Oracle prior用于设置约束条件，确保数据的完整性和一致性，包括主键、外键、唯一性等约束。
Oracle中的Prior关键字用于在查询中引用先前的行值，它的主要作用是在执行一些聚合操作时，可以访问到当前行的前一行的值，以下是Prior关键字的一些主要作用：
1、累计求和
当需要对一个分组进行累计求和时，可以使用Prior关键字来实现，计算每个部门的累计工资总额：
SELECT department_id, salary, salary + PRIOR salary AS cumulative_salary FROM employees ORDER BY department_id, salary;
2、累积计数
当需要对一个分组进行累积计数时，也可以使用Prior关键字，计算每个部门的员工数量：
SELECT department_id, COUNT(*) AS employee_count, COUNT(*) + PRIOR COUNT(*) AS cumulative_count FROM employees GROUP BY department_id ORDER BY department_id;
3、计算移动平均值
当需要计算一个分组的移动平均值时，可以使用Prior关键字，计算每个部门的工资移动平均值：
SELECT department_id, salary, salary / (1 + PRIOR salary) AS moving_average_salary FROM employees ORDER BY department_id, salary;
4、计算累积百分比
当需要计算一个分组的累积百分比时，也可以使用Prior关键字，计算每个部门的员工占总员工数的百分比：
SELECT department_id, COUNT(*) AS employee_count, COUNT(*) * 100 / (SELECT COUNT(*) FROM employees) AS cumulative_percentage,
COUNT(*) * 100 / (SELECT COUNT(*) FROM employees) + PRIOR COUNT(*) * 100 / (SELECT COUNT(*) FROM employees) AS cumulative_percentage_with_prior
FROM employees
GROUP BY department_id
ORDER BY department_id;
5、生成连续的日期序列
当需要生成一个连续的日期序列时，可以使用Prior关键字，生成一个包含每个月最后一天的日期序列：
WITH date_series AS ( SELECT TRUNC(SYSDATE, 'MM') + LEVEL 1 AS date, LEVEL 1 AS month FROM DUAL CONNECT BY LEVEL <= 12 * (TRUNC(SYSDATE, 'YY') TRUNC(SYSDATE, 'YY') 1) + 12 TRUNC(SYSDATE, 'MM') + 1 ), last_days AS ( SELECT date, month, LAST_DAY(date) AS last_day, LAST_DAY(date) date + 1 AS days_in_month, LAST_DAY(date) TRUNC(LAST_DAY(date), 'MM') + 1 AS days_in_previous_months FROM date_series ds, last_days ls WHERE ds.date = ls.last_day AND ds.month = ls.month ls.days_in_previous_months + 1 ) SELECT date, month, days_in_month, days_in_previous_months, days_in_month + PRIOR days_in_month AS cumulative_days_in_month, days_in_month + PRIOR days_in_month PRIOR days_in_month AS cumulative_days_in_month_with_prior FROM last_days;
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