好久沒寫SQL查詢語句了，想了好久也沒想出來，于是上CSDN以帖詢問，有一熱心老兄給了一段：
CREATE TABLE store (
bookname VARCHAR(255) NOT NULL,
username VARCHAR(255) NOT NULL,
PRIMARY KEY (bookname, username)
);
INSERT INTO store (bookname, username) VALUES ('C++編程語言', 'a');
INSERT INTO store (bookname, username) VALUES ('C++對象模型', 'a');
INSERT INTO store (bookname, username) VALUES ('C++24小時通', 'a');
INSERT INTO store (bookname, username) VALUES ('C++速成', 'a');
INSERT INTO store (bookname, username) VALUES ('C++編程語言', 'b');
INSERT INTO store (bookname, username) VALUES ('C++對象模型', 'b');
INSERT INTO store (bookname, username) VALUES ('C++24小時通', 'b');
INSERT INTO store (bookname, username) VALUES ('C++速成', 'b');
INSERT INTO store (bookname, username) VALUES ('C++編程語言', 'c');
INSERT INTO store (bookname, username) VALUES ('C++對象模型', 'c');
INSERT INTO store (bookname, username) VALUES ('C++24小時通', 'c');
INSERT INTO store (bookname, username) VALUES ('C++速成', 'c');
INSERT INTO store (bookname, username) VALUES ('C++編程語言', 'd');
INSERT INTO store (bookname, username) VALUES ('C++24小時通', 'd');
INSERT INTO store (bookname, username) VALUES ('C++速成', 'd');
INSERT INTO store (bookname, username) VALUES ('C++編程語言', '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++編程語言'
)
and bookname <> 'C++編程語言'
group by bookname
having count(*) >= 0
order by count(*) desc
limit 2;
看完不禁啞然。這不是以前就用過的么？怎么現在都不會了？
想起以前在學校，最擅長的大概就是數據庫了吧，接下來被老師拉去做C開發。找的第一個工作是WEB開發的，做了一段時間沒什么興趣。接下來是java開發，2個項目后又因為客戶端視頻控件的原因，回到C語言。然后又是服務器開發，這次用C++了。幾個月后又因為部分項目外包的原因，開始學習python開發。最近又回到WEB開發了。
感覺一直是換來換去，自己都搞不準該往哪個方向走，學的東西倒是多了，不過大都不深入。長此以往，我還能說自己“熟練掌握”什么？