Mysql 我们的OP：D@tombom很抱歉我不是每天都检查SE。但我很感激你为解释所做的工作。谢谢 +----------------+-----------+------------+------------+--------------------------

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我们的OP：D@tombom很抱歉我不是每天都检查SE。但我很感激你为解释所做的工作。谢谢

+----------------+-----------+------------+------------+----------------------------+------+-------+
| uid            | last_name | first_name | dob        | email                      | rank | grade |
+----------------+-----------+------------+------------+----------------------------+------+-------+
| 13428700000001 | Smith     | John       | 1990-12-03 | ...@gmail.com              |   99 |     4 |
| 13428721960000 | Li        | Kai Li     | 1979-02-15 | ...@miryclay.com           |   12 |     2 |
| 13428722180001 | Zhang     | Xi Xiong   | 1993-11-09 | ...@163.com                |    5 |     5 |
| 13428739950000 | Zhou      | Ji Hai     | 1991-06-06 | ...@msn.com                |  234 |     1 |
| 13428739950001 | Pan       | Yao        | 1992-05-12 | ...@email.com              |   43 |     2 |
| 13428740010001 | Jin       | Denny      | 1994-06-02 | ...@yahoo.com              |  198 |     3 |
| 13428740010002 | Li        | Fonzie     | 1991-02-02 | ...@hotmail.com            |   75 |     3 |
| 13428743370000 | Ma        | Haggar     | 1991-08-16 | ...@haggars.com            |   47 |     4 |
| 13428743590001 | Ren       | Jenny      | 1990-03-29 | ...@email.com              |    5 |     2 |
| 13428774040000 | Chen      | Dragon     | 1999-04-12 | ...@aol.com                |   23 |     5 |
| 13428774260001 | Wang      | Doctor     | 1996-09-30 | ...@yahoo.com              |    1 |     5 |
| 13430100000000 | Chanz     | Heyvery    | 1994-04-04 | ...@gmail.com              |  107 |     2 |
+----------------+-----------+------------+------------+----------------------------+------+-------+

SELECT s1.*
FROM students s1
LEFT JOIN students s2 ON s1.rank < s2.rank
WHERE s2.uid IS NULL;

drop table if exists students;
CREATE TABLE students
    (`uid` bigint, `last_name` varchar(5), `first_name` varchar(8), `dob` varchar(10), `email` varchar(16), `rank` int, `grade` int)
;

INSERT INTO students
    (`uid`, `last_name`, `first_name`, `dob`, `email`, `rank`, `grade`)
VALUES
    (13428700000001, 'Smith', 'John', '1990-12-03', '...@gmail.com', 99, 4),
    (13428721960000, 'Li', 'Kai Li', '1979-02-15', '...@miryclay.com', 12, 2),
    (13428722180001, 'Zhang', 'Xi Xiong', '1993-11-09', '...@163.com', 5, 5),
    (13428739950000, 'Zhou', 'Ji Hai', '1991-06-06', '...@msn.com', 234, 1),
    (13428739950001, 'Pan', 'Yao', '1992-05-12', '...@email.com', 43, 2),
    (13428740010001, 'Jin', 'Denny', '1994-06-02', '...@yahoo.com', 198, 3),
    (13428740010002, 'Li', 'Fonzie', '1991-02-02', '...@hotmail.com', 75, 3),
    (13428743370000, 'Ma', 'Haggar', '1991-08-16', '...@haggars.com', 47, 4),
    (13428743590001, 'Ren', 'Jenny', '1990-03-29', '...@email.com', 5, 2),
    (13428774040000, 'Chen', 'Dragon', '1999-04-12', '...@aol.com', 23, 5),
    (13428774260001, 'Wang', 'Doctor', '1996-09-30', '...@yahoo.com', 1, 5),
    (13430100000000, 'Chanz', 'Heyvery', '1994-04-04', '...@gmail.com', 107, 2)
;

| ID | SELECT_TYPE |    TABLE | TYPE | POSSIBLE_KEYS |    KEY | KEY_LEN |    REF | ROWS |       EXTRA |
-------------------------------------------------------------------------------------------------------
|  1 |     PRIMARY | students |  ALL |        (null) | (null) |  (null) | (null) |   12 | Using where |
|  2 |    SUBQUERY | students |  ALL |        (null) | (null) |  (null) | (null) |   12 |             |

| ID | SELECT_TYPE | TABLE | TYPE | POSSIBLE_KEYS |    KEY | KEY_LEN |    REF | ROWS |       EXTRA |
----------------------------------------------------------------------------------------------------
|  1 |      SIMPLE |    s1 |  ALL |        (null) | (null) |  (null) | (null) |   12 |             |
|  1 |      SIMPLE |    s2 |  ALL |        (null) | (null) |  (null) | (null) |   12 | Using where |

drop table if exists students;
CREATE TABLE students
    (`uid` bigint, `last_name` varchar(5), `first_name` varchar(8), `dob` varchar(10), `email` varchar(16), `rank` int, `grade` int
    , key rankkey(rank)
    )
;

| ID | SELECT_TYPE |    TABLE |   TYPE | POSSIBLE_KEYS |     KEY | KEY_LEN |    REF |   ROWS |                        EXTRA |
-----------------------------------------------------------------------------------------------------------------------------
|  1 |     PRIMARY | students |    ref |       rankkey | rankkey |       5 |  const |      1 |                  Using where |
|  2 |    SUBQUERY |   (null) | (null) |        (null) |  (null) |  (null) | (null) | (null) | Select tables optimized away |

| ID | SELECT_TYPE | TABLE | TYPE | POSSIBLE_KEYS |    KEY | KEY_LEN |    REF | ROWS |       EXTRA |
----------------------------------------------------------------------------------------------------
|  1 |      SIMPLE |    s1 |  ALL |        (null) | (null) |  (null) | (null) |   12 |             |
|  1 |      SIMPLE |    s2 |  ALL |       rankkey | (null) |  (null) | (null) |   12 | Using where |

drop table if exists students;
CREATE TABLE students
    (`uid` bigint, `last_name` varchar(5), `first_name` varchar(8), `dob` varchar(10), `email` varchar(16), `rank` int, `grade` int
    , key rankkey(rank)
    , primary key(uid)
    );

| ID | SELECT_TYPE |    TABLE |   TYPE | POSSIBLE_KEYS |     KEY | KEY_LEN |    REF |   ROWS |                        EXTRA |
-----------------------------------------------------------------------------------------------------------------------------
|  1 |     PRIMARY | students |    ref |       rankkey | rankkey |       5 |  const |      1 |                  Using where |
|  2 |    SUBQUERY |   (null) | (null) |        (null) |  (null) |  (null) | (null) | (null) | Select tables optimized away |

| ID | SELECT_TYPE | TABLE |  TYPE | POSSIBLE_KEYS |     KEY | KEY_LEN |    REF | ROWS |                                EXTRA |
-------------------------------------------------------------------------------------------------------------------------------
|  1 |      SIMPLE |    s1 |   ALL |        (null) |  (null) |  (null) | (null) |   12 |                                      |
|  1 |      SIMPLE |    s2 | index |       rankkey | rankkey |       5 | (null) |   12 | Using where; Using index; Not exists |

SELECT s1.*
FROM students s1
LEFT JOIN students s2 ON s1.rank < s2.rank
WHERE s2.last_name IS NULL;

| ID | SELECT_TYPE | TABLE | TYPE | POSSIBLE_KEYS |    KEY | KEY_LEN |    REF | ROWS |       EXTRA |
----------------------------------------------------------------------------------------------------
|  1 |      SIMPLE |    s1 |  ALL |        (null) | (null) |  (null) | (null) |   12 |             |
|  1 |      SIMPLE |    s2 |  ALL |       rankkey | (null) |  (null) | (null) |   12 | Using where |

SELECT s1.*
FROM students s1
LEFT JOIN students s2 ON s1.rank < s2.rank
WHERE s2.rank IS NULL;

| ID | SELECT_TYPE | TABLE |  TYPE | POSSIBLE_KEYS |     KEY | KEY_LEN |    REF | ROWS |                    EXTRA |
-------------------------------------------------------------------------------------------------------------------
|  1 |      SIMPLE |    s1 |   ALL |        (null) |  (null) |  (null) | (null) |   12 |                          |
|  1 |      SIMPLE |    s2 | index |       rankkey | rankkey |       5 | (null) |   12 | Using where; Using index |