adding db lec3 notes

cst363/lec/lec3.md

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+# lec3
+
+## Relational Algebra and its relation to SQL
+
+### SELECT
+Used to select rows from some table.
+
+> `SELECT [...] FROM [...]`
+
+> Relational symbol: sigma
+
+### Projection
+Picks out the columns of a (set of) table(s)
+
+> `PROJECTION [table...]`
+
+> Relational symbol: pi
+
+### Union
+Adds the rows of two tables into some new table. 
+Removes duplicates from resultant table as well.
+
+> `UNION table[table...]`
+
+> Relational symbol: 
+
+### Relational Algebra on paper
+
+Even though conceptually this maps straight to SQL there is a handwritten way to express this on paper. The following is a _plain_ english way of reading these pape operations.
+
+> /select_(concatenation of fields)[target table]
+
+> /project_(_list of fields_)[target table]
+
+> {resultant table} /union {resultant table}
+
+## Cartesian Product
+
+We take the _crossproduct_ of two relations(tables)
+
+Take the __N'th__ row of the first and combine linearly with the rows from the second table.
+The result should be a new table with R1_(rowcount) * R2_(rowcount).
+This result isn't something that we would particularly care about since there is so much data which is now mixed throughout.
+
+## SQL 
+
+Declarative language which mainly queries databases, and deals with schemas. 
+
+> Data-Definition language (DDL)
+* Create/modify/delete schemas
+* define integrity contraints 
+* define views
+* drop tables
+
+> Data Manipulation Language (DML)
+* Queries and whatnot
+
+### Define a relation Schema
+
+```
+create table tableName (
+	fieldName	type(argv...),
+	...
+	fieldName1	type(length) not null,
+	primary_key(fieldName,[...]),
+	foreign_key(fieldName,[...]) references otherTable
+
+);
+```
+#### Contraints
+
+`not null`: Requires that the field not be null when data is being inserted.
+
+`primary key`: Can be used inline to show that some field is a primary key on its own.
+
+### Field Types
+
+`varchar(length)`: _variable_ length string of characters. If we have `6` as a maxiumum then we may only end reserving `4` bytes(words) of space somewhere.
+
+`char(length)`: _Fixed_ length string of characters. If we have `5` then we must have a string length of `5`.
+
+`numeric(p,d)`: Fixed-point number of `p` digits with `d` digits to the right of the decimal.
+