2.SQL 中的if和case语法

IF expression THEN  
  statements;  
ELSEIF expression THEN  
  statements;  
...  
ELSE  
  statements;  
END IF;

CASE case-expression  
 WHEN when_expression_1 THEN commands  
 WHEN when_expression_2 THEN commands  
 ...  
 ELSE commands  
END CASE;