Question

Having a bit of trouble dealing with not exist clause in a query.

I have three tables:

Meal

no_meal integer
type_meal varchar(255)

Consumed

no_meal integer
designation varchar(255)
quantity integer

Food

designation varchar(255)
quantity integer
type varchar(255)

Quantity in food is quantity stored and quantity in consumed is consumed quantity so, they will differ and thus natural inner join won’t cut it.

• Type of food can be ‘drink’ ‘meat’ ‘fish’ and so on.
• Type of meal can be ‘Breakfast’ ‘Lunch’ ‘Dinner’

I want a query that will tell me the drinks that have been consumed in all types of meals.

I’ve been toggling and testing code but can’t get it right. At the moment I’m trying this:

SELECT Consumed.designation
FROM Consumed
WHERE NOT EXISTS

(SELECT type_meal
 FROM Consumed, Food, Meal
 WHERE Consumed.designation = Food.designation
 AND Consumed.no_meal = Meal.no_meal
 AND type = 'Drink'
 ORDER BY Food.designation)

EXCEPT

(SELECT type_meal
 FROM Meal);

How the hell do I get it right?

Thanks for input.

EDIT:

I’ll add some data in order to make it clearer.

Food

Steak   100    Meat
Water   200    Drink
Coca cola    300   Drink

Meal

0001     Breakfast
0002     Lunch
0003     Dinner

Consumed

0001     Water       50
0002     Steak       20
0001     Coca cola   20
0003     Water        5
0002     Water       15

Now, I want to know which drink has been consumed in every meal, which will only give water.

Hope I cleared some minds about the problem

Answer 1

Twist your mind a little bit, and think with double negatives ….

I want […] the drinks that have been consumed in all types of meals.

You want all drinks for which there are NO meal types for which that drink is NOT part of the meal type:

select distinct f.designation 
from food f 
where 
  type = 'Drink' and 
  not exists (
    select * 
    from meal m1
    where not exists (
      select *  
      from meal m2
      join consumed c on c.no_meal = m2.no_meal
      where 
        m1.no_meal = m2.no_meal and
        c.designation = f.designation
    )
  ) 

This is called relational division.