Oracle 从多个表中选择最大值答案

【问题标题】：Oracle select max from multiply tablesOracle 从多个表中选择最大值
【发布时间】：2020-05-28 18:12:03
【问题描述】：

我有这个问题。它不起作用。我需要从 3 个表中的字段中选择 MAX，其中年份 = 2002。这段代码给了我“order_number”：invalid identifier”。但它存在于表中。出了什么问题。使用别名的同样错误：

select MAX(ROUND(to_number(replace(substr(order_number,0, INSTR(a.order_number,'-')-1),'.',',')))) from(
SELECT e.order_date FROM employee_trip_orders e
where EXTRACT(YEAR FROM e.order_date)='2020' 
union all
SELECT p.order_date FROM postgraduate_trip_orders p
where EXTRACT(YEAR FROM p.order_date)='2020' 
union all
SELECT s.order_date FROM student_trip_orders s
where EXTRACT(YEAR FROM s.order_date)='2020' 
) a

提前致谢）

【问题讨论】：

标签： sql oracle select max union

【解决方案1】：

您从未在联合子查询中选择 order_number，因此 Oracle 告诉您此列在外部选择中不可用。试试这个版本：

SELECT MAX(ROUND(TO_NUMBER(REPLACE(SUBSTR(order_number, 0, INSTR(order_number, '-') - 1), '.', ','))))
FROM
(
    SELECT order_number FROM employee_trip_orders EXTRACT(YEAR FROM order_date) = 2020 UNION ALL
    SELECT order_number FROM postgraduate_trip_orders EXTRACT(YEAR FROM order_date) = 2020 UNION ALL
    SELECT order_number FROM student_trip_orders EXTRACT(YEAR FROM order_date) = 2020
) t;

【讨论】：