板凳-------Mysql cookbook学习 （八--2）
3.10 在用户程序中使用null作为比较参数
mysql> select * from taxpayer;
+---------+--------+
| name | id |
+---------+--------+
| bernina | 198-48 |
| bertha | NULL |
| ben | NULL |
| NULL | 475-83 |
| baidu | 111+55 |
+---------+--------+
5 rows in set (0.00 sec)
3.11 结果集排序
mysql> select * from mail where size > 100000 order by size;
+---------------------+---------+---------+---------+---------+---------+
| t | srcuser | srchost | dstuser | dsthost | size |
+---------------------+---------+---------+---------+---------+---------+
| 2006-05-12 12:48:13 | tricia | mars | gene | venus | 194925 |
| 2006-05-15 10:25:52 | gene | mars | tricia | saturn | 998532 |
| 2006-05-14 17:03:01 | tricia | saturn | phil | venus | 2394482 |
+---------------------+---------+---------+---------+---------+---------+
3 rows in set (0.00 sec)
mysql> select * from mail where dstuser = 'tricia'-> order by srchost, srcuser;
+---------------------+---------+---------+---------+---------+--------+
| t | srcuser | srchost | dstuser | dsthost | size |
+---------------------+---------+---------+---------+---------+--------+
| 2006-05-15 10:25:52 | gene | mars | tricia | saturn | 998532 |
| 2006-05-14 11:52:17 | phil | mars | tricia | saturn | 5781 |
| 2006-05-17 12:49:23 | phil | mars | tricia | saturn | 873 |
| 2006-05-11 10:15:08 | barb | saturn | tricia | mars | 58274 |
| 2006-05-13 13:59:18 | barb | saturn | tricia | venus | 271 |
+---------------------+---------+---------+---------+---------+--------+
5 rows in set (0.00 sec)
mysql> select * from mail where size > 50000 order by size desc;
+---------------------+---------+---------+---------+---------+---------+
| t | srcuser | srchost | dstuser | dsthost | size |
+---------------------+---------+---------+---------+---------+---------+
| 2006-05-14 17:03:01 | tricia | saturn | phil | venus | 2394482 |
| 2006-05-15 10:25:52 | gene | mars | tricia | saturn | 998532 |
| 2006-05-12 12:48:13 | tricia | mars | gene | venus | 194925 |
| 2006-05-14 14:42:21 | barb | venus | barb | venus | 98151 |
| 2006-05-11 10:15:08 | barb | saturn | tricia | mars | 58274 |
+---------------------+---------+---------+---------+---------+---------+
5 rows in set (0.00 sec)
3.12 使用视图来简化查询
mysql> select-> date_format(t, '%M %e %Y') as date_sent,-> concat(srcuser, '@', srchost) as sender,-> concat(dstuser, '@', dsthost) as recipient,-> size from mail;
+-------------+---------------+---------------+---------+
| date_sent | sender | recipient | size |
+-------------+---------------+---------------+---------+
| May 11 2006 | barb@saturn | tricia@mars | 58274 |
| May 12 2006 | tricia@mars | gene@venus | 194925 |
| May 12 2006 | phil@mars | phil@saturn | 1048 |
| May 13 2006 | barb@saturn | tricia@venus | 271 |
| May 14 2006 | gene@venus | barb@mars | 2291 |
| May 14 2006 | phil@mars | tricia@saturn | 5781 |
| May 14 2006 | barb@venus | barb@venus | 98151 |
| May 14 2006 | tricia@saturn | phil@venus | 2394482 |
| May 15 2006 | gene@mars | gene@saturn | 3824 |
| May 15 2006 | phil@venus | phil@venus | 978 |
| May 15 2006 | gene@mars | tricia@saturn | 998532 |
| May 15 2006 | gene@saturn | gene@mars | 3856 |
| May 16 2006 | gene@venus | barb@mars | 613 |
| May 16 2006 | phil@venus | barb@venus | 10294 |
| May 17 2006 | phil@mars | tricia@saturn | 873 |
| May 19 2006 | gene@saturn | gene@venus | 23992 |
+-------------+---------------+---------------+---------+
16 rows in set (0.00 sec)
mysql> select * from mail_view;
+--------------+---------------+---------------+---------+
| date_sent | sender | recipient | size |
+--------------+---------------+---------------+---------+
| May 11, 2006 | barb@saturn | tricia@mars | 58274 |
| May 12, 2006 | tricia@mars | gene@venus | 194925 |
| May 12, 2006 | phil@mars | phil@saturn | 1048 |
| May 13, 2006 | barb@saturn | tricia@venus | 271 |
| May 14, 2006 | gene@venus | barb@mars | 2291 |
| May 14, 2006 | phil@mars | tricia@saturn | 5781 |
| May 14, 2006 | barb@venus | barb@venus | 98151 |
| May 14, 2006 | tricia@saturn | phil@venus | 2394482 |
| May 15, 2006 | gene@mars | gene@saturn | 3824 |
| May 15, 2006 | phil@venus | phil@venus | 978 |
| May 15, 2006 | gene@mars | tricia@saturn | 998532 |
| May 15, 2006 | gene@saturn | gene@mars | 3856 |
| May 16, 2006 | gene@venus | barb@mars | 613 |
| May 16, 2006 | phil@venus | barb@venus | 10294 |
| May 17, 2006 | phil@mars | tricia@saturn | 873 |
| May 19, 2006 | gene@saturn | gene@venus | 23992 |
+--------------+---------------+---------------+---------+
16 rows in set (0.00 sec)
mysql> select date_sent, sender, size from mail_view-> where size > 100000 order by size;
+--------------+---------------+---------+
| date_sent | sender | size |
+--------------+---------------+---------+
| May 12, 2006 | tricia@mars | 194925 |
| May 15, 2006 | gene@mars | 998532 |
| May 14, 2006 | tricia@saturn | 2394482 |
+--------------+---------------+---------+
3 rows in set (0.00 sec)
3.13 多表查询
mysql> select * from profile;
+----+---------+------------+-------+-----------------------+------+
| id | name | birth | color | foods | cats |
+----+---------+------------+-------+-----------------------+------+
| 1 | Fred | 1970-04-13 | black | lutefisk,fadge,pizza | 1 |
| 2 | Mort | 1969-09-30 | white | burrito,curry,eggroll | 3 |
| 3 | Brit | 1957-12-01 | red | burrito,curry,pizza | 1 |
| 4 | Carl | 1973-11-02 | red | eggroll,pizza | 4 |
| 5 | Sean | 1963-07-04 | blue | burrito,curry | 5 |
| 6 | Alan | 1965-02-14 | red | curry,fadge | 1 |
| 7 | Mara | 1968-09-17 | green | lutefisk,fadge | 1 |
| 8 | Shepard | 1975-09-02 | black | curry,pizza | 2 |
| 9 | Dick | 1952-08-20 | green | lutefisk,fadge | 0 |
| 10 | Tony | 1960-05-01 | white | burrito,pizza | 0 |
| 11 | Alison | 1973-01-12 | blue | eggroll | 4 |
| 12 | De'Mont | 1973-01-12 | NULL | eggroll | 4 |
| 13 | De'Mont | 1973-01-12 | NULL | eggroll | 4 |
| 14 | De'Mont | 1973-01-12 | NULL | eggroll | 4 |
| 15 | De'Mont | 1973-01-12 | NULL | eggroll | 4 |
| 16 | De'Mont | 1973-01-12 | NULL | eggroll | 4 |
| 17 | Amabel | NULL | NULL | NULL | NULL |
| 18 | De'Mont | 1980-12-12 | NULL | eggroll | 8 |
| 19 | Juan | NULL | NULL | NULL | NULL |
+----+---------+------------+-------+-----------------------+------+
19 rows in set (0.00 sec)
mysql> select * from profile_contact order by profile_id, service;
+------------+---------+---------------+
| profile_id | service | contact_name |
+------------+---------+---------------+
| 1 | AIM | user1-aimid |
| 1 | MSN | user1-msnid |
| 2 | AIM | user2-aimid |
| 2 | MSN | user2-msnid |
| 2 | Yahoo | user2-yahooid |
| 4 | Yahoo | user4-yahooid |
+------------+---------+---------------+
6 rows in set (0.00 sec)
mysql> select id, name, service, contact_name-> from profile inner join profile_contact on id = profile_id;
+----+------+---------+---------------+
| id | name | service | contact_name |
+----+------+---------+---------------+
| 1 | Fred | AIM | user1-aimid |
| 1 | Fred | MSN | user1-msnid |
| 2 | Mort | AIM | user2-aimid |
| 2 | Mort | MSN | user2-msnid |
| 2 | Mort | Yahoo | user2-yahooid |
| 4 | Carl | Yahoo | user4-yahooid |
+----+------+---------+---------------+
6 rows in set (0.00 sec)
mysql> select * from profile_contact-> where profile_id = (select id from profile where name = 'Mort');
+------------+---------+---------------+
| profile_id | service | contact_name |
+------------+---------+---------------+
| 2 | AIM | user2-aimid |
| 2 | MSN | user2-msnid |
| 2 | Yahoo | user2-yahooid |
+------------+---------+---------------+
3 rows in set (0.00 sec)
3.14 从查询结果集头或尾取出部分行
mysql> select * from profile limit 1;
+----+------+------------+-------+----------------------+------+
| id | name | birth | color | foods | cats |
+----+------+------------+-------+----------------------+------+
| 1 | Fred | 1970-04-13 | black | lutefisk,fadge,pizza | 1 |
+----+------+------------+-------+----------------------+------+
1 row in set (0.00 sec)
mysql> select * from profile limit 10;
+----+---------+------------+-------+-----------------------+------+
| id | name | birth | color | foods | cats |
+----+---------+------------+-------+-----------------------+------+
| 1 | Fred | 1970-04-13 | black | lutefisk,fadge,pizza | 1 |
| 2 | Mort | 1969-09-30 | white | burrito,curry,eggroll | 3 |
| 3 | Brit | 1957-12-01 | red | burrito,curry,pizza | 1 |
| 4 | Carl | 1973-11-02 | red | eggroll,pizza | 4 |
| 5 | Sean | 1963-07-04 | blue | burrito,curry | 5 |
| 6 | Alan | 1965-02-14 | red | curry,fadge | 1 |
| 7 | Mara | 1968-09-17 | green | lutefisk,fadge | 1 |
| 8 | Shepard | 1975-09-02 | black | curry,pizza | 2 |
| 9 | Dick | 1952-08-20 | green | lutefisk,fadge | 0 |
| 10 | Tony | 1960-05-01 | white | burrito,pizza | 0 |
+----+---------+------------+-------+-----------------------+------+
10 rows in set (0.00 sec)
首先使用order by 对查询结果进行排序，再使用limit从查询结果中取出最大或最小特征的行。
mysql> select * from profile order by birth limit 1;
+----+--------+-------+-------+-------+------+
| id | name | birth | color | foods | cats |
+----+--------+-------+-------+-------+------+
| 17 | Amabel | NULL | NULL | NULL | NULL |
+----+--------+-------+-------+-------+------+
1 row in set (0.00 sec)mysql> select * from profile order by birth desc limit 1;
+----+---------+------------+-------+---------+------+
| id | name | birth | color | foods | cats |
+----+---------+------------+-------+---------+------+
| 18 | De'Mont | 1980-12-12 | NULL | eggroll | 8 |
+----+---------+------------+-------+---------+------+
1 row in set (0.00 sec)mysql> select name, date_format(birth, '%m-%d') as birthday-> from profile order by birthday limit 1;
+------+----------+
| name | birthday |
+------+----------+
| Juan | NULL |
+------+----------+
1 row in set (0.00 sec)
3.15 在结果集中间选取部分行 185/951 Thursday, May 29, 2025
Select from：从哪些表中筛选
on：关联多表查询时，去除笛卡尔积
where：从表中筛选的条件
group by：分组依据
having：在统计结果中再次筛选
order by：排序
limit：分页
以下是对 SQL 查询执行逻辑的专业描述，并附 MySQL 可执行示例：
一、FROM 阶段（表关联处理）
1. 笛卡尔积生成（CROSS JOIN）
原理：生成所有表行的组合。若表 A 有m行，表 B 有n行，则结果包含m×n行。
示例表结构：
sql
CREATE TABLE A (id INT PRIMARY KEY, name VARCHAR(10));
CREATE TABLE B (id INT PRIMARY KEY, value INT);
INSERT INTO A VALUES (1,'Alice'), (2,'Bob');
INSERT INTO B VALUES (1,100), (2,200);执行逻辑：
sql
SELECT * FROM A CROSS JOIN B; -- 生成4行结果（2×2）结果：
plaintext
| id | name | id | value |
|----|-------|----|-------|
| 1 | Alice | 1 | 100 |
| 1 | Alice | 2 | 200 |
| 2 | Bob | 1 | 100 |
| 2 | Bob | 2 | 200 |2. ON 条件筛选
原理：通过ON子句过滤无效行。
示例：
sql
SELECT * FROM A JOIN B ON A.id = B.id; -- 仅保留id匹配的行结果：
plaintext
| id | name | id | value |
|----|-------|----|-------|
| 1 | Alice | 1 | 100 |
| 2 | Bob | 2 | 200 |3. 外部行添加（外连接）
左连接示例：
sql
SELECT * FROM A LEFT JOIN B ON A.id = B.id; -- 保留A的未匹配行结果（若 B 中无匹配行，B 的字段为NULL）：
plaintext
| id | name | id | value |
|----|-------|----|-------|
| 1 | Alice | 1 | 100 |
| 2 | Bob | 2 | 200 |
| 3 | Carol | NULL| NULL | -- 假设A新增行(3,'Carol')二、WHERE 阶段（行过滤）
原理：基于FROM结果筛选行。
示例：
sql
SELECT * FROM A JOIN B ON A.id = B.id WHERE B.value > 150;结果：
plaintext
| id | name | id | value |
|----|------|----|-------|
| 2 | Bob | 2 | 200 |三、GROUP BY 阶段（分组操作）
原理：按指定列分组，非分组列需用聚合函数。
示例：
sql
SELECT A.name, SUM(B.value)
FROM A JOIN B ON A.id = B.id
GROUP BY A.name;结果：
plaintext
| name | SUM(B.value) |
|-------|-------------|
| Alice | 100 |
| Bob | 200 |四、HAVING 阶段（分组过滤）
原理：筛选符合条件的分组。
示例：
sql
SELECT A.name, SUM(B.value)
FROM A JOIN B ON A.id = B.id
GROUP BY A.name
HAVING SUM(B.value) > 150;结果：
plaintext
| name | SUM(B.value) |
|------|-------------|
| Bob | 200 |五、SELECT 阶段（字段提取）
原理：提取指定列或表达式。
示例：
sql
SELECT A.name AS user, B.value*1.1 AS adjusted_value
FROM A JOIN B ON A.id = B.id;结果：
plaintext
| user | adjusted_value |
|-------|----------------|
| Alice | 110.0 |
| Bob | 220.0 |六、DISTINCT 阶段（去重）
原理：移除完全重复的行。
示例：
sql
SELECT DISTINCT value FROM B;结果（若 B 中有重复值100,100,200）：
plaintext
| value |
|-------|
| 100 |
| 200 |七、ORDER BY 阶段（排序）
原理：按指定列排序。
示例：
sql
SELECT * FROM B ORDER BY value DESC;结果：
plaintext
| id | value |
|----|-------|
| 2 | 200 |
| 1 | 100 |八、LIMIT 阶段（分页截取）
原理：限制返回行数。
示例：
sql
SELECT * FROM B LIMIT 1; -- 返回第1行结果：
plaintext
| id | value |
|----|-------|
| 1 | 100 |完整执行顺序示例
查询语句：
sql
SELECT A.name, SUM(B.value) AS total
FROM A
LEFT JOIN B ON A.id = B.id
WHERE A.id > 0
GROUP BY A.name
HAVING total > 50
ORDER BY total DESC
LIMIT 1;
执行步骤：
FROM+ON：左连接 A 和 B，保留 A 的未匹配行。
WHERE：过滤出 A.id>0 的行（若所有 A.id>0，则无影响）。
GROUP BY：按 A.name 分组。
HAVING：筛选总和 > 50 的分组。
SELECT：提取 A.name 和 SUM (B.value)。
ORDER BY：按总和降序排列。
LIMIT：取第 1 行。
结果（假设数据如前）：
plaintext
| name | total |
|------|-------|
| Bob | 200 |
执行顺序总结
plaintext
FROM → ON → 外连接处理 → WHERE → GROUP BY → HAVING → SELECT → DISTINCT → ORDER BY → LIMIT
二、SELECT 语句的底层执行逻辑（以 MySQL 为例）
1. 执行顺序与阶段解析
SQL 语句的书写顺序（如 SELECT ... FROM ... WHERE ... GROUP BY ... HAVING ... ORDER BY ... LIMIT）与实际执行顺序不同。以下是按执行顺序排列的核心阶段（附实例说明）：
执行阶段 说明 实例（MySQL 可执行）
1. FROM 阶段 - 解析表名，生成初始结果集（多张表时先计算笛卡尔积）
- 处理 JOIN 逻辑（内连接 / 外连接） sql<br>SELECT *<br>FROM orders o<br>LEFT JOIN users u ON o.user_id = u.id; -- 先执行 FROM + ON 条件<br>
2. ON 过滤 - 对 JOIN 生成的临时表应用 ON 条件（仅在 JOIN 时生效） 同上，ON o.user_id = u.id 在此阶段执行过滤。
3. 添加外部行 - 处理外连接（左 / 右 / 全连接），补充主表中不满足 ON 条件的行（值为 NULL） 左连接时，users 表中无匹配的 orders 行会保留，对应字段为 NULL。
4. WHERE 过滤 - 对 FROM 阶段生成的临时表应用 WHERE 条件（行级过滤） sql<br>SELECT *<br>FROM orders o<br>LEFT JOIN users u ON o.user_id = u.id<br>WHERE o.order_date > '2023-01-01'; -- WHERE 在此阶段执行<br>
5. GROUP BY 分组 - 按指定字段分组，生成分组后的临时表（行合并） sql<br>SELECT user_id, SUM(amount) AS total<br>FROM orders<br>WHERE order_date > '2023-01-01'<br>GROUP BY user_id; -- 分组在此阶段执行<br>
6. HAVING 过滤 - 对分组后的结果应用 HAVING 条件（分组级过滤） sql<br>SELECT user_id, SUM(amount) AS total<br>FROM orders<br>WHERE order_date > '2023-01-01'<br>GROUP BY user_id<br>HAVING total > 1000; -- HAVING 在此阶段执行<br>
7. SELECT 投影 - 计算 SELECT 中的表达式，提取目标字段（列选择与计算） sql<br>SELECT user_id, CONCAT('用户', user_id) AS user_label -- CONCAT 在此阶段计算<br>FROM users;<br>
8. DISTINCT 去重 - 过滤重复行，仅保留唯一记录 sql<br>SELECT DISTINCT user_id<br>FROM orders;<br>
9. ORDER BY 排序 - 对结果集按指定字段排序（内存排序或文件排序） sql<br>SELECT *<br>FROM orders<br>ORDER BY order_date DESC; -- 排序在此阶段执行<br>
10. LIMIT 分页 - 截取指定行数的结果（最后一步执行） sql<br>SELECT *<br>FROM orders<br>ORDER BY order_date DESC<br>LIMIT 10 OFFSET 20; -- LIMIT 在此阶段执行<br>
2. 关键逻辑说明
ON 与 WHERE 的区别：
ON 用于 表关联时的条件过滤，发生在 FROM 阶段，可访问未过滤的原始表数据（如外连接中主表的 NULL 值）。
WHERE 用于 对最终临时表的行级过滤，发生在 FROM 和 JOIN 之后，无法访问外连接中未匹配的 NULL 行（会被提前过滤）。
示例：
sql
-- 左连接中，ON 可保留主表 NULL 行，WHERE 会过滤 NULL 行
SELECT *
FROM orders o
LEFT JOIN users u ON o.user_id = u.id AND u.status = 'active'; -- ON 条件中包含过滤逻辑
-- 等价于：先左连接，再保留 u.status='active' 的匹配行，主表 NULL 行保留 SELECT *
FROM orders o
LEFT JOIN users u ON o.user_id = u.id
WHERE u.status = 'active'; -- WHERE 会过滤掉 u 为 NULL 的行（主表无匹配的记录被删除） 分组与聚合的逻辑：
GROUP BY 会将相同分组字段的行合并为一行，并通过聚合函数（如 SUM、COUNT）计算分组值。
HAVING 必须依赖 GROUP BY，用于过滤分组后的结果（例如 “筛选总金额超过 1000 的用户分组”）。
性能优化关键点：
尽早过滤：将过滤条件（如 WHERE）放在靠前的阶段（避免处理无效数据）。
避免 SELECT *：仅选择需要的字段，减少数据传输和计算量。
合理使用索引：在 JOIN、WHERE、ORDER BY 字段上创建索引，加速过滤和排序。
三、总结
SELECT 语句的底层逻辑是 「按阶段逐步处理数据，从表关联到最终结果集生成」，执行顺序与书写顺序不同。理解这一逻辑有助于优化 SQL 性能，避免逻辑错误（如误用 ON 和 WHERE）。实际开发中，可通过 MySQL 的 EXPLAIN 语句分析执行计划，验证优化效果。
mysql> select count(*) from profile;
+----------+
| count(*) |
+----------+
| 19 |
+----------+
1 row in set (0.02 sec)mysql> select sql_calc_found_rows * from profile order by name limit 5;
+----+--------+------------+-------+---------------------+------+
| id | name | birth | color | foods | cats |
+----+--------+------------+-------+---------------------+------+
| 6 | Alan | 1965-02-14 | red | curry,fadge | 1 |
| 11 | Alison | 1973-01-12 | blue | eggroll | 4 |
| 17 | Amabel | NULL | NULL | NULL | NULL |
| 3 | Brit | 1957-12-01 | red | burrito,curry,pizza | 1 |
| 4 | Carl | 1973-11-02 | red | eggroll,pizza | 4 |
+----+--------+------------+-------+---------------------+------+
5 rows in set, 1 warning (0.00 sec)mysql> select found_rows();
+--------------+
| found_rows() |
+--------------+
| 19 |
+--------------+
1 row in set, 1 warning (0.01 sec)3.16 选择合适的limit参数
mysql> select sql_calc_found_rows * from profile order by name limit 5;
+----+--------+------------+-------+---------------------+------+
| id | name | birth | color | foods | cats |
+----+--------+------------+-------+---------------------+------+
| 6 | Alan | 1965-02-14 | red | curry,fadge | 1 |
| 11 | Alison | 1973-01-12 | blue | eggroll | 4 |
| 17 | Amabel | NULL | NULL | NULL | NULL |
| 3 | Brit | 1957-12-01 | red | burrito,curry,pizza | 1 |
| 4 | Carl | 1973-11-02 | red | eggroll,pizza | 4 |
+----+--------+------------+-------+---------------------+------+
5 rows in set, 1 warning (0.00 sec)mysql> select found_rows();
+--------------+
| found_rows() |
+--------------+
| 19 |
+--------------+
1 row in set, 1 warning (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 (0.02 sec)mysql> select name, wins from al_winner-> order by wins desc, name-> limit 5;
+----------------+------+
| name | wins |
+----------------+------+
| Mulder, Mark | 21 |
| Clemens, Roger | 20 |
| Moyer, Jamie | 20 |
| Garcia, Freddy | 18 |
| Hudson, Tim | 18 |
+----------------+------+
5 rows in set (0.00 sec)mysql> select name, wins from al_winner-> order by wins desc, name-> limit 3, 1;
+----------------+------+
| name | wins |
+----------------+------+
| Garcia, Freddy | 18 |
+----------------+------+
1 row in set (0.00 sec)mysql> select name, wins from al_winner-> where wins >= 18-> order by wins desc, name;
+----------------+------+
| name | wins |
+----------------+------+
| Mulder, Mark | 21 |
| Clemens, Roger | 20 |
| Moyer, Jamie | 20 |
| Garcia, Freddy | 18 |
| Hudson, Tim | 18 |
+----------------+------+
5 rows in set (0.00 sec)mysql> SELECT name, wins-> FROM al_winner-> WHERE wins >= (-> SELECT wins-> FROM al_winner-> ORDER BY wins DESC, name-> LIMIT 3, 1 -- 英文逗号+英文括号-> )-> ORDER BY wins DESC, name;
+----------------+------+
| name | wins |
+----------------+------+
| Mulder, Mark | 21 |
| Clemens, Roger | 20 |
| Moyer, Jamie | 20 |
| Garcia, Freddy | 18 |
| Hudson, Tim | 18 |
+----------------+------+
5 rows in set (0.00 sec)mysql> SELECT DISTINCT wins-> FROM al_winner-> ORDER BY wins DESC -- 仅保留 wins 列-> LIMIT 3, 1;
+------+
| wins |
+------+
| 17 |
+------+
1 row in set (0.00 sec)mysql> SELECT wins-> FROM (-> SELECT wins,-> DENSE_RANK() OVER (ORDER BY wins DESC) AS rnk-> FROM al_winner-> ) t-> WHERE rnk = 4 -- 直接获取第4名的 wins-> LIMIT 1;
+------+
| wins |
+------+
| 17 |
+------+
1 row in set (0.01 sec)
总结
使用 DISTINCT 时：ORDER BY 必须仅引用 SELECT 列表中的列。
窗口函数：处理排名问题更直观，推荐优先使用（如 RANK()、DENSE_RANK()）。
性能提示：若表数据量大，添加 INDEX(wins) 可加速排序和去重。
mysql> select name, wins from al_winner-> where wins >= 17-> 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 |
+----------------+------+
10 rows in set (0.00 sec)
原查询的问题在于子查询的 ORDER BY name 与 DISTINCT wins 不兼容。通过移除 name 解决了这个问题，同时确保主查询逻辑不变。
若你的 MySQL 版本支持窗口函数（8.0+），强烈推荐使用方案 2，它更健壮且易于理解。
mysql> SELECT name, wins-> FROM al_winner-> WHERE wins >= (-> SELECT DISTINCT wins -- 仅选择 wins 列-> FROM al_winner-> ORDER BY wins DESC -- 仅按 wins 排序-> LIMIT 3, 1 -- 获取第4高的 wins 值-> )-> ORDER BY wins DESC, name; -- 主查询排序正常
+----------------+------+
| 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 |
+----------------+------+
10 rows in set (0.00 sec)mysql> SELECT name, wins-> FROM al_winner-> WHERE wins >= (-> SELECT wins-> FROM (-> SELECT DISTINCT wins,-> DENSE_RANK() OVER (ORDER BY wins DESC) AS rnk -- 计算排名-> FROM al_winner-> ) t-> WHERE rnk = 4 -- 直接获取第4名的 wins-> );
+----------------+------+
| name | wins |
+----------------+------+
| Abbott, Paul | 17 |
| Clemens, Roger | 20 |
| Garcia, Freddy | 18 |
| Hudson, Tim | 18 |
| Mays, Joe | 17 |
| Moyer, Jamie | 20 |
| Mulder, Mark | 21 |
| Mussina, Mike | 17 |
| Sabathia, C.C. | 17 |
| Zito, Barry | 17 |
+----------------+------+
10 rows in set (0.00 sec)
3.17 当limit需要“错误”的排列顺序时做什么
mysql> select name, birth from profile order by birth desc limit 4;
+---------+------------+
| name | birth |
+---------+------------+
| De'Mont | 1980-12-12 |
| Shepard | 1975-09-02 |
| Carl | 1973-11-02 |
| Alison | 1973-01-12 |
+---------+------------+
4 rows in set (0.00 sec)mysql> select count(*) from profile;
+----------+
| count(*) |
+----------+
| 19 |
+----------+
1 row in set (0.00 sec)mysql> select name, birth from profile order by birth desc limit 6, 4;
+---------+------------+
| name | birth |
+---------+------------+
| De'Mont | 1973-01-12 |
| De'Mont | 1973-01-12 |
| De'Mont | 1973-01-12 |
| Fred | 1970-04-13 |
+---------+------------+
4 rows in set (0.00 sec)mysql> select * from-> (select name, birth from profile order by birth desc limit 5) as t-> order by birth;
+---------+------------+
| name | birth |
+---------+------------+
| De'Mont | 1973-01-12 |
| Alison | 1973-01-12 |
| Carl | 1973-11-02 |
| Shepard | 1975-09-02 |
| De'Mont | 1980-12-12 |
+---------+------------+
5 rows in set (0.00 sec)
3.18 从表达式中计算limit值
步骤解释：
定义变量 @limit_val。
使用 CONCAT 构建 SQL 字符串，将变量值嵌入其中。
通过 PREPARE 和 EXECUTE 执行动态生成的 SQL，确保变量被正确解析为整数。
mysql> SET @limit_val = 5 + 5;
Query OK, 0 rows affected (0.00 sec)mysql> SET @sql = CONCAT('SELECT * FROM profile LIMIT ', @limit_val);
Query OK, 0 rows affected (0.00 sec)mysql>
mysql> PREPARE stmt FROM @sql;
Query OK, 0 rows affected (0.00 sec)
Statement preparedmysql> EXECUTE stmt;
+----+---------+------------+-------+-----------------------+------+
| id | name | birth | color | foods | cats |
+----+---------+------------+-------+-----------------------+------+
| 1 | Fred | 1970-04-13 | black | lutefisk,fadge,pizza | 1 |
| 2 | Mort | 1969-09-30 | white | burrito,curry,eggroll | 3 |
| 3 | Brit | 1957-12-01 | red | burrito,curry,pizza | 1 |
| 4 | Carl | 1973-11-02 | red | eggroll,pizza | 4 |
| 5 | Sean | 1963-07-04 | blue | burrito,curry | 5 |
| 6 | Alan | 1965-02-14 | red | curry,fadge | 1 |
| 7 | Mara | 1968-09-17 | green | lutefisk,fadge | 1 |
| 8 | Shepard | 1975-09-02 | black | curry,pizza | 2 |
| 9 | Dick | 1952-08-20 | green | lutefisk,fadge | 0 |
| 10 | Tony | 1960-05-01 | white | burrito,pizza | 0 |
+----+---------+------------+-------+-----------------------+------+
10 rows in set (0.00 sec)mysql> DEALLOCATE PREPARE stmt;
使用存储过程（简化动态查询）
若需要频繁执行类似查询，可创建存储过程：
mysql> DELIMITER $$
mysql> CREATE PROCEDURE GetProfile(IN limit_val INT)-> BEGIN-> SELECT * FROM profile LIMIT limit_val;-> END$$
Query OK, 0 rows affected (0.04 sec)mysql> DELIMITER ;
mysql>
mysql> -- 调用存储过程
mysql> CALL GetProfile(10); -- 直接传入数值
+----+---------+------------+-------+-----------------------+------+
| id | name | birth | color | foods | cats |
+----+---------+------------+-------+-----------------------+------+
| 1 | Fred | 1970-04-13 | black | lutefisk,fadge,pizza | 1 |
| 2 | Mort | 1969-09-30 | white | burrito,curry,eggroll | 3 |
| 3 | Brit | 1957-12-01 | red | burrito,curry,pizza | 1 |
| 4 | Carl | 1973-11-02 | red | eggroll,pizza | 4 |
| 5 | Sean | 1963-07-04 | blue | burrito,curry | 5 |
| 6 | Alan | 1965-02-14 | red | curry,fadge | 1 |
| 7 | Mara | 1968-09-17 | green | lutefisk,fadge | 1 |
| 8 | Shepard | 1975-09-02 | black | curry,pizza | 2 |
| 9 | Dick | 1952-08-20 | green | lutefisk,fadge | 0 |
| 10 | Tony | 1960-05-01 | white | burrito,pizza | 0 |
+----+---------+------------+-------+-----------------------+------+
10 rows in set (0.01 sec)Query OK, 0 rows affected (0.21 sec)mysql> -- 或使用变量
mysql> SET @limit = 5 + 5;
Query OK, 0 rows affected (0.00 sec)mysql> CALL GetProfile(@limit);
+----+---------+------------+-------+-----------------------+------+
| id | name | birth | color | foods | cats |
+----+---------+------------+-------+-----------------------+------+
| 1 | Fred | 1970-04-13 | black | lutefisk,fadge,pizza | 1 |
| 2 | Mort | 1969-09-30 | white | burrito,curry,eggroll | 3 |
| 3 | Brit | 1957-12-01 | red | burrito,curry,pizza | 1 |
| 4 | Carl | 1973-11-02 | red | eggroll,pizza | 4 |
| 5 | Sean | 1963-07-04 | blue | burrito,curry | 5 |
| 6 | Alan | 1965-02-14 | red | curry,fadge | 1 |
| 7 | Mara | 1968-09-17 | green | lutefisk,fadge | 1 |
| 8 | Shepard | 1975-09-02 | black | curry,pizza | 2 |
| 9 | Dick | 1952-08-20 | green | lutefisk,fadge | 0 |
| 10 | Tony | 1960-05-01 | white | burrito,pizza | 0 |
+----+---------+------------+-------+-----------------------+------+
10 rows in set (0.00 sec)Query OK, 0 rows affected (0.06 sec)
如果你需要频繁执行类似查询，创建存储过程更简洁：
– 创建存储过程
DELIMITER $$
CREATE PROCEDURE GetProfile(IN limit_val INT)
BEGINSELECT * FROM profile LIMIT limit_val;
END$$
DELIMITER ;-- 调用存储过程
CALL GetProfile(10); -- 直接传入数值
-- 或使用变量
SET @limit = 5 + 5;
CALL GetProfile(@limit);
-------------------------------------------------------------
使用 PREPARE 语句
若需要动态生成 LIMIT 参数，可结合 预处理语句（Prepared Statements）：
优点：安全处理动态参数，避免 SQL 注入。
mysql> SET @skip = 5;
Query OK, 0 rows affected (0.00 sec)mysql> SET @show = 5;
Query OK, 0 rows affected (0.00 sec)mysql> SET @sql = CONCAT('SELECT * FROM profile LIMIT ', @skip, ', ', @show);
Query OK, 0 rows affected (0.00 sec)mysql>
mysql> PREPARE stmt FROM @sql;
Query OK, 0 rows affected (0.01 sec)
Statement preparedmysql> EXECUTE stmt;
+----+---------+------------+-------+----------------+------+
| id | name | birth | color | foods | cats |
+----+---------+------------+-------+----------------+------+
| 6 | Alan | 1965-02-14 | red | curry,fadge | 1 |
| 7 | Mara | 1968-09-17 | green | lutefisk,fadge | 1 |
| 8 | Shepard | 1975-09-02 | black | curry,pizza | 2 |
| 9 | Dick | 1952-08-20 | green | lutefisk,fadge | 0 |
| 10 | Tony | 1960-05-01 | white | burrito,pizza | 0 |
+----+---------+------------+-------+----------------+------+
5 rows in set (0.00 sec)mysql> DEALLOCATE PREPARE stmt;
Query OK, 0 rows affected (0.00 sec)