Securing Open Source Databases and other web application related queries

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http://www.linuxquestions.org – Hi, How do Open Source Databases fare when it comes to security features? How effective is encrypting data stored in the database as a security measure? I am working on the design of a web application, wherein security is very important. My previous assignments were more related to networks :-I, hence i am reading up all i can. Based on what i have gathered from google search results, Transparent Data Encryption (TDE) seems to quite popular when it comes to Oracle/MS SQL, what would be equivalent for Open Source RDBMS? One of the replies at this link mentions that data at rest should be (HowTos)