Refresher/SQL

This is a refresher course for those who need to review basic knowledge of SQL.

For a comprehensive tutorial, go to http://www.1keydata.com/sql/sql.html.

Basic Concepts

Constraint: an input filter
Denormalizing: performance improvement through reduction in number of joins needed for data processing
Group by and order by clauses: expensive due to creation of temporary tables
Indexes' tradeoffs: faster retrieval, slower updates
Join: a logical connection between two tables
Normalizing: eliminating redundant information and making it easier for future updates
Primary key: a combination of columns which uniquely specify a row
Trigger: the code automatically executed in response to certain table events
Where vs having: where is a restriction and having is a filter
Wildcard: % using LIKE

Inner join: intersection, e.g. SELECT Person.LastName, Person.FirstName, Sales.OrderNo FROM Person INNER JOIN Sales ON Person.P_Id=Sales.P_Id ORDER BY Person.LastName
Outer join: union, e.g.
Foreign key: primary key of another table.

For a comprehensive tutorial, go to http://www.1keydata.com/sql/sql.html.