Complex SQL design book

SQL for Smarties by Joe Celko. I like the way it covers methods for representing datasets that you'd not immediately think would fit into a relational model.

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If you're talking SQL Server, the series of SQL books by Solid Quality Mentors are well worth a read.

I personally can recommend:

• Inside Microsoft SQL Server 2005: T-SQL Programming
• Inside Microsoft SQL Server 2005: T-SQL Querying
• Inside Microsoft SQL Server 2005: Query Tuning and Optimization

I think they're a great set to have on your bookcase.

for T-SQL, take a look at look at books by Itzik Ben-Gan:

http://www.solidq.com/insidetsql/