SQL Conditional JOIN column [duplicate]

问题

This question already has answers here:

SQL Conditional AND (4 answers)

Closed 5 years ago.

I want to determine the 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, reported_date.

I want to check against an exchange rate based on the date. I know the reported_date is never null, but it's my last resort, so I have a priority order for which to join against the exchange_rate table. I'm not quite sure how to do this with a CASE statement, if that's even the right 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

回答1:

Try 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 coalesce() function returns the first non-NULL value in the list.

回答2:

The CASE statement could 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

来源：https://stackoverflow.com/questions/25255240/sql-conditional-join-column