SQL練習14：從titles表獲取按照title進行分組，注意對于重復的emp_no進行忽略
題目鏈接：牛客網
題目描述
從titles表獲取按照title進行分組，每組個數大于等于2，給出title以及對應的數目t，
注意對于重復的emp_no進行忽略(即emp_no重復的title不計算，title對應的數目t不增加)，
CREATE TABLE IF NOT EXISTS `titles` (
`emp_no` int(11) NOT NULL,
`title` varchar(50) NOT NULL,
`from_date` date NOT NULL,
`to_date` date DEFAULT NULL);
INSERT INTO titles VALUES(10001,'Senior Engineer','1986-06-26','9999-01-01');
INSERT INTO titles VALUES(10002,'Staff','1996-08-03','9999-01-01');
INSERT INTO titles VALUES(10003,'Senior Engineer','1995-12-03','9999-01-01');
INSERT INTO titles VALUES(10004,'Engineer','1986-12-01','1995-12-01');
INSERT INTO titles VALUES(10004,'Senior Engineer','1995-12-01','9999-01-01');
INSERT INTO titles VALUES(10005,'Senior Staff','1996-09-12','9999-01-01');
INSERT INTO titles VALUES(10005,'Staff','1989-09-12','1996-09-12');
INSERT INTO titles VALUES(10006,'Senior Engineer','1990-08-05','9999-01-01');
INSERT INTO titles VALUES(10007,'Senior Staff','1996-02-11','9999-01-01');
INSERT INTO titles VALUES(10007,'Staff','1989-02-10','1996-02-11');
INSERT INTO titles VALUES(10008,'Assistant Engineer','1998-03-11','2000-07-31');
INSERT INTO titles VALUES(10009,'Assistant Engineer','1985-02-18','1990-02-18');
INSERT INTO titles VALUES(10009,'Engineer','1990-02-18','1995-02-18');
INSERT INTO titles VALUES(10009,'Senior Engineer','1995-02-18','9999-01-01');
INSERT INTO titles VALUES(10010,'Engineer','1996-11-24','9999-01-01');
INSERT INTO titles VALUES(10010,'Engineer','1996-11-24','9999-01-01');
解法
這題的解法和上一題類似，只需要在使用COUNT()時添加DISTINCT來確保emp_no的唯一性即可，
SELECT title, count(DISTINCT emp_no) t
FROM titles
GROUP BY title
HAVING t >= 2
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