if not exists (select * FROM INFORMATION_SCHEMA.COLUMNS where Table_Name='cnt_contrato'  and column_name = 'id_subtipoDocumento_solicitud')
begin  
ALTER TABLE [cnt_contrato] ADD [id_subtipoDocumento_solicitud] int NOT NULL DEFAULT 0 FOREIGN KEY (id_subtipoDocumento_solicitud) REFERENCES doc_subTipoDocumento (id)
end