如何检查变量是否为 NULL，然后使用 MySQL 存储过程设置它？

Question

我有一个 MySQL 存储过程，可以从表中找到最大值。

如果没有值，我想将变量设置为昨天的日期。

DECLARE current_procedure_name CHAR(60) DEFAULT 'accounts_general';
DECLARE last_run_time datetime DEFAULT NULL;
DECLARE current_run_time datetime DEFAULT NOW();

-- Define the last run time
SET last_run_time := (SELECT MAX(runtime) 
FROM dynamo.runtimes WHERE procedure_name = @current_procedure_name);

-- if there is no last run time found then use yesterday as starting point
IF(@last_run_time IS NULL) THEN
    SET last_run_time := DATE_SUB( NOW(), INTERVAL 1 DAY);
END IF;

SELECT @last_run_time;

问题是@last_run_time 始终为NULL。

以下代码由于某种原因没有被执行

IF(last_run_time IS NULL) THEN
    SET last_run_time := DATE_SUB( NOW(), INTERVAL 1 DAY);
END IF;

如何正确设置变量@last_run_time？

Answer 1

@last_run_time 是一个9.4. User-Defined Variables 和last_run_time datetime 一个13.6.4.1. Local Variable DECLARE Syntax，是不同的变量。

试试：SELECT last_run_time;

更新

例子：

/* CODE FOR DEMONSTRATION PURPOSES */
DELIMITER $$

CREATE PROCEDURE `sp_test`()
BEGIN
    DECLARE current_procedure_name CHAR(60) DEFAULT 'accounts_general';
    DECLARE last_run_time DATETIME DEFAULT NULL;
    DECLARE current_run_time DATETIME DEFAULT NOW();

    -- Define the last run time
    SET last_run_time := (SELECT MAX(runtime) FROM dynamo.runtimes WHERE procedure_name = current_procedure_name);

    -- if there is no last run time found then use yesterday as starting point
    IF(last_run_time IS NULL) THEN
        SET last_run_time := DATE_SUB(NOW(), INTERVAL 1 DAY);
    END IF;

    SELECT last_run_time;

    -- Insert variables in table2
    INSERT INTO table2 (col0, col1, col2) VALUES (current_procedure_name, last_run_time, current_run_time);
END$$

DELIMITER ;