如何为UNION构建"空列"？

Question

根据我的理解"UNION中的每个SELECT语句必须具有相同的列数.列也必须具有相似的数据类型.此外,每个SELECT语句中的列必须具有相同的顺序." 那么,如果第一个SELECT的列数多于第二个可以生成的列数,那该怎么办？这就是我的意思:让我说我想

SELECT "City", "Country", "Continent" from table1  
UNION  
SELECT "City", "Country" from table2     

...假设表2不包含名为"Continent"的列,但是对于我的需要,来自table2的记录在该列中具有空白或NULL是正常的.我正在使用dashDB.

Answer 1

您始终可以添加"虚拟"列:

SELECT "City", "Country", "Continent" from table1  
UNION  
SELECT "City", "Country", NULL AS "Continent" from table2 

Answer 2

您好，
您可以使用，

 SELECT "City", "Country", "Continent" from table1  
UNION  
SELECT "City", "Country", ' ' as "Continent"  from table2

或者

 SELECT "City", "Country", "Continent" from table1  
    UNION  
    SELECT 

"City", "Country", NULL as "Continent"  from table2

它将表 2 中的“Continent”视为空