MySQL 8.0

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select version();

version()

8.0.27

create table customer_type_total_avg(USER_ID int,CID int,type varchar(20),PERCENTAGECOMPLETED int);
insert into customer_type_total_avg values(1,6,'external',12);
insert into customer_type_total_avg values(1,6,'external',50);
insert into customer_type_total_avg values(1,6,'internal',50);
insert into customer_type_total_avg values(1,6,'external',84);
insert into customer_type_total_avg values(1,6,'internal',100);
insert into customer_type_total_avg values(1,6,'external',100);
select avg(case when type='internal' and PERCENTAGECOMPLETED=50 THEN 0 ELSE PERCENTAGECOMPLETED END) AS AVG from
customer_type_total_avg

AVG

57.6667