在一列中合并两列

Question

I have column 1 and column 2, and want to combine them into column 3 of the same table. If the column 2 is null then display column 1 value, if column 1 is null, them display column 2 data. If both of them are null, then display null. I tried two things:

1) using CONCAT SELECT CONCAT(Column1, Column2) AS Column3 FROM TEST_ATTRIBUTES .

It just merges the columns only when both of them are not null. otherwise it just prins null.

2) using (column1 + column 2).

SELECT (Column1 + Column2) AS Column3 FROM TEST_ATTRIBUTES .

doesn't show desired output.

I'm writing this code in java. Thanks

Answer 1

use COALESCE() , this doesn't concatenate but returns the first non-null value from the list.

SELECT Column1, 
       Column2, 
       COALESCE(Column1, Column2) AS Column3 
FROM   TEST_ATTRIBUTES

• SQLFiddle Demo

if there are chances that both of them are null,

SELECT Column1, 
       Column2, 
       IF(Column1 IS NULL AND Column2 IS NULL, NULL, CONCAT(COALESCE(Column1,''), COALESCE(Column2,''))) AS Column3 
FROM   TEST_ATTRIBUTES

• SQLFiddle Demo