How to join two tables by dependent match keys in BigQuery?

I have two tables in BigQuery First one is a list of rates. Rates have default values with source equal -1 for each combo code - offer. Apart from combo code - offer, some rates have specified source

Second table has same columns as first table except rates + any other data.

My goal join rates by matched code - offer - source otherwise use default rate by matched code - offer with source equal -1

In example query returns default rates only:

    WITH t1 AS (SELECT 21 as source, 'SA' as code, 'offer1' as offer, 2.4 as rate 
            UNION ALL
            SELECT 33, 'SA', 'offer1', 2.5
            UNION ALL
            SELECT 39, 'SA', 'offer1', 2.1
            UNION ALL
            SELECT -1, 'SA', 'offer1', 3
            UNION ALL
            SELECT -1, 'SA', 'offer2', 4
            UNION ALL
            SELECT 47, 'YN', 'offer1', 2.7
            UNION ALL
            SELECT -1, 'YN', 'offer1', 5.4
            UNION ALL
            SELECT -1, 'YN', 'offer2', 0.9
            UNION ALL
            SELECT -1, 'RE', 'offer1', 5.7
            UNION ALL
            SELECT -1, 'RE', 'offer2', 3.4),
t2 as (SELECT 21 as source, 'SA' as code, 'offer1' as offer, "any data" as other_columns
        UNION ALL SELECT 21, 'SA', 'offer1', "any data"
        UNION ALL SELECT 21, 'SA', 'offer1', "any data"
        UNION ALL SELECT 21, 'SA', 'offer2', "any data"
        UNION ALL SELECT 47, 'YN', 'offer1', "any data"
        UNION ALL SELECT 47, 'YN', 'offer2', "any data"
        UNION ALL SELECT 50, 'YN', 'offer1', "any data"
        UNION ALL SELECT 47, 'YN', 'offer2', "any data"
        UNION ALL SELECT 78, 'RE', 'offer1', "any data"
        UNION ALL SELECT 66, 'RE', 'offer2', "any data")
        
        
SELECT t2.*, rate FROM t2
LEFT JOIN t1 ON t1.offer = t2.offer AND t1.code = t2.code AND IF (t1.source = t1.source AND rate IS NULL, t1.source = t2.source, t1.source = - 1)

Next query returns rates with specified source and null when source did not match

SELECT t2.*, rate FROM t2
    LEFT JOIN t1 ON t1.offer = t2.offer AND t1.code = t2.code AND IF (t1.source = t1.source AND rate IS NOT NULL, t1.source = t2.source, t1.source = - 1) 

How can I join rates correct?

Answer

You can left join twice and use conditional logic:

select t2.*, coalesce(t11.rate, t12.rate) rate
from t2
left join t1 t11
    on  t11.code = t2.code
    and t11.offer = t2.offer 
    and t11.source = t2.source
left join t1 t12 
    on  t12.code = t2.code
    and t12.offer = t2.offer
    and t12.source = -1
    and t11.code is null