Extending PostgreSQL – A better CONCAT operator

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PostgreSQL users have the option of extending the set of functions and operators available. If you have a common SQL or programmatic routine, custom functions can be an effective way to more succinctly and efficiently accomplish your tasks. Likewise, custom operators can be created to call these functions (or existing built-in functions) in order to make more efficient and legible SQL statements. PostgreSQL provides four kinds of functions: query language functions (functions written in SQL), procedural language functions (functions written in, for example, PL/pgSQL or PL/Tcl), internal functions, C-language functions.

Creating custom functions or operators in PostgreSQL is not difficult at all. Here is an example of a user-defined operator that I find very useful.

A better CONCAT operator for PostgreSQL

In PostgreSQL, (unlike Oracle for example) concatenation with a null column (using || operator) nulls the result.
Here’s a quick example:

As you can see, the rows containing NULL values are omitted (concatenation result is NULL). This might not be the behavior you expect. There are several solutions to handle this situation, including the use of the SQL CASE WHEN expression, COALESCE or ARRAY_TO_STRING built-in functions.
But a more interesting and elegant approach is to make use of the PostgreSQL extensibility and to create an alternate version of the CONCAT operator.
First, we define a function which takes two TEXT values, and returns:
– One value if one of them is null (null if both are null)
– Concatenation of the values with a space between them if neither is null

Then we create a custom operator “||+

Using this new operator in a query is very easy and will produce a more natural result.

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7 Comments
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  • David Frankson says:

    I was looking for a replacement for SQL Server’s + operator so it would treat NULL as ”, and could work with ints, floats, etc. I had to define 3 functions to over the operator. Posting here to save someone else time:

    CREATE OR REPLACE FUNCTION concat1(text,anyelement)
    RETURNS text AS
    $BODY$

    SELECT COALESCE($1,”) || COALESCE(CAST($2 as text),”)

    $BODY$
    LANGUAGE sql IMMUTABLE;

    CREATE OR REPLACE FUNCTION concat2(anyelement, text)
    RETURNS text AS
    $BODY$

    SELECT COALESCE($1,”) || COALESCE(CAST($2 as text),”)

    $BODY$
    LANGUAGE sql IMMUTABLE;

    CREATE OR REPLACE FUNCTION concat3(text, text)
    RETURNS text AS
    $BODY$

    SELECT COALESCE($1,”) || COALESCE($2,”)

    $BODY$
    LANGUAGE sql IMMUTABLE;

    CREATE OPERATOR + (LEFTARG = text, RIGHTARG = anyelement, PROCEDURE = concat);
    CREATE OPERATOR + (LEFTARG = anyelement, RIGHTARG = text, PROCEDURE = concat2);
    CREATE OPERATOR + (LEFTARG = text, RIGHTARG = text, PROCEDURE = concat3);

    November 18, 2010 at 11:31 pm

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