Check out example codes for "using sum function in mysql for two diffrenet conditions". It will help you in understanding the concepts better.

Code Example 1

SELECT sum( mark ) FROM `student`

Code Example 2

SELECT 
    SUM(CASE
        WHEN status = 'Shipped' THEN quantityOrdered
    END) qty_shipped,
    SUM(CASE
        WHEN status = 'Resolved' THEN quantityOrdered
    END) qty_resolved,
    SUM(CASE
        WHEN status = 'Cancelled' THEN quantityOrdered
    END) qty_cancelled,
    SUM(CASE
        WHEN status = 'On Hold' THEN quantityOrdered
    END) qty_on_hold,
    SUM(CASE
        WHEN status = 'Disputed' THEN quantityOrdered
    END) qty_on_disputed,
    SUM(CASE
        WHEN status = 'In Process' THEN quantityOrdered
    END) qty_in_process
FROM
    orderdetails
        INNER JOIN
    orders USING (orderNumber);

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