data-modeling

In SQL and Relational Theory (C.J. Date, 2009) chapter 4 advocates avoiding duplicate rows, and also to avoid NULL attributes in the data we store. While I have no troubles avoiding duplicate rows, I am struggling to see how I can model data without making use of NULL . Take the following, for example - which is a bit from work. We have an artist table, which has, amongst other columns, a gender column. This is a foreign key to the gender table. However, for some artists, we don't know their gender - for example we've been given a list of new music which has no descriptions of the artist. How,

Say I offer user to check off languages she speaks and store it in a db. Important side note, I will not search db for any of those values, as I will have some separate search engine for search. Now, the obvious way of storing these values is to create a table like UserLanguages ( UserID nvarchar(50), LookupLanguageID int ) but the site will be high load and we are trying to eliminate any overhead where possible, so in order to avoid joins with main member table when showing results on UI, I was thinking of storing languages for a user in the main table, having them comma separated, like "12

Someone told me that it was bad design to have loops in the datamodel. I have heard this before a couple of times but didn't pay much attention. For example you have entities User, Project, Activity. A project is owned by a User, so we have a one-to-many relationship from user to Project. An activity can be assigned to a single User, another one-to-many relationship from User to Activity. Of course a project is defined by a set of activities, another one-to-many relationship from Project to Activity. Thus a loop is formed. I asked this guy why is it bad design but he told me he didn't know

Let's say we have a root collection named 'todos'. Every document in this collection has: title : String subcollection named todo_items Every document in the subcollection todo_items has title : String completed : Boolean I know that querying in Cloud Firestore is shallow by default, which is great, but is there a way to query the todos and get results that include the subcollection todo_items automatically? In other words, how do I make the following query include the todo_items subcollection? db.collection('todos').onSnapshot((snapshot) => { snapshot.docChanges.forEach((change) => { // ... }

I have 2 tables in my database orders and orderHistory . ----------------- ----------------------- | orders | | orderHistory | ----------------- ----------------------- | orderID (PK) | | historyLineID (PK) | | orderDate | | status | | price | | quantity | ----------------- ----------------------- Now an order can have multiple history lines . However, a history line can't exist on its own. I heard this is called a weak entity and therefore the PK from orders must be part of the PK of table orderHistory . Questions Is this really a correct weak entity relationship? Is there other ways to