How to write an SQL aggregate function/query - sql

Question

I have a query that displays the total value (sum of amount) for each day.
The query:
SELECT CAST(date AS DATE), SUM(amount) AS total_amount FROM table
WHERE date BETWEEN '2019-01-01 00:00:00' AND '2019-12-31 00:00:00'
GROUP BY CAST(date AS DATE)
The CAST is to abbreviate the datetime format to just a date.
Now I want to select only the day which has the highest sum with the max function.
To do this I tried writing the following aggregate query:
SELECT s.date, s.total_amount
FROM (SELECT CAST(date AS DATE), SUM(amount) AS total_amount FROM table
WHERE date BETWEEN '2019-01-01 00:00:00' AND '2019-12-31 00:00:00'
GROUP BY CAST(date AS DATE)) s
WHERE s.total_amount = (SELECT MAX(s.total_amount) FROM table)
This does not work. I know the problem is with the final WHERE clause, but I need help with making it work.

Answer 1

Use ORDER BY with LIMIT :
SELECT CAST(date AS DATE), SUM(amount) AS total_amount
FROM table
WHERE date BETWEEN '2019-01-01 00:00:00' AND '2019-12-31 00:00:00'
GROUP BY CAST(date AS DATE)
ORDER BY total_amount DESC
LIMIT 1;
If you are working with SQL Server then you can use TOP :
SELECT TOP (1) CAST(date AS DATE), SUM(amount) AS total_amount
FROM table
WHERE date BETWEEN '2019-01-01 00:00:00' AND '2019-12-31 00:00:00'
GROUP BY CAST(date AS DATE)
ORDER BY total_amount DESC;
If you want ties then you can use window function :
SELECT t.*
FROM (SELECT CAST(date AS DATE), SUM(amount) AS total_amount,
RANK() OVER (ORDER BY SUM(amount) DESC) as Seq
FROM table
WHERE date BETWEEN '2019-01-01 00:00:00' AND '2019-12-31 00:00:00'
GROUP BY CAST(date AS DATE)
) t
WHERE seq = 1;
You can use CTE :
WITH CTE AS (
SELECT CAST(date AS DATE), SUM(amount) AS total_amount
FROM table
WHERE date BETWEEN '2019-01-01 00:00:00' AND '2019-12-31 00:00:00'
GROUP BY CAST(date AS DATE)
)
SELECT c.*
FROM CTE C
WHERE C.total_amount = (SELECT MAX(total_amount) FROM CTE);
Note : If your DBMS doesn't support CTE expression then you need repeat the SELECT statement in Subquery.
SELECT CAST(date AS DATE), SUM(amount) AS total_amount
FROM table
WHERE date BETWEEN '2019-01-01 00:00:00' AND '2019-12-31 00:00:00'
GROUP BY CAST(date AS DATE)
HAVING SUM(amount) = (SELECT MAX(total_amount)
FROM (SELECT CAST(date AS DATE), SUM(amount) AS total_amount
FROM table
WHERE date BETWEEN '2019-01-01 00:00:00' AND '2019-12-31 00:00:00'
GROUP BY CAST(date AS DATE)
) t
);

Answer 2

If you are using SQL Server then you can use TOP
SELECT TOP 1 CAST(date AS DATE), SUM(amount) AS total_amount
FROM table
WHERE date BETWEEN '2019-01-01 00:00:00' AND '2019-12-31 00:00:00'
GROUP BY CAST(date AS DATE)
ORDER BY total_amount DESC
Use window function row_number() - should work with MySQL 8.0, PostgreSQL, Oracle and SQL Server.
select
date,
total_amount
from
(
SELECT
CAST(date AS DATE) as date,
SUM(amount) AS total_amount,
row_number() over (order by SUM(amount) desc) as rnk
FROM table
WHERE date BETWEEN '2019-01-01 00:00:00' AND '2019-12-31 00:00:00'
GROUP BY CAST(date AS DATE)
) val
where rnk = 1

Answer 3

SELECT s.dt, s.total_amount
FROM (SELECT CAST(date AS DATE) as dt, SUM(amount) AS total_amount
FROM table
WHERE CAST(date as date) BETWEEN '2019-01-01' AND '2019-12-31'
GROUP BY CAST(date AS DATE)) s
WHERE s.total_amount = (Select max(total_amount)
FROM (SELECT CAST(date AS DATE) as dt, SUM(amount) AS total_amount
FROM table
WHERE CAST(date as date) BETWEEN '2019-01-01' AND '2019-12-31'
GROUP BY CAST(date AS DATE)) ss )