条件分岐

case

 SELECT CASE 1 WHEN 1 THEN "one" WHEN 2 THEN "two" ELSE "more" END;

if

 SELECT IF(2>1, 'YES', 'NO');  --'YES'
 UPDATE users SET class = IF(age >= 20, 'adult', 'child');

参考

• http://dev.mysql.com/doc/refman/4.1/ja/control-flow-functions.html
• http://dev.mysql.com/doc/refman/5.1/ja/control-flow-functions.html