mysql - SQL 中值未声明变量

Question

我正在尝试创建一个存储过程来获取表中的中位年龄，但出现未声明的变量错误。我的代码是:

DELIMITER //
CREATE PROCEDURE MedianAge()
BEGIN

SET @row_count = (SELECT COUNT(*) FROM employee);
SET @median_index = (@row_count/2);

SELECT  TIMESTAMPDIFF(YEAR, bdate, CURDATE()) AS age 
FROM employee ORDER BY bdate DESC 
LIMIT median_index, median_index;


END //
DELIMITER ;

我收到错误:

Error Code: 1327. Undeclared variable: median_index

据我所知，我已经正确声明了变量，但不确定为什么 SELECT 语句不起作用。

Answer 1

您正在使用用户定义的变量(以@ 为前缀)。但是，在存储过程中，您应该使用局部变量(不带前缀)。这些变量必须在使用前声明。区别很好解释here .

你会得到这样的东西:

DELIMITER //
CREATE PROCEDURE MedianAge()
BEGIN

DECLARE row_cnt INT unsigned;
DECLARE median_index INT unsigned;

SET row_cnt = (SELECT COUNT(*) FROM employees);
SET median_index = (row_cnt/2);

SELECT  TIMESTAMPDIFF(YEAR, bdate, CURDATE()) AS age 
FROM employee ORDER BY bdate DESC 
LIMIT median_index, median_index;


END //
DELIMITER ;

此外，请记住 row_count 是 MySQL 中的保留字(函数名)。最好避免将其用作变量名。