BigQuery - 在一个列或另一个列上左连接两个表

Question

I want to perform a left join on two tables. The field I will join by is an email address. As the table I on the left has two email fields which may be different, I want that if the joining by the first email fails and returns null values, perform a second join on the other email field. Lastly, I want to throw away those entries which have not been matched with any of the two joins.

I have thought of doing something like this:

SELECT *
FROM a
LEFT JOIN b
ON
    a.address1 = b.email
    OR a.address2 = b.email

However, this returned an error message saying LEFT OUTER JOIN cannot be used without a condition that is an equality of fields from both sides of the join .

What is the correct way of achieving this?

Answer 1

You might be able to phrase your logic using a union:

SELECT * FROM a LEFT JOIN b ON a.address1 = b.email
UNION
SELECT * FROM a LEFT JOIN b ON a.address2 = b.email;

Answer 2

OR in JOIN conditions makes is really hard to optimize queries. So, BigQuery makes it hard to use OR .

I think you can rephrase the query by doing:

SELECT *
FROM a CROSS JOIN
     UNNEST(ARRAY[address1, address2]) address LEFT JOIN
     b
     ON address = b.email;

Answer 3

Consider below (BigQuery) approach

SELECT * EXCEPT(row_key, priority)
FROM (
  SELECT *, TO_JSON_STRING(a) row_key, IF(b.email IS NULL, 3, 1) priority
  FROM `project.dataset.tableA` a LEFT JOIN `project.dataset.tableB` b
  ON a.address1 = b.email
  UNION ALL
  SELECT *, TO_JSON_STRING(a), IF(b.email IS NULL, 3, 2) 
  FROM `project.dataset.tableA` a LEFT JOIN `project.dataset.tableB` b 
  ON a.address2 = b.email
)
WHERE true 
QUALIFY ROW_NUMBER() OVER(PARTITION BY row_key ORDER BY priority NULLS LAST) = 1     

Or you can use less verbose / more generic (so you can avoid redundant code fragments which can be useful if you need more than just two conditions) version of above

SELECT * EXCEPT(row_key, priority)
FROM (
  SELECT a.*, b.*, TO_JSON_STRING(a) row_key, IF(b.email IS NULL, 3, c.priority) priority
  FROM `project.dataset.tableA` a, 
  UNNEST([STRUCT(address1 as address, 1 as priority), (address2, 2)]) c
  LEFT JOIN `project.dataset.tableB` b
  ON c.address = b.email
)
WHERE TRUE 
QUALIFY ROW_NUMBER() OVER(PARTITION BY row_key ORDER BY priority NULLS LAST) = 1