problem:
there is table store course score of students, now write a sql to query student who has at least 2 course failed, means score<60,
sql: (mysql)
# drop table drop table if exists score; # create table create table score( id integer AUTO_INCREMENT primary key, student_num varchar(32) not null, course_name varchar(64), course_score float, unique key(student_num, course_name) ) engine=MYISAM DEFAULT CHARSET=utf8; # clear data truncate score; # prepare data insert into score(student_num, course_name, course_score) values ("001", "English", 100), ("001", "Math", 100), ("002", "English", 90), ("002", "Math", 90), ("003", "English", 20), ("003", "Math", 20), ("003", "Music", 20), ("004", "English", 20), ("004", "Math", 20), ("005", "English", 70), ("005", "Math", 30); # query select student_num, count(course_name) as cc from score where course_score < 60 group by student_num having cc>=2