根据 case when 子句的条件执行不同的准备语句

Question

我是 MySQL 的新手，我正在尝试根据 case when 子句的条件执行不同的准备语句。问题是找到 table STATION 的 LAT_N 列的中位数，我想在 LAT_N 记录的偶数和奇数时使用 Case。

这是我为奇数情况准备的声明：

SET @foo = (SELECT ROUND((COUNT(*)-1)/2) FROM STATION);
PREPARE STMT1 FROM 'SELECT ROUND(LAT_N,4) FROM STATION ORDER BY LAT_N LIMIT ?,1';
EXECUTE STMT1 USING @foo;

这是偶数：

SET @foo1 = (SELECT ROUND(COUNT(*)/2) FROM STATION);
SET @foo2 = (SELECT ROUND((COUNT(*)+2)/2) FROM STATION);
PREPARE STMT2 FROM 'SELECT ROUND((((SELECT ROUND(LAT_N,4) FROM STATION ORDER BY LAT_N LIMIT ?,1) + (SELECT ROUND(LAT_N,4) FROM STATION ORDER BY LAT_N LIMIT ?,1))/2),4)';
EXECUTE STMT2 USING @foo1, @foo2;

而我的想法和这段代码有点相似：

SELECT CASE WHEN COUNT(*)%2=0 THEN EXECUTE STMT2 USING @foo1, @foo2
            ELSE EXECUTE STMT1 USING @foo
FROM STATION
END;

但是那个代码 returns 错误 1064 (42000) 所以我想知道是否有任何替代方法。非常感谢

Answer 1

我找到了这个问题的答案，其中包括程序的使用

DELIMITER $$

CREATE PROCEDURE latmed()
BEGIN 
IF (SELECT COUNT(*) FROM STATION)%2=0 THEN
    SET @foo1 = (SELECT ROUND(COUNT(*)/2) FROM STATION);
    SET @foo2 = (SELECT ROUND((COUNT(*)+2)/2) FROM STATION);
    PREPARE STMT2 FROM 'SELECT ROUND((((SELECT ROUND(LAT_N,4) FROM STATION ORDER BY LAT_N LIMIT ?,1) + (SELECT ROUND(LAT_N,4) FROM STATION ORDER BY LAT_N LIMIT ?,1))/2),4)';
    EXECUTE STMT2 USING @foo1, @foo2;
ELSE
    SET @foo = (SELECT ROUND((COUNT(*)-1)/2) FROM STATION);
    PREPARE STMT1 FROM 'SELECT ROUND(LAT_N,4) FROM STATION ORDER BY LAT_N LIMIT ?,1';
    EXECUTE STMT1 USING @foo;
END IF;
END$$

DELIMITER ;
CALL latmed()

我觉得它有点长，所以如果有人有更短的代码可以回答我的问题。谢谢

根据 case when 子句的条件执行不同的准备语句

Executing different prepared statement based on conditions of case when clause

mysql

prepared-statement

case-when