对于COUNT调用的ELSE条件，您应该计算NULL，然后在计算中忽略它：

SELECT
    123 AS cust,
    'PRIOR' AS Range,
    COUNT(CASE WHEN fordate BETWEEN '2017-01-01' AND '2017-04-13' AND cust = 123
               THEN 1 END) AS count
FROM table1;

请注意，我省略了实际的ELSE条件，因为如果没有显式指定，默认值为NULL。此外，我将WHERE条件移到了条件计数中。这样做的效果是，即使该客户根本没有出现在表中，也始终返回一行。
选择cust看起来很奇怪，因为您要对整个表进行计数。

如果您的表中没有客户123的行，则需要“生成”这样一行。

SELECT
    c.cust, 
    'PRIOR' AS Range,
    (  SELECT COUNT(*) FROM table1 t
       WHERE t.cust = c.cust AND fordate BETWEEN '2017-01-01' AND '2017-04-13'
     ) AS count
FROM TABLE(VALUES( 123 )) AS c(cust)

或者可以使用LEFT OUTER JOIN

SELECT
    c.cust, 
    'PRIOR' AS Range,
    count(t.cust) AS count
FROM TABLE(VALUES( 123 )) AS c(cust)
LEFT OUTER JOIN table1 t
ON t.cust = c.cust
AND t.fordate BETWEEN '2017-01-01' AND '2017-04-13'
GROUP BY
   c.cust

我有一个类似的问题，我想报告满足一个条件的行数。

SELECT Count(*), 'Duplicated Contractor LGA records - post-delete'  
FROM (SELECT Company, LGA 
      FROM   detcontractorLGA 
      GROUP BY Company, LGA 
      HAVING count(*) > 1 
     )
;