How to concatenate columns in a Postgres SELECT?

Question

I have two string columns a and b in a table foo.

select a, b from foo returns values a and b<

Answer 1

The problem was in nulls in the values; then the concatenation does not work with nulls. The solution is as follows:

SELECT coalesce(a, '') || coalesce(b, '') FROM foo;

Answer 2

it is better to use CONCAT function in PostgreSQL for concatenation

eg : select CONCAT(first_name,last_name) from person where pid = 136

if you are using column_a || ' ' || column_b for concatenation for 2 column , if any of the value in column_a or column_b is null query will return null value. which may not be preferred in all cases.. so instead of this

||

use

CONCAT

it will return relevant value if either of them have value