mysql - 错误 #1241 在 Mysql Workbench 中操作数应包含 1 列

Question

我只是尝试以下查询:

CREATE DEFINER=`root`@`localhost` FUNCTION `cal1`(z DATE , y DATE) RETURNS float
BEGIN
SET @c= (SELECT a.StudentName, (b.marks/a.marks) as difference from (select Date, StudentName, marks from studenthistory2015 WHERE Date=z) as a INNER JOIN (select Date, StudentName, marks from studenthistory2015 where Date=y) as b on a.date=z and b.date=y WHERE a.date = z and b.date = y and a.StudentName=b.StudentName
);
RETURN @c;
END

执行:

选择student_history.cal1('2015-01-01', '2015-02-01');但执行后却显示错误 #1241 操作数应包含 1 列

我应该如何解决这个查询？

Answer 1

如P.Salmon说，你试图返回一组参数，包含字符串和 float (我假设)

SELECT a.StudentName, (b.marks/a.marks) as difference

虽然您实际上在函数中确实声明了您将返回一个 float ，但不是返回单个变量:RETURNS float

也许您可以将函数转换为过程，就像 detailed answer here 中提到的那样，回答与您相关的另一个问题。