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Added latest databse lectures
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‎_data/CS258.yml

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order:
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- Database_Basics.html
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- Relational_Data_Model.html
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- Relational_Database_Constraints.html
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- Relational_Database_Constraints.html
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- Structured_Query_Language.html
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- SQL_Creating_A_Schema.html
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- SQL_Retrieving_Data.html

‎modules/CS255/Joes_Notes/Informed_Search.md

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So $$cost(P) \leq cost(P'')$$ for any path $$P''$$ to a goal
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### Solution
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>Theorem: A* will always find a solution fi there is one
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>Theorem: A* will always find a solution if there is one
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The frontier always contains the initial part of a path to a goal
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‎modules/CS258/Joes_Notes/SQL_Creating_A_Schema.md

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> CS258 and this guide is based on PostgreSQL other versions may be different
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> This page shows limited select commands
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> This page provides links in the titles to the PostgreSQL documentation It is recommenced you visit for a deeper understanding of the commands
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## Create
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### [CREATE SCHEMA](https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=1722)
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<pre>CREATE SCHEMA <i>schema_name</i> [AUTHORIZATION <i>user_name</i>]</pre>
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enters a new schema into the database, a schema contains tables if no schema is specified the public schema is used.
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tables within a schema can be referenced with the dot operator
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CREATE TABLE _schema.table_ ....
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if no schema is specified then `public` is used
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<pre>
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CREATE TABLE <i>table_name</i> (...) == CREATE TABLE <i>public.table_name</i> (...)
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</pre>
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**catalogue** named collection of schemas
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### [CREATE TABLE](https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=1738)
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<pre>
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CREATE TABLE <i>table_name</i> (
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<i>column_name</i> <i>data_type</i> <i>constraints</i>
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<i>column_name</i> <i>data_type</i> <i>constraints</i>
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<i>column_name</i> <i>data_type</i> <i>constraints</i>
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...
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)
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</pre>
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#### [CREATE VIEW](https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=1792)
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Can be used to create a virtual table that is the result of a query , the view is virtual and is run on every request
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## [Constraints](https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=98)
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### [NOT NULL](https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=101)
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Specifies that data in a particular column must not be null
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```
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CREATE TABLE products(
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name text NOT NULL
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)
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```
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### [CHECK](https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=99)
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A check consists of a boolean expression that must evaluate to true for the data to be valid
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```
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CREATE TABLE products(
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price numeric CHECK (price > 0)
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)
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```
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the constraint can also be given a name this can help with error messages as the mae will be provided
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```
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CREATE TABLE products(
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price numeric CONSTRAINT positive_price CHECK (price > 0)
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)
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```
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A check can also refer to multiple columns by adding it to the end of the table
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```
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CREATE TABLE products(
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price numeric CONSTRAINT positive_price CHECK (price > 0)
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discount_price numeric CONSTRAINT positive_price CHECK (discountPrice > 0)
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check( discount_price < price)
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)
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```
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Check can oly be assumed to evaluate on update or delete so should only refer to items in the current row not other elements or table
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### [PRIMARY KEY](https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=103)
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Assignees a column as a key. multiple can be used with `PRIMARY KEY (a,b)`
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### [UNIQUE](https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=101)
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Ensures that data is unique among all the rows in the table. multiple can be used with `UNIQUE (a,b)`
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### Foreign Key [REFERENCES](https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=103)
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Ensures that referential integrity is maintained between tables
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```
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CREATE TABLE orders(
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product_id integer REFERENCES products
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)
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```
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can use multiple foreign keys with `FOREIGN KEY (b, c) REFERENCES other_table (c1, c2)`
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a foreign key must reference a UNIQUE column or a primary KEY if no column is specified the primary key will be chosen
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#### Referential Triggered Actions
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When a value that a foreign key is referencing is updated or deleted
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```
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CREATE TABLE orders(
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product_id integer REFERENCES products ON DELETE CASCADE
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)
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```
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Action | ON DELETE | ON UPDATE
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- |- |-
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RESTRICT | prevents the deletion | prevents the update
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CASCADE | any row(s) referencing deleted rows are also deleted | any row(s) referencing updated rows are changed to the new reference
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SET NULL | sets the reference to null | sets the reference to null
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SET DEFAULT | Sets the reference to the default value | Sets the reference to the default value
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## DROP
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**!! WARNING these commands can destroy large amounts of data**
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## [DROP SCHEMA](https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=1843)
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Removes a schema from the database
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```
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DROP SCHEMA mystuff
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```
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## [DROP TABLE](https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=1849)
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Removes a table from the database
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```
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DROP TABLE mytable
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```
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## Other
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### Default
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used to define a default value if none is provided
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```
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CREATE TABLE meal(
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drink text DEFAULT "water"
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)
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```

‎modules/CS258/Joes_Notes/SQL_Retrieving_Data.md

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> CS258 and this guide is based on PostgreSQL other versions may be different
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> This page shows limited select commands
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> This page provides links in the titles to the PostgreSQL documentation It is recommenced you visit for a deeper understanding of the commands
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## Data duplication
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SQL Databases allow tables to contain duplicates of tuples. known as **Multi-set** or **bag** semantics
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This has some advantages for checking for duplicates of a particular column e.g counting the number fo people who live at a particular address. It is also computationally expensive to remove duplicates and so should only ber done if required.
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SQL query results are also tables
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## [SELECT](https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=1927)
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Select is one of the only commands used to retrieve data from a database and has a lot of additional commands and extensions to define a full selection.
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### Basic
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<pre>
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SELECT <<i>attribute1</i>,<i>attribute2</i>, ...> FROM <<i>table</i>> WHERE (<i>condition</i>)
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</pre>
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### Attributes
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These are the attributes to select form the table when filtered by the where clause
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If using multiple tables `table.attribute` can be used to specify witch table the attribute comes from]
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If there are several attributes with a similar or the came name then aliasing can be used with `AS`
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```
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SELECT shop AS Teashop FROM teas
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```
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Mathematical expressions can also be used with name aliasing
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```
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SELECT price*1.38 AS PriceInDollars FROM teas
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```
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the character `*` can be used to select all the attributes
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### Table
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Name aliasing can also be used with tables
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```
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SELECT s.studentID FROM students AS s
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```
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### WHERE
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where is used to filter the table to produces a reduces output and operates on standard SQL condition statements
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```
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SELECT studentID FROM students WHERE yearOfStudy = 2
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```
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```
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SELECT tea as expensiveTea FROM teas WHERE price > 4.5
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```
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### ORDER BY
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this allows an order to be applied to the output of an SQL result
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<pre>
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ORDER BY <i>expression</i> [ASC | DESC]
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</pre>
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```
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SELECT tea as expensiveTea FROM teas ORDER BY price DESC
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```
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### DISTINCT | ALL
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It can be desirable to only have unique items in a query result adding `DISTINCT` removes any duplicates
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```
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SELECT DISTINCT address FROM customers
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```

‎modules/CS258/Joes_Notes/Structured_Query_Language.md

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SQL or Structured query Language is a high lever **declarative language**
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Using SQL the DBMS can be told what to do ant it will find the optimal ways to perform the request
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SQL is both a
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* Data definition Language
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* Data manipulation Language
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Formal definition to SQL terms
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Relational | SQL
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-|-
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relation | Table
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Tuple | Row
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Attribute | Columns
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Standards have evolved over time and there are now many different standards. SQL but operate against the chosen DBMS to the standard will be defined form that.
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> CS258 and this guide will focus on PostgreSQl
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## DataTypes
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Data Types are used to describe the domain and provide information on how to interpret attributes. SQL has many different data types the full list ofr PostgreSQL can be found [here](https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=179) (p.179)
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| Name | Description |
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|-|-|
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integer | standard integer ~$$ \plusmn 2^{32}$$ *|
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bigint | Large range integer ~ $$\plusmn 2^{63}$$ * |
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numeric | near exact number $$131072$$ digits before decimal $$16383$$ digits after |
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char(n) | Fixed length character
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varchar(n)| Variable length character with limit
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text | Variable unlimited length
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boolean | true or false
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real | implementation of IEEE 754
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<small>* approximate for exact specification see the documentation</small>
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ANSI data types are a standard and may DBMS will recognize ANSI data types
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## Comparison Operators
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sql uses the standard comparison operators
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operator | Desc
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-|-
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= | Equal to
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\> | Greater than
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< | Less than
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\>= | Greater than or equal to
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<= | Less than or equal to
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<> | Not equal to
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Will return TRUE, FALSE or UNKNOWN
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UNKNOWN is returned when a comparison os performed with a NULL value
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use
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`IS NULL` and `IS NOT NULL` when comparing NULL values
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## Logical Operators
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operator | desc
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-|-
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AND | logical and
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OR | logical or
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NOT | logical not
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## Pattern Matching
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https://www.postgresql.org/files/documentation/pdf/14/postgresql-14-A4.pdf#page=281
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Sql has simple sting pattern matching using the keywords `LIKE` and `NOT LIKE`
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pattern | desc
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-|-
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_ | a single character
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% | a sequence of 0 or more characters
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'C' | A single character c
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if selecting a literal `%` or `_` a `\` can be used ans an escape character
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`ILIKE` can be used in postgresQL to make the match case insensitive
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### SIMILAR TO
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this command is available to use with regular expressions

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