好久没写SQL查询语句了，想了好久也没想出来，于是上CSDN以帖询问，有一热心老兄给了一段：
CREATE TABLE store (
bookname VARCHAR(255) NOT NULL,
username VARCHAR(255) NOT NULL,
PRIMARY KEY (bookname, username)
);
INSERT INTO store (bookname, username) VALUES ('C++编程语言', 'a');
INSERT INTO store (bookname, username) VALUES ('C++对象模型', 'a');
INSERT INTO store (bookname, username) VALUES ('C++24小时通', 'a');
INSERT INTO store (bookname, username) VALUES ('C++速成', 'a');
INSERT INTO store (bookname, username) VALUES ('C++编程语言', 'b');
INSERT INTO store (bookname, username) VALUES ('C++对象模型', 'b');
INSERT INTO store (bookname, username) VALUES ('C++24小时通', 'b');
INSERT INTO store (bookname, username) VALUES ('C++速成', 'b');
INSERT INTO store (bookname, username) VALUES ('C++编程语言', 'c');
INSERT INTO store (bookname, username) VALUES ('C++对象模型', 'c');
INSERT INTO store (bookname, username) VALUES ('C++24小时通', 'c');
INSERT INTO store (bookname, username) VALUES ('C++速成', 'c');
INSERT INTO store (bookname, username) VALUES ('C++编程语言', 'd');
INSERT INTO store (bookname, username) VALUES ('C++24小时通', 'd');
INSERT INTO store (bookname, username) VALUES ('C++速成', 'd');
INSERT INTO store (bookname, username) VALUES ('C++编程语言', 'e');
INSERT INTO store (bookname, username) VALUES ('C++速成', 'e');
select bookname, count(*) from store where username in
(
select username from store where bookname = 'C++编程语言'
)
and bookname <> 'C++编程语言'
group by bookname
having count(*) >= 0
order by count(*) desc
limit 2;
看完不禁哑然。这不是以前就用过的么？怎么现在都不会了？
想起以前在学校，最擅长的大概就是数据库了吧，接下来被老师拉去做C开发。找的第一个工作是WEB开发的，做了一段时间没什么兴趣。接下来是java开发，2个项目后又因为客户端视频控件的原因，回到C语言。然后又是服务器开发，这次用C++了。几个月后又因为部分项目外包的原因，开始学习python开发。最近又回到WEB开发了。
感觉一直是换来换去，自己都搞不准该往哪个方向走，学的东西倒是多了，不过大都不深入。长此以往，我还能说自己“熟练掌握”什么？