改进代码var req []*Total res := db.Scopes(models.Paginate(page, pageSize)).Raw("SELECT * FROM flaws WHERE facility_id = ? "+ "UNION SELECT * FROM issues WHERE facility_id = ? "+ "UNION SELECT * FROM records WHERE facility_id = ? "+ "UNION SELECT * FROM qualities WHERE facility_id = ? "+ "UNION SELECT * FROM inspections WHERE facility_id = ?", code, code, code, code, code).Find(&req)

这段代码可以优化为使用GORM的联合查询（Union）功能，避免使用Raw SQL语句。

可以尝试这样改进代码：

err := db.Where("facility_id = ?", code).
    Select("id, created_at, updated_at, field1, field2, NULL as field3, NULL as field4, NULL as field5").
    Table("flaws").
    Union(db.Where("facility_id = ?", code).
        Select("id, created_at, updated_at, field1, NULL as field2, field3, field4, NULL as field5").
        Table("issues")).
    Union(db.Where("facility_id = ?", code).
        Select("id, created_at, updated_at, field1, NULL as field2, NULL as field3, NULL as field4, field5").
        Table("records")).
    Union(db.Where("facility_id = ?", code).
        Select("id, created_at, updated_at, field1, field2, NULL as field3, NULL as field4, NULL as field5").
        Table("qualities")).
    Union(db.Where("facility_id = ?", code).
        Select("id, created_at, updated_at, field1, NULL as field2, field3, field4, field5").
        Table("inspections")).
    Order("created_at desc").
    Limit(pageSize).
    Offset((page - 1) * pageSize).
    Find(&req).Error

在这个示例中，我们使用了 `Select` 函数选择每个表中的字段，并将不需要的字段设置为 `NULL`。然后我们使用 `Union` 函数将所有查询合并到一起，并使用 `Order`、`Limit` 和 `Offset` 函数进行分页和排序。

这种方法比使用 `Raw` 函数更安全，因为它可以防止 SQL 注入攻击，并且更易于维护。