您可以在WHERE子句中重复整个CASE语句，如下所示：

select
    case
        when x then y 
        when a then b 
        when c then d 
        else 'NOT IN RANGE'
    end as foo
from
    table t 
where 
    case
        when x then y 
        when a then b 
        when c then d 
        else 'NOT IN RANGE'
    end <> 'NOT IN RANGE'

也可以使用如下子查询（或CTE）：

select * 
from (
    select
        case
            when x then y 
            when a then b 
            when c then d 
            else 'NOT IN RANGE'
        end as foo
    from
        table t 
) 
where foo <> 'NOT IN RANGE'

如果不能看到完整的查询，就不可能给出“最优”的解决方案，最简单的解决方案是将条件添加为HAVING子句：

HAVING `C4630` <> 'NOT IN RANGE'

我对你的日期格式感到困惑。如果这应该是一个MySQL查询，它们应该是yyyy-mm-dd。可以说，即使在SQL Server上，它们也应该是这种格式，因为SQL Server支持不明确的本地日期格式。
为什么要使用RTRIM和CAST？数据应该以正确的格式存储，并在输入时进行清理，如果无法做到这一点，请定期清理数据，而不是对每个查询都进行清理。
明智的做法是将seasons移到它们自己的表中，而不是在查询时定义它们，这样就可以简单地连接到seasons，我在这里使用了一个子查询，但希望您能理解：

SELECT
    /* other columns */
    COALESCE(seasons.season, FTBL_TEST_SCORES.test_admin_period) AS `C4630`
FROM all_the_other_tables
LEFT JOIN (

    SELECT 'Bay Area' AS region, 'FALL' AS season, '2021-08-07' AS start, '2021-09-08' AS end UNION ALL
    SELECT 'Bay Area'          , 'WINTER'        , '2021-11-27'         , '2021-12-15'        UNION ALL
    SELECT 'Bay Area'          , 'SPRING'        , '2022-03-04'         , '2022-03-22'        UNION ALL
        
    SELECT 'Central Valley'    , 'FALL'          , '2021-07-31'         , '2021-09-01'        UNION ALL
    SELECT 'Central Valley'    , 'WINTER'        , '2021-11-27'         , '2021-12-15'        UNION ALL
    SELECT 'Central Valley'    , 'SPRING'        , '2022-02-19'         , '2022-03-08'        UNION ALL
        
    SELECT 'Los Angeles'       , 'FALL'          , '2021-08-14'         , '2021-09-15'        UNION ALL
    SELECT 'Los Angeles'       , 'WINTER'        , '2021-11-27'         , '2021-12-15'        UNION ALL
    SELECT 'Los Angeles'       , 'SPRING'        , '2022-03-04'         , '2022-03-22'

) AS seasons
    ON dtbl_school_dates.local_school_year = '2021-2022'
    AND dtbl_schools_ext.region = seasons.region
    AND dtbl_school_dates.date_value BETWEEN seasons.start AND seasons.end
WHERE (
    (dtbl_school_dates.local_school_year = '2021-2022' AND seasons.season IS NOT NULL) OR
    dtbl_school_dates.local_school_year <> '2021-2022'
);