SQL 如何为 UNION 构建“虚拟列”？
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How to build "dummy columns" for UNION?
提问by Mike Pala
from what I understand "each SELECT statement within the UNION must have the same number of columns. The columns must also have similar data types. Also, the columns in each SELECT statement must be in the same order." Well what if the first SELECT has more columns than the 2nd one can generate. Here's what I mean: let's say I want to
根据我的理解，“UNION 中的每个 SELECT 语句必须具有相同数量的列。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。” 好吧，如果第一个 SELECT 的列比第二个可以生成的列多怎么办。这就是我的意思：假设我想要
SELECT "City", "Country", "Continent" from table1
UNION
SELECT "City", "Country" from table2
...let's say table 2 does not contain a column called "Continent" but for my needs it's fine for the records that come from table2 to have a blank or NULL in that column. I am using dashDB.
...假设表 2 不包含名为“Continent”的列，但根据我的需要，来自 table2 的记录在该列中具有空白或 NULL 是可以的。我正在使用 dashDB。
回答by Tim Schmelter
You can always add "virtual" columns:
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
回答by Jim Macaulay
Hi,
You can use,
您好，
您可以使用，
SELECT "City", "Country", "Continent" from table1
UNION
SELECT "City", "Country", ' ' as "Continent" from table2
or
或者
SELECT "City", "Country", "Continent" from table1
UNION
SELECT
"City", "Country", NULL as "Continent" from table2
It considers "Continent" as null in table2
它将表 2 中的“Continent”视为空
