A difference between statement and decision coverage

Question

Statement coverage is said to make sure that every statement in the code is executed at least once.
Decision/branch coverage is said to test that each branch/output of a decisions is tested, i.e. all statements in both false/true branches will be executed.
But is it not the same? In Statement coverage I need to execute all statements so I guess it can be only done by running all possible ways. I know I am missing something here..

Answer 1

The answer by Paul isn't quite right, at least I think so (according to ISTQB's definitions). There's quite significant difference between statement, decision/branch and condition coverage. I'll use the sample from the other answer but modified a bit, so I can show all three test coverage examples. Tests written here gives 100% test coverage for each type.

if(a || b)) {
    test1 = true;
}
else {
    if(c) {
      test2 = true
    }
}

We have two statements here - if(a||b) and if(c), to fully explain those coverage differences:

1. statement coverage have to test each statement at least once, so we need just two tests:
   • a=true b=false - that gives us path if(a||b) true -> test1 = true
   • a=false, b=false and c=true - that gives us path: if(a||b) false -> else -> if(c) -> test2=true.

This way we executed each and every statement.

2. branch/decision coverage needs one more test:

   • a=false, b=false, c=false - that leads us to that second if but we are executing false branch from that statement, that wasn't executed in statement coverage

   That way we have all the branches tested, meaning we went through all the paths.

3. condition coverage needs another test:

   • a=false, b=true - that leads through the same path as first test but executes the other decision in OR statement (a||b) to go through it.

That way we have all conditions tested, meaning that we went through all paths (branches) and triggered it with each condition we could - first 'if' statement was true in first test because of a=true triggered it and in the last test because b=true triggered it. Of course someone can argue that case with a=true and b=true should be tested as well, but when we will check how 'or' works then we can see it isn't needed and also variable c can be of any value as in those tests it is not evaluated.

At least I interpreted it this way. If someone is still interested :)

EDIT: In most sources I found lately decision/branch coverage terms are equivalent and the term I described as decision coverage is in fact condition coverage hence that update of the answer.