SQL中的SUM函数是用来求和的，它可以返回查询结果中某列的总和，它可以接受多个参数，比如一个列名，一个表名，一个条件等。SUM函数的使用方法如下：
1. 求某一列的总和
SELECT SUM(column_name) FROM table_name;
2. 求某一列满足某一条件的总和
SELECT SUM(column_name) FROM table_name WHERE condition;
3. 求多列的总和
SELECT SUM(column_name1) + SUM(column_name2) + ... FROM table_name;
4. 求多列满足某一条件的总和
SELECT SUM(column_name1) + SUM(column_name2) + ... FROM table_name WHERE condition;
5. 使用GROUP BY子句求某一列的总和
SELECT column_name1, SUM(column_name2) FROM table_name GROUP BY column_name1;
6. 使用GROUP BY子句求多列的总和
SELECT column_name1, SUM(column_name2) + SUM(column_name3) + ... FROM table_name GROUP BY column_name1;