When you merge two tables, there may be rows they don't have in common. This can create the appearance of missing data or empty rows. Here's what happens with some common merge situations in Fusion Tables.
Let's say our first table has country populations:
Here's a second table with the country's official language:
Now merge these two tables using the "Country" and "Name" columns. In Fusion Tables, the first table dictates what rows appear in the merged table. (Database buffs will recognize this as a left-outer join.)
Here's the result:
Notice a few things missing?
The second table didn't have a row for Brazil, so the new column is empty.
Although the second table had a row for Canada, the first didn't, so Canada doesn't appear in the merged table. (If having Canada is more important to you, try starting the merge from the second table.)
Both tables have a row for Denmark, although one is lowercase. Because the merge ignores case when matching strings, it succeeds in matching "Denmark" with "denmark".