Data Integration aims to combine distributed information conforming to different data models and provide interfaces for accessing such information in an unfied view. The Mediator Environment for Multiple Information Sources (MOMIS), aims to construct synthesized, integrated descriptions of information coming from multiple heterogeneous sources. We show how the MOMIS data integration system can deal with all the twelve queries of the THALIA benchmark, a public available testbed and benchmark for information integration systems. We propose new techniques that consider the optimization of full outerjoin operation in MOMIS. Full outerjoin is used in data integration systems for merging multiple records representing the same real-world object into a single, consistent, and clean representation. A query manager that merge information using full outerjoin can benefit easily from this optimization technique. We made a study of different data integration systems which implement the notion of data quality. We propose a table that summarizes relationships between data anomalies and data quality dimensions. Finally, we focus on quality-driven query processing.