板凳-------Mysql cookbook学习（十一--------10)

13.6 生成随机数 7/13/2025 10:42:00 PM

sql 复制代码

mysql> select rand(), rand(), rand();
+--------------------+--------------------+--------------------+
| rand()             | rand()             | rand()             |
+--------------------+--------------------+--------------------+
| 0.6048015268564239 | 0.9033102261864675 | 0.7021465810967111 |
+--------------------+--------------------+--------------------+
1 row in set (0.01 sec)

mysql> SHOW COLUMNS FROM t;
+-------+-------------+------+-----+---------+-------+
| Field | Type        | Null | Key | Default | Extra |
+-------+-------------+------+-----+---------+-------+
| name  | varchar(64) | YES  |     | NULL    |       |
| value | int         | YES  |     | NULL    |       |
| date  | date        | YES  |     | NULL    |       |
+-------+-------------+------+-----+---------+-------+
3 rows in set (0.01 sec)

mysql> SELECT name, value, date, RAND(), RAND(10), RAND(20) FROM t;
+-------+-------+------------+---------------------+---------------------+---------------------+
| name  | value | date       | RAND()              | RAND(10)            | RAND(20)            |
+-------+-------+------------+---------------------+---------------------+---------------------+
| name1 |    38 | 1999-01-01 |  0.8008022576577982 |  0.6570515219653505 | 0.15888261251047497 |
| name2 |    40 | 2000-12-31 |  0.8975715757325027 | 0.12820613023657923 |  0.6355305003333189 |
| name3 |    42 | 2013-02-28 | 0.08545113642276371 |  0.6698761160204896 |  0.7010046948688149 |
| name4 |    44 | 2018-01-02 |  0.7345409856499554 |  0.9647622201263553 |  0.5984320040777623 |
+-------+-------+------------+---------------------+---------------------+---------------------+
4 rows in set (0.00 sec)

13.7 随机化行集合

sql 复制代码

mysql> select name from t order by rand();
+-------+
| name  |
+-------+
| name4 |
| name3 |
| name1 |
| name2 |
+-------+
4 rows in set (0.00 sec)

mysql> select name from t order by rand();
+-------+
| name  |
+-------+
| name2 |
| name3 |
| name4 |
| name1 |
+-------+
4 rows in set (0.00 sec)

13.8 从行集合中随机选择条目

sql 复制代码

mysql> select n from die order by rand() limit 1;
+---+
| n |
+---+
| 4 |
+---+
1 row in set (0.02 sec)

mysql> select n from die order by rand() limit 1;
+---+
| n |
+---+
| 6 |
+---+
1 row in set (0.00 sec)

mysql> select n from die order by rand() limit 1;
+---+
| n |
+---+
| 3 |
+---+
1 row in set (0.00 sec)

mysql> select n from die order by rand() limit 1;
+---+
| n |
+---+
| 5 |
+---+
1 row in set (0.00 sec)

13.9 分配等级

sql 复制代码

mysql> SELECT value FROM t ORDER BY value DESC;
+-------+
| value |
+-------+
|    44 |
|    42 |
|    40 |
|    38 |
+-------+
4 rows in set (0.00 sec)

1.	在 MySQL 中，以下常见关键字需要特别注意（建议用反引号包裹）：
o	rank
o	order
o	group
o	desc
o	table
o	index 等

rank 是 MySQL 的保留关键字（用于窗口函数中的排名操作），直接作为列别名会导致语法错误。解决方法是用反引号 ` 将 rank 包裹起来，或者使用其他非保留关键字作为列名。
mysql> SELECT @rownum := @rownum + 1 AS `rank`, value
    -> FROM t
    -> ORDER BY value DESC;
+------+-------+
| rank | value |
+------+-------+
|    1 |    44 |
|    2 |    42 |
|    3 |    40 |
|    4 |    38 |
+------+-------+
4 rows in set, 1 warning (0.00 sec)

mysql> set @rank = 0, @prev_val = null;
Query OK, 0 rows affected (0.00 sec)

mysql> select @rank := if(@prev_val = value, @rank, @rank+1) as `rank`,
    -> @prev_val := value as value
    -> from t order by value desc;
+------+-------+
| rank | value |
+------+-------+
|    0 |    44 |
|    1 |    42 |
|    2 |    40 |
|    3 |    38 |
+------+-------+
4 rows in set, 2 warnings (0.00 sec)

mysql> SELECT
    ->   @rownum := @rownum + 1 AS `row`,
    ->   @rank := IF(@prev_val != value, @rownum, @rank) AS `rank`,
    ->   @prev_val := value AS `value`
    -> FROM t
    -> ORDER BY value DESC;
+------+------+-------+
| row  | rank | value |
+------+------+-------+
|    1 |    0 |    44 |
|    2 |    2 |    42 |
|    3 |    3 |    40 |
|    4 |    4 |    38 |
+------+------+-------+
4 rows in set, 3 warnings (0.00 sec)

如果你使用MySQL 8.0及以上版本，推荐使用窗口函数：
mysql> SELECT
    ->   ROW_NUMBER() OVER (ORDER BY value DESC) AS row_num,
    ->   DENSE_RANK() OVER (ORDER BY value DESC) AS ranking,
    ->   value AS score_value
    -> FROM t;
+---------+---------+-------------+
| row_num | ranking | score_value |
+---------+---------+-------------+
|       1 |       1 |          44 |
|       2 |       2 |          42 |
|       3 |       3 |          40 |
|       4 |       4 |          38 |
+---------+---------+-------------+
4 rows in set (0.01 sec)

mysql> select name, wins from al_winner order by wins desc, name;
+----------------+------+
| name           | wins |
+----------------+------+
| Mulder, Mark   |   21 |
| Clemens, Roger |   20 |
| Moyer, Jamie   |   20 |
| Garcia, Freddy |   18 |
| Hudson, Tim    |   18 |
| Abbott, Paul   |   17 |
| Mays, Joe      |   17 |
| Mussina, Mike  |   17 |
| Sabathia, C.C. |   17 |
| Zito, Barry    |   17 |
| Buehrle, Mark  |   16 |
| Milton, Eric   |   15 |
| Pettitte, Andy |   15 |
| Radke, Brad    |   15 |
| Sele, Aaron    |   15 |
+----------------+------+
15 rows in set (0.01 sec)

mysql> set @rownum = 0, @rank = 0, @prev_val = null;
Query OK, 0 rows affected (0.00 sec)

mysql> select @rownum := @rownum + 1 as `row`,
    -> @rank := if(@prev_val != wins, @rownum, @rank) as `rank`,
    -> name,
    -> @prev_val := wins as wins
    -> from al_winner order by wins desc;
+------+------+----------------+------+
| row  | rank | name           | wins |
+------+------+----------------+------+
|    1 |    1 | Mulder, Mark   |   21 |
|    2 |    2 | Clemens, Roger |   20 |
|    3 |    2 | Moyer, Jamie   |   20 |
|    4 |    4 | Garcia, Freddy |   18 |
|    5 |    4 | Hudson, Tim    |   18 |
|    6 |    6 | Abbott, Paul   |   17 |
|    7 |    6 | Mays, Joe      |   17 |
|    8 |    6 | Mussina, Mike  |   17 |
|    9 |    6 | Sabathia, C.C. |   17 |
|   10 |    6 | Zito, Barry    |   17 |
|   11 |   11 | Buehrle, Mark  |   16 |
|   12 |   12 | Milton, Eric   |   15 |
|   13 |   12 | Pettitte, Andy |   15 |
|   14 |   12 | Radke, Brad    |   15 |
|   15 |   12 | Sele, Aaron    |   15 |
+------+------+----------------+------+
15 rows in set, 3 warnings (0.00 sec)

第14章：处理重复项

14.0 引言

14.1 防止在表中发生重复

sql 复制代码

mysql> describe person;
+------------+--------------+------+-----+---------+-------+
| Field      | Type         | Null | Key | Default | Extra |
+------------+--------------+------+-----+---------+-------+
| tax_id     | int unsigned | NO   | PRI | NULL    |       |
| last_name  | char(20)     | NO   |     | NULL    |       |
| first_name | char(20)     | NO   |     | NULL    |       |
| address    | char(40)     | YES  |     | NULL    |       |
+------------+--------------+------+-----+---------+-------+
4 rows in set (0.01 sec)
1.	插入数据时需要确保：
INSERT INTO person (tax_id, last_name, first_name, address)
VALUES (123456, 'Smith', 'John', '123 Main St');

mysql> select * from person;
+--------+-----------+------------+-------------+
| tax_id | last_name | first_name | address     |
+--------+-----------+------------+-------------+
| 123456 | Smith     | John       | 123 Main St |
+--------+-----------+------------+-------------+
1 row in set (0.00 sec)

mysql> INSERT INTO person (tax_id, last_name, first_name, address)
    -> VALUES (123456, 'Smith', 'John', '123 Main St');
ERROR 1062 (23000): Duplicate entry '123456' for key 'person.PRIMARY'
mysql> INSERT ignore INTO person (tax_id, last_name, first_name, address)
    -> VALUES (123456, 'Smith', 'John', '123 Main St');
Query OK, 0 rows affected, 1 warning (0.00 sec)

mysql> replace into person (tax_id, last_name, first_name, address)
    -> VALUES (123456, 'Smith', 'John', '123 Main St');
Query OK, 1 row affected (0.01 sec)

mysql> select * from person;
+--------+-----------+------------+-------------+
| tax_id | last_name | first_name | address     |
+--------+-----------+------------+-------------+
| 123456 | Smith     | John       | 123 Main St |
+--------+-----------+------------+-------------+
1 row in set (0.00 sec)

14.2 处理向表中装载行时出现的重复错误

sql 复制代码

mysql> select * from poll_vote;
Empty set (0.01 sec)

mysql> describe poll_vote;
+--------------+--------------+------+-----+---------+----------------+
| Field        | Type         | Null | Key | Default | Extra          |
+--------------+--------------+------+-----+---------+----------------+
| poll_id      | int unsigned | NO   | PRI | NULL    | auto_increment |
| candidate_id | int unsigned | NO   | PRI | NULL    |                |
| vote_count   | int unsigned | YES  |     | NULL    |                |
+--------------+--------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

mysql> insert into poll_vote (poll_id, candidate_id,vote_count) values(14, 2, 1)
    -> on duplicate key update vote_count = vote_count + 1;
Query OK, 1 row affected (0.01 sec)

mysql> select * from poll_vote;
+---------+--------------+------------+
| poll_id | candidate_id | vote_count |
+---------+--------------+------------+
|      14 |            2 |          1 |
+---------+--------------+------------+
1 row in set (0.00 sec)

mysql> insert into poll_vote (poll_id, candidate_id,vote_count) values(14, 2, 1)
    -> on duplicate key update vote_count = vote_count + 1;
Query OK, 2 rows affected (0.01 sec)

mysql> select * from poll_vote;
+---------+--------------+------------+
| poll_id | candidate_id | vote_count |
+---------+--------------+------------+
|      14 |            2 |          2 |
+---------+--------------+------------+
1 row in set (0.00 sec)

14.3 计数和识别重复项

sql 复制代码

mysql> select * from catalog_list;
+-----------+-------------+--------------------------+
| last_name | first_name  | street                   |
+-----------+-------------+--------------------------+
| Isaacson  | Jim         | 515 Fordam St., Apt. 917 |
| Baxter    | Wallace     | 57 3rd Ave.              |
| McTavish  | Taylor      | 432 River Run            |
| Pinter    | Marlene     | 9 Sunset Trail           |
| BAXTER    | WALLACE     | 57 3rd Ave.              |
| Brown     | Bartholomew | 432 River Run            |
| Pinter    | Marlene     | 9 Sunset Trail           |
| Baxter    | Wallace     | 57 3rd Ave., Apt 102     |
| Isaacson  | Jim         | 515 Fordam St., Apt. 917 |
| Baxter    | Wallace     | 57 3rd Ave.              |
| McTavish  | Taylor      | 432 River Run            |
| Pinter    | Marlene     | 9 Sunset Trail           |
| BAXTER    | WALLACE     | 57 3rd Ave.              |
| Brown     | Bartholomew | 432 River Run            |
| Pinter    | Marlene     | 9 Sunset Trail           |
| Baxter    | Wallace     | 57 3rd Ave., Apt 102     |
+-----------+-------------+--------------------------+
16 rows in set (0.01 sec)

mysql> select count(*) as `rows` from catalog_list;
+------+
| rows |
+------+
|   16 |
+------+
1 row in set (0.00 sec)

mysql> select count(distinct last_name, first_name) as 'distinct names'
    -> from catalog_list;
+----------------+
| distinct names |
+----------------+
|              5 |
+----------------+
1 row in set (0.00 sec)

mysql> select count(*) - count(distinct last_name, first_name) as 'distinct names'
    -> from catalog_list;
+----------------+
| distinct names |
+----------------+
|             11 |
+----------------+
1 row in set (0.00 sec)

mysql> SELECT
    ->     COUNT(DISTINCT last_name, first_name) / COUNT(*) AS 'unique',
    ->     1 - (COUNT(DISTINCT last_name, first_name) / COUNT(*)) AS 'nonunique'
    -> FROM catalog_list;
+--------+-----------+
| unique | nonunique |
+--------+-----------+
| 0.3125 |    0.6875 |
+--------+-----------+
1 row in set (0.00 sec)

mysql> SELECT
    ->     COUNT(DISTINCT CONCAT(last_name, '|', first_name)) / COUNT(*) AS 'unique',
    ->     1 - (COUNT(DISTINCT CONCAT(last_name, '|', first_name)) / COUNT(*)) AS 'nonunique'
    -> FROM catalog_list;
+--------+-----------+
| unique | nonunique |
+--------+-----------+
| 0.3125 |    0.6875 |
+--------+-----------+
1 row in set (0.00 sec)

mysql> SELECT
    ->     ROUND(COUNT(DISTINCT last_name, first_name) / COUNT(*) * 100, 2) AS 'unique_percent',
    ->     ROUND((1 - COUNT(DISTINCT last_name, first_name) / COUNT(*)) * 100, 2) AS 'duplicate_percent'
    -> FROM catalog_list;
+----------------+-------------------+
| unique_percent | duplicate_percent |
+----------------+-------------------+
|          31.25 |             68.75 |
+----------------+-------------------+
1 row in set (0.00 sec)

mysql> SELECT
    ->     COUNT(DISTINCT full_name) / COUNT(*) AS 'unique',
    ->     1 - (COUNT(DISTINCT full_name) / COUNT(*)) AS 'nonunique'
    -> FROM (
    ->     SELECT CONCAT(last_name, '|', first_name) AS full_name
    ->     FROM catalog_list
    -> ) AS t;
+--------+-----------+
| unique | nonunique |
+--------+-----------+
| 0.3125 |    0.6875 |
+--------+-----------+
1 row in set (0.00 sec)

mysql> select count(*) as repetitions, last_name, first_name
    -> from catalog_list
    -> group by last_name, first_name
    -> having repetitions > 1;
+-------------+-----------+-------------+
| repetitions | last_name | first_name  |
+-------------+-----------+-------------+
|           2 | Isaacson  | Jim         |
|           6 | Baxter    | Wallace     |
|           2 | McTavish  | Taylor      |
|           4 | Pinter    | Marlene     |
|           2 | Brown     | Bartholomew |
+-------------+-----------+-------------+
5 rows in set (0.00 sec)

mysql> create table tmpp6
    -> select count(*) as count, last_name, first_name from catalog_list
    -> group by last_name, first_name having count > 1;
Query OK, 5 rows affected (0.04 sec)
Records: 5  Duplicates: 0  Warnings: 0

mysql> select catalog_list.*
    -> from tmpp6 inner join catalog_list using(last_name, first_name)
    -> order by last_name, first_name;
+-----------+-------------+--------------------------+
| last_name | first_name  | street                   |
+-----------+-------------+--------------------------+
| Baxter    | Wallace     | 57 3rd Ave.              |
| BAXTER    | WALLACE     | 57 3rd Ave.              |
| Baxter    | Wallace     | 57 3rd Ave., Apt 102     |
| Baxter    | Wallace     | 57 3rd Ave.              |
| BAXTER    | WALLACE     | 57 3rd Ave.              |
| Baxter    | Wallace     | 57 3rd Ave., Apt 102     |
| Brown     | Bartholomew | 432 River Run            |
| Brown     | Bartholomew | 432 River Run            |
| Isaacson  | Jim         | 515 Fordam St., Apt. 917 |
| Isaacson  | Jim         | 515 Fordam St., Apt. 917 |
| McTavish  | Taylor      | 432 River Run            |
| McTavish  | Taylor      | 432 River Run            |
| Pinter    | Marlene     | 9 Sunset Trail           |
| Pinter    | Marlene     | 9 Sunset Trail           |
| Pinter    | Marlene     | 9 Sunset Trail           |
| Pinter    | Marlene     | 9 Sunset Trail           |
+-----------+-------------+--------------------------+
16 rows in set (0.00 sec)

板凳-------Mysql cookbook学习 （十一--------10)

板凳-------Mysql cookbook学习（十一--------10)