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

Question

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;

Answer 1

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;

Answer 2

It is better if you count the rows because it gets the number of input rows.
This should work:
BEGIN;
INSERT INTO my_table_name
SELECT
nextval('my_table_name_SEQ'),
'Some website URL',
'Some image URL',
'Some website name',
'Y',
'Y'
WHERE
(SELECT COUNT(*) FROM my_table_name)>0
COMMIT;
END;

Answer 3

Inserts won't overwrite, so I'm not understanding that part of your question.
Below are two ways to insert multiple rows; the second example is a single sql statement:
create table test (col1 int,
col2 varchar(10)
) ;
insert into test select 1, 'A' ;
insert into test select 2, 'B' ;
insert into test (col1, col2)
values (3, 'C'),
(4, 'D'),
(5, 'E') ;
select * from test ;
1 "A"
2 "B"
3 "C"
4 "D"
5 "E"