SQL if query functionality - mysql

Question

I want to write a query to determine the success rate for every day for each different mode. I did write a query that'll just group by date and mode which serves my purpose but one of my seniors wrote this following query which also works but I am just unable to understand how the if clause is working. I'll add a bit of the query here -
SELECT
dt,
sum(if(mode='A',success,0))AS a_s,
sum(if(mode='A',total,0))AS a_t,
sum(if(mode='B',success,0))AS b_s,
sum(if(mode='B',total,0))AS b_t,
sum(if(mode='C',success,0))AS c_s,
sum(if(mode='C',total,0))AS c_t,
sum(if(mode='D',success,0))AS d_s,
sum(if(mode='D',total,0))AS d_t,
sum(if(mode NOT in('A','B','C','D'),success,0))AS other_s,
sum(if(mode NOT in('A','B','C','D'),total,0))AS other_t
FROM
(SELECT
mode,
date(addedon)AS dt,
sum(if(status in('success','partial'),1,0))AS success,
count(*)AS total
FROM `a_huge_ass_table`
WHERE `studentid`=159633 AND addedon>'2021-01-15'
GROUP BY mode,date(addedon)
)AS t
Here I am unable to understand how sum(if(mode='A',success,0))AS a_s, - this if clause is working. If the condition is true then the clause is returning success? how does that work does adding success also somehow verify that its status is a success case? I cant find this on google.

Answer 1

First, if() is not standard SQL. I recommend rewrite this using case:
sum(case when mode = 'A' then success else 0 end) as a_s,
sum(case when mode = 'A' then total else 0 end) as a_t,
and so on.
Second, this query is missing the final group by dt. Otherwise it produces one row, rather than a separate row for each dt value.
This is called conditional aggregation. Every row in the final result set represents a group of rows from the subquery. Within this group, some have mode = 'A' and some do not. For the ones with mode = 'A' the above sums the value of success and total.
There is no need for a subquery by the way. That just slows down the query. I would recommend writing the query as:
SELECT date(addedon) as dt
SUM( mode = 'A' AND status IN ('success', 'partial') ) as a_success,
SUM( mode = 'A' ) as a_total,
. . .
FROM `a_huge_ass_table`
WHERE studentid = 159633 AND addedon >= '2021-01-15'
GROUP BY date(addedon);
Note that this uses a MySQL extension where boolean expressions are treated as integers, with "1" for true and "0" for false.