Debugging

After syntax errors cleared up

1. Query runs; results wrong
2. Join problems
3. Know data: count, characterize population

Population defined by WHERE clause

People:	10
Adresses for people with addresses:	13
People with adresses:	9
Addresses of all 10 people:	14

Population test:

select count(*) "count",
       count(distinct name_id) "dist"
  from name
 where name_seriesno = name_now(name_id);

    
count dist
----- ----
    6    6

Numbers should match

If they don't, step through the joins:

1. Add one join at a time
2. Test population again
3. Join problem located when numbers don't match

Join problems;

1. Population loss; "falling through join"

   Ex: From 6 to 5 after adding join

2. Population gain: "mulplying through a join"

   Ex: 6 and 7: one item counted twice

3. 5 and 7: both loss and gain

Correction:

1. Identify row(s) causing problem
2. Compare to join conditions
3. Determine cause