitter'),
+  day    enum('monday','tuesday','wednesday')
+);
+
 CREATE TABLE magic (
   id         int PRIMARY KEY NOT NULL AUTO_INCREMENT,
   id_two     int NOT NULL,