Every business above a certain size has the same argument in a meeting: two systems, two numbers, and an hour spent working out which one to believe. That's an integration problem, not a reporting one.
Copying records between systems takes a day. Deciding which system is right when they disagree, and proving afterwards that nothing was lost, is the actual work.
Reliable extraction from each system, including the ones with no real API where the only route out is a scheduled export.
Working out that three records are the same customer, which is the difference between one view of a relationship and three partial ones.
A documented contract between systems, so the meaning of a field doesn't live only in the head of whoever built the integration.
Formats, units, currencies, and time zones normalised once, centrally, rather than patched differently in each consuming system.
Batch or real-time per data type, with a defined answer to which system wins when both have changed the same record.
Automated checks that totals match on both sides, so a silent failure surfaces the same day rather than at quarter end.
The integration most companies actually run is a spreadsheet, a scheduled export, and one person who knows how to fix it when it breaks.
Where a clean interface is needed rather than a pipeline, see API development, and the modelling underneath sits with database design. Once the data lands in one place, reporting on it is data warehouse and BI. Trading partner documents are their own discipline — EDI integration.
List the systems and the report nobody trusts. We'll map where the numbers diverge and what it takes to make them agree permanently.