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.

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