Question

Let’s say we have a database with a table that has many other associated tables. If you diagrammed the database, this would be the table at the center with many foreign key relationships spiraling out of it.

To make it more concrete, let’s say the two records in this central table are Initech and Contoso. Initech and Contoso are both associated with many other records in associated tables like Employees, AccountingTransactions, etc. Let’s say the two merged (Initech bought Contoso) and from a data standpoint, it really is as simple as merging all the records. What’s the easiest way to take all of Contoso’s related records, make them point to Initech and then delete Contoso?

UPDATE with CASCADE comes tantalizingly close, but it obviously can’t work without turning off constraints and then turning them back on (yuck).

Is there a nice generic way to do this without hunting down every single linked table and migrating them one by one? This has to be a common requirement. It’s come up in two places in this project and can be summed up with: Entity A needs to control everything Entity B current controls. How can I make it happen?

Before Merge:

Companies
ID Name
1  Contoso
2  Initech

Employees
ID Name CompanyId
1  Bob  1 
2  Ted  2

After Merge:

Companies
ID Name
2  Initech

Employees
ID Name CompanyId
1  Bob  2 
2  Ted  2

All my attempts at searching only turned up questions about merging separate databases… so sorry if this has been asked before.

Answer 1

This query is likely vendor-dependent, but in MySQL:

UPDATE Employees e, Cars c, OtherEntity o
  SET e.CompanyId = 2, c.CompanyId = 2, o.CompanyID = 2
  WHERE e.CompanyID = 1 OR c.CompanyId = 1 OR o.CompanyId = 1;