使用输出参数调用存储过程

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这里我给出了存储过程GetAvgOut":

Herewith I have given the stored procedure "GetAvgOut":

delimiter // DROP PROCEDURE IF EXISTS GetAvgOut// CREATE DEFINER = 'MailIntimator'@'127.0.0.1' PROCEDURE GetAvgOut(OUT average INT,IN col VARCHAR(30),IN tbl VARCHAR(30)) READS SQL DATA COMMENT 'returns average' BEGIN SET @userVar = CONCAT(' SELECT AVG( ' , col , ' ) FROM ' , tbl ); PREPARE stmt FROM @userVar; EXECUTE stmt; END; // delimiter ;

我尝试使用以下方法调用上述过程，

I tried calling the aforeseen procedure using,

CALL GetAvgOut(@a,'Population','city'); SELECT @a;

"select @a" 返回空值.如何获得分配给输出参数@a"的平均值?

"select @a" returns null. How can I get the average which is assigned to out parameter "@a"?

推荐答案

您没有在选择中设置 OUT 参数.

you are not setting the OUT parameter in your select.

尝试将您的声明更新为:

try updating your statement to:

SET @userVar = CONCAT('SELECT AVG( ' , col , ' ) INTO average FROM ' , tbl );