MYSQL:连接 2 个包含存储函数的 select 语句时出错

Question

下午好，

我创建了一个存储函数来模拟 MySQL 中“SHOW GRANTS FOR”命令的行为。脚本定义如下:

CREATE FUNCTION get_grants(p_user text, p_host text, p_type text, p_obj text) 
   RETURNS text 
   NOT DETERMINISTIC 
   READS SQL DATA
BEGIN
...
END;

如果我在 select 语句中使用该函数，它会正确执行。例如:

SELECT User as USER, Host as HOST, 
      'GLOBAL' as PRIV_TYPE, NULL as DATABASE_NAME,   
       mydatabase.get_grants(User, Host, 'GLOBAL', '') as PRIVILEGES
  FROM mysql.user

但是，当我使用 UNION 或 UNION ALL 语句连接 2 个使用不同参数值调用函数的 select 语句时，如下所示:

SELECT User as USER, Host as HOST, 
       'GLOBAL' as PRIV_TYPE, NULL as DATABASE_NAME,   
       mydatabase.get_grants(User, Host, 'GLOBAL', '') as PRIVILEGES
  FROM mysql.user
UNION ALL
SELECT User as USER, Host as HOST,
       'DB' as PRIV_TYPE, Db as DATABASE_NAME,
       mydatabase.get_grants(User, Host, 'DB', Db) as PRIVILEGES
  FROM mysql.db
 ORDER BY USER, HOST, DATABASE_NAME IS NULL, DATABASE_NAME;

我收到以下错误:#1222 - 使用的 SELECT 语句具有不同的列数。

谁能解释这里发生了什么以及如何解决这个问题？

提前致谢。

Answer 1

我认为是 UNION ALL 导致了这个问题。 通常，如果您使用 UNION ALL，列数是否不同并不重要，但使用 UNION 时它们必须相等。 因此，请尝试使用 UNION ALL 并将 NULL 删除为 DATABASE_NAME 或仅使用 UNION 并将 NULL 保留为 DATABASE_NAME

SELECT User as USER, Host as HOST, 
       'GLOBAL' as PRIV_TYPE, NULL as DATABASE_NAME,   
       mydatabase.get_grants(User, Host, 'GLOBAL', '') as PRIVILEGES
  FROM mysql.user
UNION
SELECT User as USER, Host as HOST,
       'DB' as PRIV_TYPE, Db as DATABASE_NAME,
       mydatabase.get_grants(User, Host, 'DB', Db) as PRIVILEGES
  FROM mysql.db
 ORDER BY USER, HOST, DATABASE_NAME IS NULL, DATABASE_NAME;

或

SELECT User as USER, Host as HOST, 
       'GLOBAL' as PRIV_TYPE,   
       mydatabase.get_grants(User, Host, 'GLOBAL', '') as PRIVILEGES
  FROM mysql.user
UNION ALL
SELECT User as USER, Host as HOST,
       'DB' as PRIV_TYPE, Db as DATABASE_NAME,
       mydatabase.get_grants(User, Host, 'DB', Db) as PRIVILEGES
  FROM mysql.db
 ORDER BY USER, HOST, DATABASE_NAME IS NULL, DATABASE_NAME;