一、单一的求和。
select sum(value) as value from table where user_id = 1 and type = 6 and type_son = 2
value 为求和的字段。
as 后面是 sum 求和后给它一个名称。
二、SQL语句中嵌套语句多条件求和。
select
(select sum(value) from table where type = 6 and type_son = 1) as xj0,
(select sum(value) from table where type = 6 and type_son = 2) as xj1,
(select sum(value) from table where type = 3 and type_son = 3) as xj2,
(select sum(value) from table where type = 4 and type_son = 3) as xj3
from table where user_id = 1 limit 0,1
as 后面是 sum 求和后给它一个名称，这样就不会冲突。
三、与第二个一样，但是不采取语句嵌套的方式求和，而是使用 sum 判断求和。
select
sum(IF(type = 6 and type_son = 1,value,NULL)) as xj0,
sum(IF(type = 6 and type_son = 2,value,NULL)) as xj1,
sum(IF(type = 3 and type_son = 0,value,NULL)) as xj2,
sum(IF(type = 4 and type_son = 3,value,NULL)) as xj3
from table where user_id = 1
sum(IF('条件判断'，'求和的字段'，'NULL不计算')) as '别名'