MYSQL:对具有特定值的行求和

Question

我有一个包含许多行的表格，其中包含不同类型的值。每行都有两列:FieldName 和 FieldValue 我需要对所有行的计数器类型

求和

SELECT FieldValue as somma FROM `my_values` as value WHERE value.`FieldName` ='counter'

我试过这个查询

select sum(value.somma) as a from (
SELECT FieldValue as somma FROM `my_values` as value WHERE value.`FieldName` ='counter'
)

但是我有以下错误:

#1248 - Every derived table must have its own alias

Answer 1

你应该使用一个简单的 sum()

SELECT sum(FieldValue) as somma 
FROM `my_values`
WHERE FieldName` ='counter'

或者如果您需要使用别名，则以适当的方式为列添加前缀

SELECT sum(value.FieldValue) as somma 
FROM `my_values` as value WHERE value.`FieldName` ='counter'