How to concatenate columns in a Postgres SELECT?

Solution 1:

With string type columns like character(2) (as you mentioned later), the displayed concatenation just works because, quoting the manual:

[...] the string concatenation operator (||) accepts non-string input, so long as at least one input is of a string type, as shown in Table 9.8. For other cases, insert an explicit coercion to text [...]

Bold emphasis mine. The 2nd example (select a||', '||b from foo) works for any data types since the untyped string literal ', ' defaults to type text making the whole expression valid in any case.

For non-string data types, you can "fix" the 1st statement by casting at least one argument to text. (Any type can be cast to text):

SELECT a::text || b AS ab FROM foo;

Judging from your own answer, "does not work" was supposed to mean "returns NULL". The result of anything concatenated to NULL is NULL. If NULL values can be involved and the result shall not be NULL, use concat_ws() to concatenate any number of values (Postgres 9.1 or later):

SELECT concat_ws(', ', a, b) AS ab FROM foo;

Separators are only added between non-null values, i.e. only where necessary.

Or concat() if you don't need separators:

SELECT concat(a, b) AS ab FROM foo;

No need for type casts here since both functions take "any" input and work with text representations.

More details (and why COALESCE is a poor substitute) in this related answer:

• Combine two columns and add into one new column

Regarding update in the comment

+ is not a valid operator for string concatenation in Postgres (or standard SQL). It's a private idea of Microsoft to add this to their products.

There is hardly any good reason to use character(n) (synonym: char(n)). Use text or varchar. Details:

• Any downsides of using data type "text" for storing strings?
• Best way to check for "empty or null value"

Solution 2:

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;

Solution 3:

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