在模板函数参数之间强制实现所需的关系(Enforcing desired relationship between template function parameters)

 为什么OP查询返回错误1054 
 
 返回的错误是因为从UNION的结果中分配给列的名称取自第一个SELECT。 
 
 您可以通过运行一个简单的示例来观察： 
 
 SELECT 1 AS one 
  UNION
 SELECT 2 AS two 
 
 该查询返回的结果集将包含单个列，分配给该列的名称将为one ，即第一个SELECT中的列名称。 这解释了为什么从查询中收到错误的原因。 
 
 
 返回没有匹配的行的一种方法 
 
 要返回table1中col1值，该值与table2 col1列中的任何值都不匹配... 
 
 使用反连接模式的一个选项... 
 
    SELECT t1.col1
      FROM table1 t1
      LEFT
      JOIN table2 t2
        ON t2.col1 = t1.col1
     WHERE t2.col1 IS NULL
 
 LEFT JOIN操作返回table1中的所有行，以及table2中找到的任何“匹配”行。 “技巧”是WHERE子句中的谓词...来自table2的任何“匹配”行将在col1中具有非NULL值。 因此，如果我们排除了找到匹配项的所有行，那么我们会留下table1中没有匹配项的行。 
 
 如果我们想从table2中获取table1没有“匹配”行的行，我们可以执行相同的操作，只需翻转表的顺序即可。 
 
 如果我们组合这两个集合，但只想要一个“不匹配”值的“不同”列表，我们可以使用UNION集合运算符： 
 
    SELECT t1.col1
      FROM table1 t1
      LEFT
      JOIN table2 t2
        ON t2.col1 = t1.col1
     WHERE t2.col1 IS NULL
     UNION 
    SELECT s2.col1
      FROM table2 s2
      LEFT
      JOIN table1 s1
        ON s1.col1 = s2.col1
     WHERE s1.col1 IS NULL
 
 - 
 
 找出不匹配值来自哪个表 
 
 有时，我们想知道哪个查询返回了值; 我们可以通过在每个查询中包含一个文字值作为鉴别器来实现。 
 
    SELECT 'table1' AS src
         , t1.col1
      FROM table1 t1
      LEFT
      JOIN table2 t2
        ON t2.col1 = t1.col1
     WHERE t2.col1 IS NULL
     UNION 
    SELECT 'table2' AS src
         , s2.col1
      FROM table2 s2
      LEFT
      JOIN table1 s1
        ON s1.col1 = s2.col1
     WHERE s1.col1 IS NULL
    ORDER BY 2
 
 
 查找不匹配行的不同（通常性能较差）方法 
 
 一个完全不同的方法，返回一个等效的结果，将做这样的事情： 
 
SELECT q.col1
  FROM ( SELECT 't1' AS src, t1.col1 FROM table1 t1 
          UNION
         SELECT 't2' AS src, t2.col1 FROM table2 t2
       ) q
 GROUP BY q.col1
HAVING COUNT(DISTINCT q.src) < 2
ORDER BY q.col1
 
 （内联视图q将被“物化”为派生表，因此这种方法对于大型集合来说可能是昂贵的，并且这种方法不会利用col1上的索引来执行匹配。）另外一个小的区别是反连接方法：如果两个表中都存在NULL，则将省略col1值NULL。 除此之外，结果集是等效的。 

Why Error 1054 is being returned by OP query
 
The error that's being returned is because the name assigned to a column from the result of a UNION is taken from the first SELECT.
 
You can observe this by running a simple example:
 
 SELECT 1 AS one 
  UNION
 SELECT 2 AS two 
 
The resultset returned by that query will contain a single column, the name assigned to the column will be one, the column name from the first SELECT. This explains why you are getting the error from your query.
 
 
One way to return rows with no match
 
To return values of col1 from table1 which do not match any value in the col1 column from table2...
 
one option to use an anti-join pattern...
 
    SELECT t1.col1
      FROM table1 t1
      LEFT
      JOIN table2 t2
        ON t2.col1 = t1.col1
     WHERE t2.col1 IS NULL
 
The LEFT JOIN operation returns all rows from table1, along with any "matching" rows found in table2. The "trick" is the predicate in the WHERE clause... any "matching" rows from table2 will have a non-NULL value in col1. So, if we exclude all of the rows where we found a match, we're left with rows from table1 that didn't have a match.
 
If we want to get rows from table2 that don't have a "matching" row in table1, we can do the same thing, just flipping the order of the tables.
 
If we combine the two sets, but only want a "distinct" list of "not matched" values, we can use the UNION set operator:
 
    SELECT t1.col1
      FROM table1 t1
      LEFT
      JOIN table2 t2
        ON t2.col1 = t1.col1
     WHERE t2.col1 IS NULL
     UNION 
    SELECT s2.col1
      FROM table2 s2
      LEFT
      JOIN table1 s1
        ON s1.col1 = s2.col1
     WHERE s1.col1 IS NULL
 
--
 
Finding out which table the non-matched value is from
 
Sometimes, we want to know which query returned the value; we can get that by including a literal value as a discriminator in each query.
 
    SELECT 'table1' AS src
         , t1.col1
      FROM table1 t1
      LEFT
      JOIN table2 t2
        ON t2.col1 = t1.col1
     WHERE t2.col1 IS NULL
     UNION 
    SELECT 'table2' AS src
         , s2.col1
      FROM table2 s2
      LEFT
      JOIN table1 s1
        ON s1.col1 = s2.col1
     WHERE s1.col1 IS NULL
    ORDER BY 2
 
 
A different (usually less performant) approach to finding non-matching rows
 
An entirely different approach, to returning an equivalent result, would be do something like this:
 
SELECT q.col1
  FROM ( SELECT 't1' AS src, t1.col1 FROM table1 t1 
          UNION
         SELECT 't2' AS src, t2.col1 FROM table2 t2
       ) q
 GROUP BY q.col1
HAVING COUNT(DISTINCT q.src) < 2
ORDER BY q.col1
 
(The inline view q will be "materialized" as a derived table, so this approach can be expensive for large sets, and this approach won't take advantage of indexes on col1 to perform the matching.) One other small difference between this and the anti-join approach: this will omit a col1 value of NULL if a NULL exists in both tables. Aside from that, the resultset is equivalent.