Gorm Support UNION and UNION ALL

Describe the feature

Motivation

Related Issues

Comment From: itmisx

common API

db.Table("(? UNION ALL ?) as user",db.Table("user1"),db.Table("user2")).Find(&users)

DAO API

unionDB := db.Table("(? UNION ALL ?) as user",db.Table("user1"),db.Table("user2"))
u:=db.Use(unionDB).User
tx:=u.WithContext(context.Background).Find()