INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (10001, ‘1953-09-02’, ‘Georgi’, ‘Facello’, ‘M’, ‘1986-06-26’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (10002, ‘1964-06-02’, ‘Bezalel’, ‘Simmel’, ‘F’, ‘1985-11-21’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (10003, ‘1959-12-03’, ‘Parto’, ‘Bamford’, ‘M’, ‘1986-08-28’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (10004, ‘1954-05-01’, ‘Chirstian’, ‘Koblick’, ‘M’, ‘1986-12-01’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (10005, ‘1955-01-21’, ‘Kyoichi’, ‘Maliniak’, ‘M’, ‘1989-09-12’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (10006, ‘1953-04-20’, ‘Anneke’, ‘Preusig’, ‘F’, ‘1989-06-02’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (10007, ‘1957-05-23’, ‘Tzvetan’, ‘Zielinski’, ‘F’, ‘1989-02-10’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (10008, ‘1958-02-19’, ‘Saniya’, ‘Kalloufi’, ‘M’, ‘1994-09-15’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (10011, ‘1972-02-29’, ‘He’, ‘Rick’, ‘M’, ‘1991-02-20’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (64887, ‘1961-05-15’, ‘Rafols’, ‘Suomi’, ‘F’, ‘1987-06-05’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (64888, ‘1962-05-16’, ‘Kristof’, ‘Marchegay’, ‘F’, ‘1988-11-10’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (64889, ‘1953-05-08’, ‘Emdad’, ‘Pauthner’, ‘M’, ‘1985-09-07’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (64890, ‘1957-08-21’, ‘Leni’, ‘Kilgore’, ‘M’, ‘1995-08-31’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (64891, ‘1954-09-27’, ‘Ioana’, ‘Pepe’, ‘F’, ‘1990-08-19’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (64892, ‘1964-12-05’, ‘Mayumi’, ‘Tyugu’, ‘F’, ‘1988-03-19’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (64893, ‘1953-11-27’, ‘Yoshimitsu’, ‘Billawala’, ‘F’, ‘1986-06-07’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (64894, ‘1955-09-19’, ‘Mori’, ‘Weedman’, ‘F’, ‘1999-01-29’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (64895, ‘1962-02-24’, ‘Dietrich’, ‘Foote’, ‘M’, ‘1988-08-07’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (64896, ‘1960-12-31’, ‘Ingemar’, ‘Schieder’, ‘M’, ‘1995-10-25’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (64897, ‘1956-04-24’, ‘Aksel’, ‘Denna’, ‘M’, ‘1986-08-31’);

INSERT INTO?employees?(emp_no,?birth_date,?first_name,?last_name,?gender,?hire_date) VALUES (64898, ‘1961-04-13’, ‘Sudhanshu’, ‘Hutton’, ‘M’, ‘1996-02-09’);

1:實(shí)現(xiàn)row_number.

?

2:取前3條：

?

3：對(duì)first_name排序取前三條：

?

在沒有排序時(shí)得到的事正確的三條數(shù)據(jù)，但是排序后得到的是21條(全部數(shù)據(jù))，奇怪。

猜測：

(1)執(zhí)行了where但是在排序時(shí)是將所有的數(shù)據(jù)都拿出來排序的

(2)沒有執(zhí)行where，直接排序輸出了。即跳過了where.

看explain:

?

MySQL官方手冊(cè)解釋：

Using where

A WHERE clause is used to restrict which rows to match against the next table or send to the client. Unless you specifically intend to fetch or examine all rows from the table, you may have something wrong in your query if the Extra value is not Using where and the table join type is ALL or index.

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