MYSQL：从两个不同的表中选择VARCHAR字段中的最大值

Question

我是MYSQL的新手，有两个表，即表A和表B.只需要两个不同表中的Max值即可。
预期产量：

AG/2016-17/P/046

  table_A                                 
       In_No
     AG/2016-17/P/01
     AG/2016-17/P/029
     AG/2016-17/P/030

     table_B
      In_No
 AG/2016-17/P/01
 AG/2016-17/P/046
 AG/2016-17/P/015

SQL：

select MAX(bv) from(
SELECT MAX(CAST(SUBSTRING(In_No, 14, length(In_No)-3) bv AS UNSIGNED)) FROM table_A
union all
SELECT MAX(CAST(SUBSTRING(In_No, 14, length(In_No)-3) bv AS UNSIGNED)) FROM table_B) as a

出问题了，它没有显示正确的输出，请建议我，谢谢。

Answer 1

尝试这个：

SELECT In_No, substring_index(In_No, '/', 2) AS In_No_sub
FROM (
    SELECT In_No FROM table_A
    UNION ALL
    SELECT In_No FROM table_B
) t
ORDER BY substring_index(In_No, '/', -1) + 0 DESC
LIMIT 1

SQLFiddle演示

Answer 2

尝试这个

select MAX(bv) 
from(
    SELECT MAX(CAST(SUBSTRING(In_No, 14, length(In_No)) as UNSIGNED INTEGER)) as bv FROM table_A
    union all
    SELECT MAX(CAST(SUBSTRING(In_No, 14, length(In_No)) as UNSIGNED INTEGER)) as bv FROM table_B
)  as a