COALESCE or CASE more efficient and/or standard

问题

In terms of x compared to y.

• Does x comply with sql standards better? [apologies if subjective]
• Is x more efficient than y?
• Or are these scripts completely different and to be used in different contexts?

x

    SELECT * 
    FROM   a 
           INNER JOIN b ON
           COALESCE(b.columntojoin, b.alternatecolumn) = a.columntojoin

y

    SELECT * 
    FROM   a 
           INNER JOIN b ON
           (case when b.columntojoin is null then b.alternatecolumn else b.columntojoin end) = a.columntojoin

回答1:

COALESCE is essentially a shorthanded CASE statement.

Both are exactly the same.

There is also ISNULL in SQL Server (differs in other DBMSs), but that's actually a non-standard feature and is actually more limited that COALESCE.

回答2:

In this case I would use COALESCE ( which provides more levels than ISNULL) rather than the CASE stement.

The CASE statement seems a bit bulky here, seeing as you are only checking for NULLs anyway.

来源：https://stackoverflow.com/questions/15523000/coalesce-or-case-more-efficient-and-or-standard