如何在select语句中连接可能包含NULL值的两列？

Question

I tried with:

SELECT CONCAT(col1, col2) AS NewCol FROM tableName

It works only if col1 and col2 are not null, else NewCol becomes null, even if one of col1 and col2 is not null. I want it to display whatever value that is available.

Answer 1

Use CONCAT_WS .

SELECT CONCAT_WS('', col1, col2) AS NewCol FROM tableName

It would return an empty string in case both the values are null .

Documentation:

CONCAT_WS(separator,str1,str2,...)

CONCAT_WS() stands for Concatenate With Separator and is a special form of CONCAT(). The first argument is the separator for the rest of the arguments. The separator is added between the strings to be concatenated. The separator can be a string, as can the rest of the arguments. If the separator is NULL, the result is NULL.

Answer 2

You can use CONCAT_WS . its NULL safe. the first argument is the separator.

sample

MariaDB [l]> SELECT CONCAT_WS('',NULL,'123');
+--------------------------+
| CONCAT_WS('',NULL,'123') |
+--------------------------+
| 123                      |
+--------------------------+
1 row in set (0.00 sec)

MariaDB [l]>

Answer 3

你也可以使用

SELECT ISNULL([col1],'') + ISNULL([col2],'') AS NewCol FROM tableName