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:实现row_number.

2:取前3条：

3：对first_name排序取前三条：

在没有排序时得到的事正确的三条数据，但是排序后得到的是21条(全部数据)，奇怪。

猜测：

(1)执行了where但是在排序时是将所有的数据都拿出来排序的

(2)没有执行where，直接排序输出了。即跳过了where.

看explain:

MySQL官方手册解释：

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.