Using SQL to compare counts of identifiers from two tables

Question

I'm trying to construct a query that compares two tables with identical structure

Table 1:
ID | Data 

Table 2:
ID | Data

ID is a nonunique key (Data is repeatable but ID|Data combos are unique). I need a list of IDs where the COUNT of those IDs is greater in Table 2 than in Table 1.

So for example, if

Table 1
a | data
a | data1    
b | data2

Table 2
a | data
a | data1
b | data2
b | data3

would generate the output "b"

This feels like it should be easy, but my head is scrambled right now. I'm doing this in mysql if that affects options.

Answer 1

To get the count for each key,

select count(*) as count, ID from Table1 group by ID

So, use this as a sub-query in the from clause, and join the tables.

select tt1.ID
from (select count(*) as count, ID from Table1 group by ID) tt1
  inner join (select count(*) as count, ID from Table2 group by ID) tt2
     on tt1.ID = tt2.ID
where tt1.count < tt2.count