Why is 4/5 equal to zero?

Short answer: dividing two integers yields integer result

Since both 4 and 5 are integers, according to SQL standard, the result also has to be an integer. As the result is 0.8 it equals to zero. Firebird does not round up when value is bigger than 0.5, so you won't get result of one.

If you wish to get something like 0.8, you should use such datatype. For example, a query like 4.0/5 would work:

SQL> select 4/5 from rdb$database;
=====================
0

SQL> select 4.0/5 from rdb$database;
=====================
0.8

SQL> select 4/5.0 from rdb$database;
=====================
0.8

SQL> select 4.0/5.0 from rdb$database;
=====================
0.80

As you can see it these examples, the number of decimals in result is the sum of decimals of values involved. Here's another example to show why is that important:

SQL> select 2/3 from rdb$database;
=====================
0

SQL> select 2.0/3 from rdb$database;
=====================
0.6

SQL> select 2/3.0 from rdb$database;
=====================
0.6

SQL> select 2.0/3.0 from rdb$database;
=====================
0.66

When you work with columns instead of numeric constants, you should cast them to desired datatypes. For example:

SQL> select cast(2 as decimal(18,1))/cast(3 as decimal(18,1)) from rdb$database;
=====================
0.66

It's a common mistake to try something like this:

select cast(2/3 as decimal(18,2)) from rdb$database;
=====================
0.00

It does not work (well, it does, but not the way you would want) because CAST happens after the integer calculation.