如何选择两个表的不同值？

Question

I'm not sure it's possible but I would like to select the distinct value of two tables columns.

Consider table1 like:

╔══════╗
║ col1 ║
╠══════╣
║ AAA  ║
║ AAA  ║
║ BBB  ║
║ BBB  ║
╚══════╝

and table2 like:

╔══════╗
║ col1 ║
╠══════╣
║ AAA  ║
║ BBB  ║
║ CCC  ║
║ CCC  ║
╚══════╝

The output I would like would be:

AAA
BBB
CCC

Obviously something like:

SELECT DISTINCT table1.col1 , table2.col1 FROM table1 , table2;

won't work.

SQLFiddle

Answer 1

A simple UNION would do the tricks

SELECT col1 FROM table1 
UNION 
SELECT col1 FROM table2;

There is no need to use the DISTINCT keyword as UNION already handle duplicates.

FYI if you use UNION ALL the duplicates won't be handled anymore, then you would need to use DISTINCT .

Answer 2

只需进行union完成工作，因为它将过滤出重复的记录。

SELECT col1 FROM table1 UNION SELECT col1 FROM table2

Answer 3

I don't usually go for sub-queries, but I think this is one of those times:

SELECT DISTINCT col1
FROM (SELECT col1 FROM table1 UNION SELECT col1 FROM table2)