325 lines
6.5 KiB
Markdown
325 lines
6.5 KiB
Markdown
# SQL 讲解 —— 行转列 与 列转行
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## 行转列
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### 题目1
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#### 描述
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```
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name subject score
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张三 语文 78
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张三 数学 88
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张三 英语 98
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李四 语文 89
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李四 数学 76
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李四 英语 90
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王五 语文 99
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王五 数学 66
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王五 英语 91
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name 语文 数学 英语
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张三 78 88 98
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李四 89 76 90
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王五 99 66 91
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```
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#### 脚本
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```sql
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create table SQL_1
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(
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name varchar(20),
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subject varchar(20),
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score float
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);
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insert into SQL_1 (name, subject, score) values ('张三', '语文', 78);
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insert into SQL_1 (name, subject, score) values ('张三', '数学', 88);
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insert into SQL_1 (name, subject, score) values ('张三', '英语', 98);
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insert into SQL_1 (name, subject, score) values ('李四', '语文', 89);
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insert into SQL_1 (name, subject, score) values ('李四', '数学', 76);
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insert into SQL_1 (name, subject, score) values ('李四', '英语', 90);
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insert into SQL_1 (name, subject, score) values ('王五', '语文', 99);
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insert into SQL_1 (name, subject, score) values ('王五', '数学', 66);
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insert into SQL_1 (name, subject, score) values ('王五', '英语', 91);
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select * from SQL_1;
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```
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#### 解题步骤
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1） 确定分组列，转换列，数据列
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2） 生成伪列
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3） 做分组查询
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4） 选择合适的聚合函数
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#### 两步法
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##### 公式：
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```sql
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select 分组列,
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聚合函数(m1) as 列名1,
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聚合函数(m2) as 列名2,
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聚合函数(m3) as 列名3
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from (select *,
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case 转换列 when 转换列值1 then 数据列 else ... end as m1,
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case 转换列 when 转换列值2 then 数据列 else ... end as m2,
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case 转换列 when 转换列值3 then 数据列 else ... end as m3
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from 表名) 临时表名
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group by 分组列;
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```
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##### 解题SQL
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```sql
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select name,
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sum(m1) as 语文,
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sum(m2) as 数学,
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sum(m3) as 英语
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from (select *,
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case subject when '语文' then score else 0 end as m1,
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case subject when '数学' then score else 0 end as m2,
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case subject when '英语' then score else 0 end as m3
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from sql_1) tmp
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group by name;
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```
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#### 一步法
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##### 公式：
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```sql
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select 分组列，
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聚合函数(case 转换列 when 转换列值1 then 数据列 else ... end) as 列名1,
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聚合函数(case 转换列 when 转换列值2 then 数据列 else ... end) as 列名2,
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聚合函数(case 转换列 when 转换列值3 then 数据列 else ... end) as 列名3
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...
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from 表名
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group by 分组列;
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select 分组列，
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聚合函数(case when 转换列=转换列值1 then 数据列 else ... end) as 列名1,
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聚合函数(case when 转换列=转换列值2 then 数据列 else ... end) as 列名2,
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聚合函数(case when 转换列=转换列值3 then 数据列 else ... end) as 列名3
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...
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from 表名
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group by 分组列;
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```
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##### 解题SQL
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```sql
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select name,
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sum(case subject when '语文' then score else 0 end) as 语文,
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sum(case subject when '数学' then score else 0 end) as 数学,
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sum(case subject when '英语' then score else 0 end) as 英语
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from sql_1
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group by name;
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select name,
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sum(case when subject = '语文' then score else 0 end) as 语文,
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sum(case when subject = '数学' then score else 0 end) as 数学,
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sum(case when subject = '英语' then score else 0 end) as 英语
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from sql_1
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group by name;
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```
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### 题目2
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#### 描述
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```txt
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# 日期 结果
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# 2022-01-01 胜
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# 2022-01-01 胜
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# 2022-01-02 负
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# 2022-01-02 负
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# 2022-01-01 负
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# 2022-01-02 负
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# 2022-01-02 胜
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# 日期 胜 负
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# 2022-01-01 2 1
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# 2022-01-02 1 3
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```
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#### 脚本
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```sql
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create table SQL_2(
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ddate varchar(10), result varchar(2)
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);
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insert into SQL_2 (ddate, result) values('2022-01-01','胜');
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insert into SQL_2 (ddate, result) values('2022-01-01','胜');
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insert into SQL_2 (ddate, result) values('2022-01-02','负');
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insert into SQL_2 (ddate, result) values('2022-01-02','负');
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insert into SQL_2 (ddate, result) values('2022-01-01','负');
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insert into SQL_2 (ddate, result) values('2022-01-02','负');
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insert into SQL_2 (ddate, result) values('2022-01-02','胜');
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select * from SQL_2;
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```
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#### 解题SQL
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```sql
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select ddate,
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sum(case when result = '胜' then 1 else 0 end) as 胜,
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sum(case when result = '负' then 1 else 0 end) as 负
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from sql_2
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group by ddate;
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```
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## 列转行
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### 题目3
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#### 描述
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```txt
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name 语文 数学 英语
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张三 78 88 98
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李四 89 76 90
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王五 99 66 91
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name subject score
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张三 语文 78
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张三 数学 88
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张三 英语 98
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李四 语文 89
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李四 数学 76
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李四 英语 90
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王五 语文 99
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王五 数学 66
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王五 英语 91
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```
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#### 脚本
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```sql
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CREATE TABLE SQL_3 (
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name varchar(20),
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语文 float,
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数学 float,
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英语 float
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);
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insert into SQL_3 (name, '语文', '数学', '英语') values ('张三', 78, 88, 98);
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insert into SQL_3 (name, '语文', '数学', '英语') values ('李四', 89, 76, 90);
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insert into SQL_3 (name, '语文', '数学', '英语') values ('王五', 99, 66, 91);
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```
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#### 解题步骤
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1） 确定转换列，非转换列
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2） 生成新列
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3） 使用union或union all 进行合并
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4） 根据需要进行order by
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#### 公式
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```sql
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SELECT 非转换列, '转换列1' AS 新转换列名, 转换列1 AS 新数据列名 FROM 表名
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UNION ALL
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SELECT 非转换列, '转换列2' AS 新转换列名, 转换列2 AS 新数据列名 FROM 表名
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UNION ALL
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SELECT 非转换列, '转换列3' AS 新转换列名, 转换列3 AS 新数据列名 FROM 表名
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ORDER BY ...;
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```
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#### 解题SQL
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```sql
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SELECT name,'语文' AS subject,语文 AS score FROM SQL_3
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UNION ALL
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SELECT name,'数学' AS subject,数学 AS score FROM SQL_3
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UNION ALL
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SELECT name,'英语' AS subject,英语 AS score FROM SQL_3
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ORDER BY name ASC, subject DESC;
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```
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### 题目4
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#### 描述
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```txt
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Q1 Q2 Q3 Q4
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1000 2000 3000 4000
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季度 业绩
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Q1 1000
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Q2 2000
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Q3 3000
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Q4 4000
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```
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#### 脚本
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```sql
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CREATE TABLE SQL_4 (
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Q1 int, Q2 int, Q3 int, Q4 int
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);
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insert into SQL_4 values (1000, 2000, 3000, 4000);
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```
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#### 解题SQL
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```sql
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SELECT 'Q1' AS 季度, Q1 AS 业绩 FROM SQL_4
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UNION ALL
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SELECT 'Q2' AS 季度, Q2 AS 业绩 FROM SQL_4
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UNION ALL
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SELECT 'Q3' AS 季度, Q3 AS 业绩 FROM SQL_4
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UNION ALL
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SELECT 'Q4' AS 季度, Q4 AS 业绩 FROM SQL_4
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ORDER BY 季度;
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```
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### 技巧：
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扩展列：select ... as 新列名
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减少列：直接不写
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扩展行：union/ union all
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减少行: 聚合函数
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