DBA - MS SQL Server

Become The DBA
http://www.johnsansom.com/how-to-become-a-sql-server-dba/
http://www.idevelopment.info/

Database Documents
MySQL - http://dev.mysql.com/doc/index-topic.html
SQL Server - http://www.microsoft.com/en-us/sqlserver/default.aspx
Oracle Database - http://www.oracle.com/us/products/database/overview/index.html

DBA
http://www.mssqltips.com/sql-server-tip-category/53/dba-best-practices/
[SQL Job] http://www.mssqltips.com/sqlservertip/2561/querying-sql-server-agent-job-information/

Tutorials
http://www.studytonight.com/dbms/er-diagram.php
Locking -  Pessimistic and Optimistic locking in .NET
http://www.codeproject.com/Articles/114262/ways-of-doing-locking-in-NET-Pessimistic-and-opt

Sites
http://sqlfool.com
e.g. http://sqlfool.com/2011/06/index-defrag-script-v4-1/


All samples (sample database, project, etc..)
http://sqlserversamples.codeplex.com/

CLR and Aggregate function

Syntax
Union and Union All - Union All allows duplicates, but Union will remove duplicated rows.

Cluster and Non-Cluster Index

http://stackoverflow.com/questions/91688/what-are-the-differences-between-a-clustered-and-a-non-clustered-index

Clustered indexes are stored physically on the table. This means they are the fastest and you can only have one clustered index per table.

Non-clustered indexes are stored separately, and you can have as many as you want.

The best option is to set your clustered index on the most used unique column, usually the PK.

A clustered index is a special type of index that reorders the way records in the table are physically stored. Therefore, the table can have only one clustered index. The leaf nodes of a clustered index contain the data pages.

A non-clustered index is a special type of index in which the logical order of the index does not match the physical stored order of the rows on disk. The leaf node of a non-clustered index does not consist of the data pages. Instead, the leaf nodes contain index rows. (Read more here)

SQL Server Optimization
http://technet.microsoft.com/en-us/library/aa964133%28v=sql.90%29.aspx