MySQL—SELECT中计算列的访问值

我有一个查询，它使用

LEFT JOIN

和

GROUP\u CONCAT

生成一些计算列：

SET @this_year = YEAR(CURDATE());
SET @next_year = YEAR(CURDATE()) +1;
SET @last_year = YEAR(CURDATE()) -1;

SELECT
    m.id, m.name,

    GROUP_CONCAT(`se1`.`name` ORDER BY `se1`.`id` ASC SEPARATOR ', ') AS this_year_events,
    GROUP_CONCAT(`se2`.`name` ORDER BY `se2`.`id` ASC SEPARATOR ', ') AS next_year_months,
    GROUP_CONCAT(`se3`.`name` ORDER BY `se3`.`id` ASC SEPARATOR ', ') AS last_year_months

FROM month m

LEFT JOIN schedule_event se1 ON
    (m.start_date BETWEEN se1.start_date AND se1.end_date) OR
    (m.end_date BETWEEN se1.start_date AND se1.end_date)
    
LEFT JOIN schedule_event se2 ON
    (m.start_date BETWEEN se2.start_date AND se2.end_date) OR
    (m.end_date BETWEEN se2.start_date AND se2.end_date)
    
LEFT JOIN schedule_event se3 ON
    (m.start_date BETWEEN se3.start_date AND se3.end_date) OR
    (m.end_date BETWEEN se3.start_date AND se3.end_date)

GROUP BY `m`.`id`

结果如下所示：

id    name             this_year_events      next_year_events      last_year_events            
====================================================================================
1     January          Training, Planning    (NULL)                (NULL)               
2     February         (NULL)                (NULL)                Audit, Budget    
3     March            Team Meeting          Team Away Day         (NULL)

现在我想在此查询中添加另一列：

active\u year

。此列将根据以下条件填充相应变量的值：

DEFAULT:
    active_year = @this_year

IF this_year_events IS NULL:
    active_year = @next_year

IF next_year_events IS NULL:
    active_year = @last_year

（如果满足后续条件，

active\u year

的值将覆盖上一个值）

---

我曾尝试为3个计算列分配一个变量，但它总是输出

NULL

。我就是想不通。

你只是想要

案例

with t as (
      <your query here>
     )
select t.*,
       (case when this_year_events is not null then @this_year
             when next_year_events is not null then @next_year
             else @last_year
        end) as active_year
from t;

---

谢谢戈登，太好了。我只是想知道做

COUNT（se1.name）！=0

，而不是

MAX（se1.name）不为NULL

？@GSTAR。这些是等价的。

select . . .,
       (case when max(se1.name) is not null then @this_year
             when max(se2.name) is not null then @next_year
             else @last_year
        end)
from . . .