insert into database only when id does not exist - sql

Question

I am using Postgres, In my code below I want to insert into the database only if the id doesn't already exist.
DROP TABLE json_table;
CREATE temp TABLE json_table (
id VARCHAR(50) PRIMARY KEY,
str_col VARCHAR(500),
int_col SMALLINT,
bool_col BOOLEAN,
json_col JSON,
float_col DECIMAL
);
INSERT INTO json_table
SELECT * FROM json_populate_recordset (NULL::json_table,
'{ "insert": [
{
"id": "1",
"str_col": "Postgres bar data",
"int_col": 3151,
"bool_col": false,
"json_col": {
"data": "tutorials"
},
"float_col": 11.51099159756918
},
{
"id": "2",
"str_col": "Postgres tutorials data data",
"int_col": 4237,
"bool_col": true,
"json_col": {
"type": "type"
},
"float_col": 48.94065780742467
}
]}'::json->'insert');
SELECT * FROM json_table;

Answer 1

You can use the on conflict do nothing clause. Your id column is the primary key of the table, so this would just work:
INSERT INTO json_table
SELECT *
FROM json_populate_recordset (NULL::json_table,
'{ "insert": [
{
"id": "1",
"str_col": "Postgres bar data",
"int_col": 3151,
"bool_col": false,
"json_col": {
"data": "tutorials"
},
"float_col": 11.51099159756918
},
{
"id": "2",
"str_col": "Postgres tutorials data data",
"int_col": 4237,
"bool_col": true,
"json_col": {
"type": "type"
},
"float_col": 48.94065780742467
}
]}'::json->'insert')
ON CONFLICT (id) DO NOTHING;
Demo on DB Fiddle
Say this record was inserted before:
insert into json_table(id) values(1)
The above query executes without error, and inserts the second row only.

Answer 2

Define a unique constraint on id:
alter table t add constraint unq_json_table_id unique(id);
You can then insert using on conflict:
insert into json_table . . .
. . .
on conflict (id) do nothing;