#93 addd sections for joins, aggregations and permissions

guides/technical-guides/sql.md

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-#SQL 101
+# Relational SQL
 
-This guide is a simple guide to the fundamentals of SQL in all of its varied forms.
+This is a guide to the fundamentals of Relational SQL in all of its varied forms.
 
+## Introduction
+
+SQL, or Structured Query Language, is a programming language designed for
+managing data in relational database management systems (RDBMS). It's easy to
+use and as the most widely used database language in the world is a key tool in
+the data engineers toolbox.
+
+SQL is a powerful and versatile language that can be used for a variety of tasks,
+including:
+
+- Creating and managing databases
+- Inserting, updating, and deleting data from databases
+- Retrieving data from databases
+- Sorting and filtering data
+- Joining tables
+- Creating views
+- Creating stored procedures
+- Granting and revoking permissions
+
+## Join Types
+
+In SQL, a join is a clause used to combine data from two or more tables. There
+are four main types of joins: inner join, outer join, cross join, and self join.
+
+### Inner join 
+
+An inner join is the most common type of join. It returns all rows from the
+left table that have matching rows in the right table. The matching rows are
+determined by the join condition, which is an expression that compares values
+in the two tables.
+
+```sql
+SELECT *
+FROM customers
+INNER JOIN orders
+ON customers.customer_id = orders.customer_id;
+```
+
+This query will return all rows from the customers table that have matching rows
+in the orders table. The * in the SELECT clause means to return all columns from
+both tables.
+
+### Outer join
+
+An outer join returns all rows from the a table, including rows that do not
+have matching rows in the second table. Any columns from the second table that
+do not have a matching row will have NULL values for the columns from.
+
+There are two types of outer joins: left outer join and right outer join. Outer
+joins are almost always left.
+
+Left outer join: Returns all rows from the left table, and the matching rows
+from the right table.
+Right outer join: Returns all rows from the right table, and the matching rows
+from the left table.
+
+```sql
+SELECT *
+FROM customers
+LEFT OUTER JOIN orders
+ON customers.customer_id = orders.customer_id;
+```
+
+This query will return all rows from the customers table, including rows that
+do not have matching rows in the orders table. The rows in the customers table
+that do not have matching rows in the orders table will have NULL values for
+the columns from the orders table.
+
+```sql
+SELECT *
+FROM orders
+RIGHT OUTER JOIN customers
+ON orders.customer_id = customers.customer_id;
+```
+
+This query will return all rows from the orders table, including rows that do
+not have matching rows in the customers table. The rows in the orders table that
+do not have matching rows in the customers table will have NULL values for the
+columns from the customers table.
+
+### Cross join
+
+A cross join returns all possible combinations of rows from the left table and
+the right table. This means that the cross join will return all rows from the
+customers table, and then for each row in the customers table, it will return
+all rows in the orders table.
+
+```sql
+SELECT *
+FROM customers
+CROSS JOIN orders;
+```
+
+This query will return all possible combinations of rows from the customers table
+and the orders table.
+
+### Self join
+
+A self join is a join that joins a table to itself. This is useful for finding
+relationships between rows in the same table.
+
+```sql
+SELECT *
+FROM customers
+INNER JOIN customers AS c2
+ON customers.customer_id = c2.customer_id;
+```
+
+This query will return all rows from the customers table, and for each row, it
+will also return all rows from the customers table where the customer ID is
+different. This is useful for finding relationships between rows in the same table.
+
+## Aggregation
+
+In SQL, aggregation is the process of summarizing data from a group of rows into
+a single value. This is done using aggregate functions, which are functions that
+operate on a set of values and return a single value.
+
+Some common aggregate functions include:
+
+- COUNT: Returns the number of rows in a group.
+- SUM: Returns the sum of the values in a group.
+- AVG: Returns the average of the values in a group.
+- MIN: Returns the minimum value in a group.
+- MAX: Returns the maximum value in a group.
+
+Aggregate functions can be used in the SELECT clause of a SQL query to return
+aggregated data. For example, the following query returns the number of customers
+in each country:
+
+```sql
+SELECT country, COUNT(*) AS num_customers
+FROM customers
+GROUP BY country;
+```
+
+The `COUNT(*)` aggregate function returns the number of rows in each group, and
+the `GROUP BY` clause groups the rows by the `country` column. This query will
+return a table with two columns: `country` and `num_customers`.
+
+Aggregate functions can also be used with the WHERE clause of a SQL query to filter
+rows before they are aggregated. For example, the following query returns the
+average price of products that have been sold after `2022-01-01`:
+
+```sql
+SELECT AVG(price) AS avg_price
+FROM products
+WHERE sold_date > '2022-01-01';
+```
+
+The `AVG(price)` aggregate function returns the average price of products that
+have been sold in the period, and the `WHERE` clause filters the rows to only
+include products that have been sold in the last year.
+
+Aggregation is a powerful tool that can be used to summarize data from a database.
+By using aggregate functions, you can quickly and easily get insights into your data.
+
+## Security
+
+### Database Permissions
+
+In a relational database management system (RDMS), access and roles work by
+defining permissions for users and groups of users. These permissions determine
+what users can do with the data in the database, such as read, write, or delete.
+Access and roles are an important part of database security. By using access and
+roles, you can control who can access your data and what they can do with it.
+This helps to protect your data from unauthorized access and malicious attacks.
+
+Permissions are assigned to users and groups of users through roles. A role is a
+collection of permissions that can be assigned to users. Roles make it easy to
+manage permissions for large numbers of users. For example, a role called
+"administrator" might have the permissions to create, drop, read, write, and
+delete tables. A user could be assigned the "administrator" role to give them
+all of these permissions. Permissions should always be managed using roles.
+
+When a user tries to access a database object, the database checks the user's
+permissions to see if they are allowed to access the object. If the user does
+not have the required permissions, the database will deny access.
+
+Each RDMS have their own defined access and roles eg.
+
+- A user with the "read" permission for a table can view the data in the table.
+- A user with the "write" permission for a table can add, update, or delete data
+in the table.
+- A user with the "delete" permission for a table can delete the table.
+- A user with the "create" permission for a table can create a new table.
+- A user with the "drop" permission for a table can delete a table.