读SQL进阶教程笔记05_关联子查询
1. 关联子查询
1.1. 关联子查询和自连接在很多时候都是等价的
1.2. 使用SQL进行行间比较时，发挥主要作用的技术是关联子查询，特别是与自连接相结合的“自关联子查询”
1.3. 缺点
[*]1.3.1. 代码的可读性不好
[*]1.3.1.1. 特别是在计算累计值和移动平均值的例题里，与聚合一起使用后，其内部处理过程非常难理解
[*]1.3.2. 性能不好
[*]1.3.2.1. 特别是在SELECT子句里使用标量子查询时，性能可能会变差
2. 增长、减少、维持现状
2.1. 使用基于时间序列的表进行时间序列分析
2.2. 示例
[*]2.2.1. --求与上一年营业额一样的年份(1)：使用关联子查询
SELECT year, sale
FROM Sales S1
WHERE sale = (SELECT sale
FROM Sales S2
WHERE S2.year = S1.year -1)
ORDER BY year;
[*]2.2.2. S2.year = S1.year -1这个条件起到了将要比较的数据偏移一行的作用
[*]2.2.3. --求与上一年营业额一样的年份(2)：使用自连接
SELECT S1.year, S1.sale
FROM Sales S1,
Sales S2
WHERE S2.sale = S1.sale
AND S2.year = S1.year -1
ORDER BY year;3. 用列表展示与上一年的比较结果
3.1. 示例
[*]3.1.1. --求出是增长了还是减少了，抑或是维持现状(1)：使用关联子查询
SELECT S1.year, S1.sale,
CASE WHEN sale =
(SELECT sale
FROM Sales S2
WHERE S2.year = S1.year -1) THEN'→'--持平
WHEN sale ＞
(SELECT sale
FROM Sales S2
WHERE S2.year = S1.year -1) THEN'↑'--增长
WHEN sale ＜
(SELECT sale
FROM Sales S2
WHERE S2.year = S1.year -1) THEN'↓'--减少
ELSE'—'END AS var
FROM Sales S1
ORDER BY year;
[*]3.1.2. --求出是增长了还是减少了，抑或是维持现状(2)：使用自连接查询（最早的年份不会出现在结果里）
SELECT S1.year, S1.sale,
CASE WHEN S1.sale = S2.sale THEN'→'
WHEN S1.sale ＞ S2.sale THEN'↑'
WHEN S1.sale ＜ S2.sale THEN'↓'
ELSE'—'END AS var
FROM Sales S1, Sales S2
WHERE S2.year = S1.year -1
ORDER BY year;4. 时间轴有间断时
4.1. 和过去最临近的时间进行比较
4.2. 示例
[*]4.2.1. --查询与过去最临近的年份营业额相同的年份
SELECT year, sale
FROM Sales2 S1
WHERE sale =
(SELECT sale
FROM Sales2 S2
WHERE S2.year =
(SELECT MAX(year) --条件2：在满足条件1的年份中，年份最早的一个
FROM Sales2 S3
WHERE S1.year ＞ S3.year)) --条件1：与该年份相比是过去的年份
ORDER BY year;
[*]4.2.2. 自连接版本
SELECT S1.year AS year,
S1.year AS year
FROM Sales2 S1, Sales2 S2
WHERE S1.sale = S2.sale
AND S2.year = (SELECT MAX(year)
FROM Sales2 S3
WHERE S1.year ＞ S3.year)
ORDER BY year;
[*]4.2.3. --求每一年与过去最临近的年份之间的营业额之差(1)：结果里不包含最早的年份
SELECT S2.year AS pre_year,
S1.year AS now_year,
S2.sale AS pre_sale,
S1.sale AS now_sale,
S1.sale - S2.sale AS diff
FROM Sales2 S1, Sales2 S2
WHERE S2.year = (SELECT MAX(year)
FROM Sales2 S3
WHERE S1.year ＞ S3.year)
ORDER BY now_year;
[*]4.2.4. --求每一年与过去最临近的年份之间的营业额之差(1)：结果里不包含最早的年份
SELECT S2.year AS pre_year,
S1.year AS now_year,
S2.sale AS pre_sale,
S1.sale AS now_sale,
S1.sale - S2.sale AS diff
FROM Sales2 S1, Sales2 S2
WHERE S2.year = (SELECT MAX(year)
FROM Sales2 S3
WHERE S1.year ＞ S3.year)
ORDER BY now_year;
[*]4.2.5. 使用极值函数时会发生排序
5. 移动累计值和移动平均值
5.1. 示例
[*]5.1.1. --求累计值：使用窗口函数
SELECT prc_date, prc_amt,
SUM(prc_amt) OVER (ORDER BY prc_date) AS onhand_amt
FROM Accounts;
[*]5.1.2. 引入窗口函数的目的原本就是解决这类问题，因此这里的代码非常简洁
[*]5.1.2.1. 如果选用的数据库支持窗口函数，也可以考虑使用窗口函数
[*]5.1.3. 从性能方面来看，表的扫描和数据排序也都只进行了一次
[*]5.1.3.1. 依赖于具体的数据库的
[*]5.1.4. --求累计值：使用冯·诺依曼型递归集合
SELECT prc_date, A1.prc_amt,
(SELECT SUM(prc_amt)
FROM Accounts A2
WHERE A1.prc_date ＞= A2.prc_date ) AS onhand_amt
FROM Accounts A1
ORDER BY prc_date;
[*]5.1.5. --求移动累计值(1)：使用窗口函数
SELECT prc_date, prc_amt,
SUM(prc_amt) OVER (ORDER BY prc_date
ROWS 2 PRECEDING) AS onhand_amt
FROM Accounts;
[*]5.1.6. --求移动累计值(2)：不满3行的时间区间也输出
SELECT prc_date, A1.prc_amt,
(SELECT SUM(prc_amt)
FROM Accounts A2
WHERE A1.prc_date ＞= A2.prc_date
AND (SELECT COUNT(＊)
FROM Accounts A3
WHERE A3.prc_date
BETWEEN A2.prc_date AND A1.prc_date ) ＜= 3 )
AS mvg_sum
FROM Accounts A1
ORDER BY prc_date;
[*]5.1.7. A3.prc_date在以A2.prc_date为起点，以A1.prc_date为终点的区间内移动
[*]5.1.8. --移动累计值(3)：不满3行的区间按无效处理
SELECT prc_date, A1.prc_amt,
(SELECT SUM(prc_amt)
FROM Accounts A2
WHERE A1.prc_date ＞= A2.prc_date
AND (SELECT COUNT(＊)
FROM Accounts A3
WHERE A3.prc_date
BETWEEN A2.prc_date AND A1.prc_date ) ＜= 3
HAVING COUNT(＊) =3) AS mvg_sum --不满3行数据的不显示
FROM Accounts A1
ORDER BY prc_date;5.2. 基本思路是使用冯·诺依曼型递归集合
6. 查询重叠的时间区间
6.1. 示例
[*]6.1.1. --求重叠的住宿期间
SELECT reserver, start_date, end_date
FROM Reservations R1
WHERE EXISTS
(SELECT ＊
FROM Reservations R2
WHERE R1.reserver ＜＞ R2.reserver --与自己以外的客人进行比较
AND ( R1.start_date BETWEEN R2.start_date AND R2.end_date
--条件(1)：自己的入住日期在他人的住宿期间内
OR R1.end_date BETWEEN R2.start_date AND R2.end_date));
--条件(2)：自己的离店日期在他人的住宿期间内
[*]6.1.2. --升级版：把完全包含别人的住宿期间的情况也输出
SELECT reserver, start_date, end_date
FROM Reservations R1
WHERE EXISTS
(SELECT ＊
FROM Reservations R2
WHERE R1.reserver ＜＞ R2.reserver
AND ( ( R1.start_date BETWEEN R2.start_date
AND R2.end_date
OR R1.end_date BETWEEN R2.start_date
AND R2.end_date)
OR ( R2.start_date BETWEEN R1.start_date
AND R1.end_date
AND R2.end_date BETWEEN R1.start_date
AND R1.end_date)));
来源:https://www.cnblogs.com/lying7/p/17277869.html
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