问题:

Is there an ANSI SQL equivalent to Oracle's DECODE function?

Oracle's decode function is the IF-THEN-ELSE construct in SQL.

回答1:

A CASE expression is the ANSI SQL method, of which there are 2 varieties, "simple" and "searched":

1) Simple CASE expression:

CASE col WHEN 1 THEN 'One'
         WHEN 2 THEN 'Two'
         ELSE 'More'
         END

2) Searched CASE expression:

CASE WHEN col < 0 THEN 'Negative'
     WHEN col = 0 THEN 'Zero'
     ELSE 'Positive'
     END

CASE WHEN a=1 THEN value1
     WHEN a=2 THEN value2
     ELSE default
END

SQL Case Statement Syntax?

Please note that Oracle DECODE treats null as equal to null, while CASE(and any other comparisons) don't.

Example: Decode(a,b,1,0) will return 1 if both a and b are nulls.

Just run these 2 statements to see the difference.

select case null when null then 'Y' else 'N' end dd from dual;
select decode(null, null, 'Y', 'N') dd from dual;

adding it up null can be compared in decode whereas in case it is not.