php mysql 求和语句

$sql ="update table set sum = mathe +language";

$sql = "select sum(sum) from tables";

CREATE TABLE `cc`.`sumtable` ( 
`id` INT( 4 ) NOT NULL , 
`a1` INT( 4 ) NOT NULL , 
`a2` INT( 4 ) NOT NULL , 
`summ` INT( 8 ) NOT NULL  
) ENGINE = InnoDB

INSERT INTO `cc`.`sumtable` ( 
`id` , 
`a1` , 
`a2` , 
`summ`  
) 
VALUES ( 
'1', '5', '5', '0' 
), ( 
'2', '6', '5', '0' 
); 
*/

 
//自动计算二个字段的值赋给另一字段sql 
update sumtable set summ = a1+a2 
//求和 
SELECT sum( summ ) FROM sumtable  
sum(summ)   
21