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SQL column of conditional JOIN

I want to define a JOIN column based on the value of the current row.

So, for example, my job table has 4 date columns: offer_date, accepted_date, start_date, report_date.

I want to check the date based exchange rate. I know report_date is never null, but this is my last resort, so I have a priority order for joining with the exchange_rate table. I'm not really sure how to do it with a CASE statement, if that's even the correct approach.

INNER JOIN exchange_rate c1 ON c1.date = {{conditionally pick a column here}}

  -- use j.offer_date if not null
  -- use j.accepted_date if above is null
  -- use j.start_date if above two are null
  -- use j.reported_date if above three are null
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2 answers


Try the logic like this:

INNER JOIN
exchange_rate c1
ON c1.date = coalesce(j.offer_date, j.accepted_date, j.start_date, j.reported_date)


The function coalesce()

returns the first non-NULL value in the list.

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A CASE statement might look something like this:



INNER JOIN exchange_rate c1 ON c1.date =
CASE
    WHEN j.offer_date IS NOT NULL THEN j.offer_date
    WHEN j.accepted_date IS NOT NULL THEN j.accepted_date
    WHEN j.start_date IS NOT NULL THEN j.start_date
    ELSE j.reported_date
END
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