如何在JOIN中使用CASE语句

我试图使用基于某些case语句而有所不同的join语句。当字段lane.dest_postal_code_prefix不为NULL时，我希望联接像下面的第一段代码一样。当field lane.dest_postal_code_prefix为NULL时，我希望联接像下面的第二个代码块一样。（请注意两种情况下ON条件的差异）

我需要一些帮助，将case语句添加到join子句中。

--WHEN lane.dest_postal_code_prefix is NOT NULL
SELECT * FROM big_bucket_bridge A
    LEFT JOIN lane 
    ON
    (
    A.customer_country = lane.dest_country_code
    AND
    SUBSTRING( A.ddm_zip, 1, LENGTH( lane.dest_postal_code_prefix ) ) = 
    lane.dest_postal_code_prefix
        )

WHERE  
    snapshot_day between '2019-06-23'-22 and '2019-06-23'
    AND (is_before_cutoff_g OR (is_before_cutoff_opt_g and not under_two))
    AND row_n =1
;

--WHEN lane.dest_postal_code_prefix is NULL
SELECT * FROM big_bucket_bridge A
    LEFT JOIN lane 
    ON
    (
    A.customer_country = lane.dest_country_code
    )

WHERE  
    snapshot_day between '2019-06-23'-22 and '2019-06-23'
    AND (is_before_cutoff_g OR (is_before_cutoff_opt_g and not under_two))
    AND row_n =1
;