In literature there exist several definitions of Schema:
“Schema integration is the process of merging autonomously developed DB schema into a unified, global schema to provide transparency through a unified view” .
“Schema integration is the process of identifying the components of a database which are related to one another, selecting the best representation for the global conceptual schema, and finally, integrating the components of each intermediate schema” .
12.2. Why is Schema Integration Required?
With respect to the web warehouse, schema integration is one of its important perspectives. There are heterogeneous sources of data in World Wide Web which vary in their schema configuration. Schemas from different domains vary in their structure and terminology like property tax schema or real estate schema. It may happens that the schemas from same real world domain also differ, since the schemas are developed independently in different real world contexts. This give rise to the need of schema integration to present a global conceptual schema for integrating the components and providing a unified view. “The aim of the integration process is the development of a global schema which integrates and subsumes the local schema in such a way that (global) users are provided with a uniform and correct view of the global database. The most urgent problem in this respect is semantic heterogeneity—if it is not detected and resolved, the usage of integrated data leads to invalid results” .
12.3. Problems in Schema Integration
Mainly two problems can arise in schema integration: 
12.3.1. Identification of associations among various conceptual schemas.
12.3.2. Verification of the identified associations, whether consistent with the schemas’ semantics.
12.4. Classification of Schema Integration Process
The schema integration process can be classified into following phases: , 
An analysis of local schemata is carried out. The analysis made governs the choice of schemas to be integrated and the order of integration. A number of schemas to be integrated at a time and the amount of designer interaction are also decided in this step .
12.4.2. Schema Comparison/Matching
This step involves identifying and analyzing inter-schema relationships by determining the correspondences between the schemas. Possible conflicts are also detected in this phase.
12.4.3. Schema Conforming
In order to integrate schemas, the detected conflicts between them have to be resolved so that it was possible to merge them . It implies modification of one or both schemas that will be integrated until each phenomenon in the Universe of the Discourse (UoD) reflects the same form in both schemas.
12.4.4. Schema Merging
Schema merging results into a single global integrated schema by superimposing the local schemas.
12.5. Conflicts in Schema Integration