在mysql中,我可以只使用别名将sum添加到另一个sum中吗

不,不能在同一查询中引用列别名。您必须将其设为子查询。

SELECT id, answerA, answerB, answerC, (answerA + answerM + answerD) AS total
FROM (
    select 
        t1.id,
        sum(t2.answ = t1.answ) as answerA,
        sum(t3.answ = t1.answ) as answerM,
        sum(t4.answ = t1.answ) as answerD,
        sum(answerA + answerM + answerD) as total
    from t1
    left join t2 on t2.id = t1.t2fk
    left join t3 on t3.id = t1.t3fk
    left join t4 on t4.id = t1.t4fk
    GROUP BY t1.id
) AS subquery

或者你可以写一个等价的表达式。

select 
    t1.id,
    sum(t2.answ = t1.answ) as answerA,
    sum(t3.answ = t1.answ) as answerM,
    sum(t4.answ = t1.answ) as answerD,
    sum(t1.answ IN (t2.answ, t3.answ, t4.answ)) as total
from t1
left join t2 on t2.id = t1.t2fk
left join t3 on t3.id = t1.t3fk
left join t4 on t4.id = t1.t4fk
GROUP BY t1.id