# lec2

Covering `tables, tuples, data stuff`

## Problem Statement \
We need to be able to manipulate data easily 

> IMS had been using trees for a while a long time ago

> Rows --> __atributes__ 

> Columns --> __tuple__ 

somtimes we refer to the title of the columns to be fields 

> Table --> __relation__ 

relational instance as well for another term

> Domain 

The set of values allowed in a field

## NULL 

cant operate on it at all 

> Count

The only thing that lets you operate on NULL. Even then you only get 0 back.

## Keys Types

> Super Key 

> Candidate Key 

> Primary Key 

## Problem Statement 

The rows are not distinguishable from each other; we still have a mes of data sitting there unlabeled. Some kind of identifier is necessary to be able to access every tuple in the relational set.

### SuperKey 

Set of attr is a superkey for a table as long as that combination of fields remains unique for every tuple in the relational set. 
In other words if we have multiple fields; f1 f3 f5 might be a good  combo to use as a key into the table.

> What's a valid superkey? 

For starters anything that contains another valid superkey 
Any subset of a full tuple that can uniquely identify any row in the table.

> Can a whole row be a superkey? 
...full on brainlet.........yes

### Candidate Key 

Any super key that wouldn't be a superkey if one of the attr were removed. Say then that we have a super key that takes columns {1,3,5,6,7}, but removing anyone of the rows no longer reliably returns an arbitrary _unique_ row.

### Primary key 

Any candidate key the database designer has chosen to serve as the unique

### Foreign Key 

Set of attrs in one table that are the primary key attrs of another table. More info about this key type will come later but just know for now that it exists in the wild.

