我想能够选择记录使用自定义功能在mysql。
我尝试了以下代码，但在有数据的地方得到了空结果：

SELECT id, getAmountS(id) FROM product WHERE id=2

DELIMITER $$
CREATE FUNCTION getAmountS(pid INT(11)) RETURNS DOUBLE(12, 2) DETERMINISTIC BEGIN
    DECLARE sellvalue DOUBLE(12, 2) ;
    SELECT
        SUM( CASE WHEN s.invoicestatus = 'Active' THEN ROUND(
        (r.rate * 1 -((r.rate * 1) * r.discount / 100))+((
        r.rate * 1 -( (r.rate * 1) * r.discount / 100)) * r.gstrate / 100),
                2  ) ELSE 0 END) INTO sellvalue FROM  morder r
        LEFT JOIN sale s ON  r.saleid = s.id
        WHERE r.productid = pid;
    RETURN sellvalue;
END $$

请帮忙。
表：产品

id(INT), productname(VARCHAR), manufacturer(INT)

表：莫德

id(INT), saleid(INT), gstrate(INT), rate(DOUBLE), productid(INT), discount(DOUBLE)

表：销售

id(INT), invoiceno(VARCHAR), invoicestatus(ENUM), invoiceDate(DATE)

以下语句可以正常工作：

SELECT SUM( CASE WHEN s.invoicestatus = 'Active' THEN ROUND(
        (r.rate * 1 -((r.rate * 1) * r.discount / 100))+((
        r.rate * 1 -( (r.rate * 1) * r.discount / 100)) * r.gstrate / 100), 2  ) ELSE 0 END) sellvalue FROM  morder r
        LEFT JOIN sale s ON  r.saleid = s.id
        WHERE r.productid = 30340