3 Query by column and create a new table - sql

Question

I have students, and these students have their meals, morning and evening. I want to print the number of meals each student eats in the morning and evening.
If the number of dishes that student eats in the morning is more than one, I want to print the number in the table and the ID of the food.
FoodType when 1(morning), and when 2(evening)
StudentId FoodId FoodType
3 1 1
3 2 1
3 3 1
3 4 2
4 3 1
4 1 2
4 2 2
4 4 2
5 4 2
5 1 1
6 1 1
6 2 1
6 3 2
6 4 2
Sample out;
StudentId MorningFoodCountOrId EveningFoodCountOrId
3 3 meals 4
4 3 3 meals
5 4 1
6 2 meals 2 meals

Answer 1

Use conditional aggregation. The logic that decides if we print the number of records or their value is not intuitive, but I would phrase it as follows:
select
studentId,
case when sum(case when foodtype = 1 then 1 else 0 end) = 1
then max(case when foodtype = 1 then foodId end)
else sum(case when foodtype = 1 then 1 else 0 end)
end MorningFoodCountOrId
case when sum(case when foodtype = 2 then 1 else 0 end) = 1
then max(case when foodtype = 2 then foodId end)
else sum(case when foodtype = 2 then 1 else 0 end)
end EveningFoodCountOrId
from mytable
group by studentId
You RDMBS should be able to optimize the query by not computing the conditional sums twice.
Note: you did not specify which RDMBS you are using. If this is MySQL, then it is possible to shorten the conditional sums a little, as follows:
select
studentId,
case when sum(foodtype = 1) = 1
then max(case when foodtype = 1 then foodId end)
else sum(foodtype = 1)
MorningFoodCountOrId
case when sum(foodtype = 2) = 1
then max(case when foodtype = 2 then foodId end)
else sum(foodtype = 2)
EveningFoodCountOrId
from mytable
group by studentId

Answer 2

Here's your query, select sum() and case.. will do this
select t1.studentid
, case when t1.m <= 1 then t2.FoodId else concat(t1.m, ' meals') end MorningFoodCountOrId
, case when t1.e <= 1 then t3.FoodId else concat(t1.e, ' meals') end EveningFoodCountOrId
from(
select studentid
, sum(case when FoodType = 1 then 1 else 0 end) as m
, sum(case when FoodType = 2 then 1 else 0 end) as e
from tableA
group by studentid) t1
left join tableA t2 on t2.studentId = t1.studentId and t1.m = 1 and t2.FoodType = 1
left join tableA t3 on t3.studentId = t1.studentId and t1.e = 1 and t3.FoodType = 2
order by t1.studentid
see dbfiddle
in postgresql
, case when t1.m <= 1 then t2.FoodId::text else concat(t1.m, ' meals') end MorningFoodCountOrId
, case when t1.e <= 1 then t3.FoodId::text else concat(t1.e, ' meals') end EveningFoodCountOrId