mysql error 'NULL' at line 1

Question

I got this error when tried to execute this:

#1064 - You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'NULL' at line 1

Can't seems to find what is the problem. Appreciate if anyone can help

SET @sql = NULL;

SELECT
  GROUP_CONCAT(
    DISTINCT CONCAT (
      "SUM(IF(DATE(FROM_UNIXTIME(machine_stop)) = '",
      DATE(FROM_UNIXTIME(machine_stop)),"' ,
      (machine_start-machine_stop)/3600, 0)) AS ",
      DATE(FROM_UNIXTIME(machine_stop))
    )
  ) INTO @sql
FROM
  downtime_data
WHERE
  DATE(FROM_UNIXTIME(machine_stop)) >= DATE(NOW()) - INTERVAL 7 DAY;

SET @sql = CONCAT("SELECT
                     failure_code, ", @sql, " 
                   FROM
                     downtime_data 
                   WHERE
                     p.machine='HH1' AND
                     DATE(FROM_UNIXTIME(machine_stop)) >= DATE(NOW()) - INTERVAL 7 DAY 
                   GROUP BY
                     failure_code,
                     DATE(FROM_UNIXTIME(machine_stop))"
                 );

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;

Answer 1

如果该@sql变量NULL在第一个select语句之后仍然保留值，那么稍后您将在执行prepare语句时遇到异常。

使用以下命令查看以下选择语句 CONCAT

SET @sql := NULL; SELECT CONCAT('abc',@sql,'def');

结果是NULL。尽管您可能期望结果是abcdef。

为了得到abcdef你需要这样做

SET @sql := NULL; SELECT CONCAT('abc',IFNULL(@sql,''),'def');

如果它解决了问题，则可以尝试以下任何一种方法：

要么

1） SET @sql := '';

要么

2）如果您想保留此行SET @sql = NULL;，请像这样更改最终查询的部分SET @sql = CONCAT("SELECT failure_code ", IF(@sql IS NULL, '',CONCAT(',',@sql)),

这是最终的查询：

SET @sql = CONCAT("SELECT
                     failure_code ", IF(@sql IS NULL, '',CONCAT(',',@sql)), " 
                   FROM
                     downtime_data 
                   WHERE
                     p.machine='HH1' AND
                     DATE(FROM_UNIXTIME(machine_stop)) >= DATE(NOW()) - INTERVAL 7 DAY 
                   GROUP BY
                     failure_code,
                     DATE(FROM_UNIXTIME(machine_stop))"
                 );

PREPARE stmt FROM @sql;
EXECUTE stmt;
DEALLOCATE PREPARE stmt;