How to insert multiple rows in a pgsql table, if the table is empty?

I'm developing a Building Block for Blackboard, and have run into a database related issue.

I'm trying to insert four rows into a pgsql table, but only if the table is empty. The query runs as a post-schema update, and is therefore run whenever I re-install the building block. It is vital that I do not simply drop exsisting values and/or replace them (which would be a simple and effective solution otherwise).

Below is my existing query, that does the job, but only for one row. As I mentioned, I'm trying to insert four rows. I can't simply run the insert multiple times, as after the first run, the table would no longer be empty.

Any help will be appriciated.

BEGIN;
    INSERT INTO my_table_name 
    SELECT
        nextval('my_table_name_SEQ'),
        'Some website URL', 
        'Some image URL',
        'Some website name',
        'Y',
        'Y'
    WHERE 
        NOT EXISTS (
            SELECT * FROM my_table_name
        );
    COMMIT;
END;

Solution

I managed to fix the issue. In this post, @a_horse_with_no_name suggest using UNION ALL to solve a similar issue.

Also thanks to @Dan for suggesting using COUNT, rather than EXISTS

My final query:

BEGIN;

INSERT INTO my_table (pk1, coll1, coll2, coll3, coll4, coll5)
    SELECT x.pk1, x.coll1, x.coll2, x.coll3, x.coll4, x.coll5
        FROM (
            SELECT 
                nextval('my_table_SEQ') as pk1,
                'Some website URL' as coll1, 
                'Some image URL' as coll2,
                'Some website name' as coll3,
                'Y' as coll4,
                'Y' as coll5
            UNION
            SELECT
                nextval('my_table_SEQ'),
                'Some other website URL', 
                'Some other image URL',
                'Some other website name',
                'Y',
                'N'
            UNION
            SELECT
                nextval('my_table_SEQ'),
                'Some other other website URL', 
                'Some other other image URL',
                'Some other other website name',
                'Y',
                'N'
            UNION
            SELECT
                nextval('my_table_SEQ'),
                'Some other other other website URL', 
                'Some other other other image URL',
                'Some other other other website name',
                'Y',
                'Y'
        ) as x
    WHERE
        (SELECT COUNT(*) FROM my_table) <= 0;

    COMMIT;
END;