带有NULL的SQL CASE语句

Question

I have the following query. According to http://dev.mysql.com/doc/refman/5.0/en/case.html , null=null is false, thus the first case row will never occur. What would be an alternative query to accomplish this?

SELECT CASE c1
    WHEN NULL THEN CONCAT("Hi",name," Your value is NULL")
    WHEN "v1" THEN CONCAT("Hello ",name," The value is ",
    CASE c2
        WHEN "v11" THEN "bla"
        WHEN "v12" THEN "bla bla"
        END 
    ,")")
    WHEN "v2" THEN CONCAT("Howdy there ",name," Yep, the value is ",
    CASE c3
        WHEN "v21" THEN "beebop"
        WHEN "v22" THEN "bopbee"
        END 
    ,")")
    END
AS myLabel
FROM mytable
WHERE bla="bla";

Answer 1

So use the where <condition> form of case :

SELECT CASE WHEN c1 is NULL THEN CONCAT('Hi', name, ' Your value is NULL')
            WHEN c1 = 'v1'
            THEN CONCAT('Hello ', name, ' The value is ',
                        (CASE WHEN c2 = 'v11' THEN 'bla'
                             WHEN c2 = 'v12' THEN 'bla bla'
                         END), ')'
                        )
            WHEN c1 = 'v21
            THEN CONCAT('Howdy there ', name, ' Yep, the value is ',
                         (CASE WHEN c3 = 'v21' THEN 'beebop'
                               WHEN c3 = 'v22' THEN 'bopbee'
                          END), ')'
                       )
        END
FROM mytable
WHERE bla = 'bla';

Also, use single quotes for string (and date) constants. Don't use them for anything else, like quoting delimiters.

Answer 2

An alternative would be to use COALESCE to get a default value in case the field is NULL, and then use that default for comparison where you are comparing with NULL in your present your query, like so:

SELECT CASE COALESCE(c1,'')
WHEN '' THEN CONCAT("Hi",name," Your value is NULL")

Of course, '' may be a legitimate value on it's own, in which case you need to use another value as default. Owing to this, I would personally recommend Gordon's solution as the way to go.