题解 | 灵活使用sum()统计函数
统计作答次数
https://www.nowcoder.com/practice/45a87639110841b6950ef6a12d20175f
# 使用sum()统计未作答完试卷次数，可理解为产生了一个新的列放入sum括号里面的条件判断值后再加和 select count(*) total_pv, sum(submit_time is not null) complete_pv, (select count(distinct exam_id) from exam_record where submit_time is not null) complete_exam_cnt from exam_record;
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