The paper considers an original approach to the process of integration of complex database schemas. Apart from the proposed theoretical model of the approach, we also developed a CASE tool, named IIS*Case (Integrated Information Systems*Case, R.6.0), which enables its practical application. The basis of the approach aimed at designing and integration of database schemas and the ways of using IIS*Case are outlined. The main functionalities for database schema integration embedded in a new version of IIS*Case, which is developed in Java, are described. IIS*Case supports conceptual modelling of a database schema, generating subschemas and their integration in a relational database schema in 3NF. It is based on a methodology of gradual integration of independently designed subschemas into a database schema. The process of independent subschema design may lead to collisions in expressing the real world constraints. IIS*Case uses specialized algorithms for checking the consistency of constraints embedded in the database schema and subschemas. The paper outlines the process of detecting collisions and possible designer's actions aimed at their resolving.