How to use a CASE statement with a JOIN

Question

I am trying to use join statement that vary based on certain case statements. When the field lane.dest_postal_code_prefix is NOT NULL then I want the join to be like the first block of code below. When the field lane.dest_postal_code_prefix is NULL then I want the join to be like second block of code below. (Note the difference on the ON conditions between the two cases)

I need some help adding the case statement to the join clause.

--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
;

Answer 1

You can use one ON clause like this:

ON
(
A.customer_country = lane.dest_country_code
AND
COALESCE(SUBSTRING(A.ddm_zip, 1, LENGTH( lane.dest_postal_code_prefix )), '') = CASE 
  WHEN lane.dest_postal_code_prefix IS NOT NULL THEN lane.dest_postal_code_prefix 
  ELSE COALESCE(SUBSTRING(A.ddm_zip, 1, LENGTH( lane.dest_postal_code_prefix )), '')
END
)

In the 2nd case (else part) the condition is always true because it is:

COALESCE(SUBSTRING(A.ddm_zip, 1, LENGTH( lane.dest_postal_code_prefix )), '') = COALESCE(SUBSTRING(A.ddm_zip, 1, LENGTH( lane.dest_postal_code_prefix )), '')

so the only actual condition is

A.customer_country = lane.dest_country_code