Return one of two columns in a view - whichever one is not null

问题

I have a table with three columns:

ColumnA          ColumnB         ColumnC
AAA               NULL            123
BBB               222             NULL
CCC               NULL            NULL

I would like to create a SELECT statement which will return ColumnA, and then a second column which will either show the value of ColumnB unless ColumnB is null; otherwise it will show the value of ColumnC, even it it's NULL. Can I use an IF statement for that? Something like:

SELECT ColumnA, 
IF(ColumnB IS NULL, ColumnC, ColumnB)
FROM table

**If I get this working, the next step would be to return the value of a joined column instead of ColumnB. In effect the IF statement would be

IF(table.ColumnB IS NULL, table.ColumnC, table2.ColumnD)

回答1:

Reading to the end of your question it sounds like you need to use CASE

CASE WHEN table.ColumnB IS NULL 
     THEN table.ColumnC 
     ELSE table2.ColumnD 
END AS Whatever

回答2:

Use COALESCE

SELECT ColumnA, COALESCE(ColumnB, ColumnC) as 'Value'

来源：https://stackoverflow.com/questions/5199360/return-one-of-two-columns-in-a-view-whichever-one-is-not-null