PostgreSQL import from CSV NULL values are text - Need null - sql

Question

I had exported a bunch of tables (>30) as CSV files from MySQL database using phpMyAdmin. These CSV file contains NULL values like:
"id","sourceType","name","website","location"
"1","non-commercial","John Doe",NULL,"California"
I imported many such csv to a PostgreSQL database with TablePlus. However, the NULL values in the columns are actually appearing as text rather than null.
When my application fetches the data from these columns it actually retrieves the text 'NULL' rather than a null value.
Also SQL command with IS NULL does not retrieve these rows probably because they are identified as text rather than null values.
Is there a SQL command I can do to convert all text NULL values in all the tables to actual NULL values? This would be the easiest way to avoid re-importing all the tables.

Answer 1

PostgreSQL's COPY command has the NULL 'some_string' option that allows to specify any string as NULL value: https://www.postgresql.org/docs/current/sql-copy.html
This would of course require re-importing all your tables.
Example with your data:
The CSV:
"id","sourceType","name","website","location"
"1","non-commercial","John Doe",NULL,"California"
"2","non-commercial","John Doe",NULL,"California"
The table:
CREATE TABLE import_with_null (id integer, source_type varchar(50), name varchar(50), website varchar(50), location varchar(50));
The COPY statement:
COPY import_with_null (id, source_type, name, website, location) from '/tmp/import_with_NULL.csv' WITH (FORMAT CSV, NULL 'NULL', HEADER);
Test of the correct import of NULL strings as SQL NULL:
SELECT * FROM import_with_null WHERE website IS NULL;
id | source_type | name | website | location
----+----------------+----------+---------+------------
1 | non-commercial | John Doe | | California
2 | non-commercial | John Doe | | California
(2 rows)
The important part that transforms NULL strings into SQL NULL values is NULL 'NULL' and could be any other value NULL 'whatever string'.

Answer 2

UPDATE For whoever comes here looking for a solution
See answers for two potential solutions
One of the solutions provides a SQL COPY method which must be performed before the import itself. The solution is provided by Michal T and marked as accepted answer is the better way to prevent this from happening in the first place.
My solution below uses a script in my application (Built in Laravel/PHP) which can be done after the import is already done.
Note- See the comments in the code and you could potentially figure out a similar solution in other languages/frameworks.
Thanks to #BjarniRagnarsson suggestion in the comments above, I came up with a short PHP Laravel script to perform update queries on all columns (which are of type 'string' or 'text') to replace the 'NULL' text with NULL values.
public function convertNULLStringToNULL()
{
$tables = DB::connection()->getDoctrineSchemaManager()->listTableNames(); //Get list of all tables
$results = []; // an array to store the output results
foreach ($tables as $table) { // Loop through each table
$columnNames = DB::getSchemaBuilder()->getColumnListing($table); //Get list of all columns
$columnResults = []; // array to store the results per column
foreach ($columnNames as $column) { Loop through each column
$columnType = DB::getSchemaBuilder()->getColumnType($table, $column); // Get the column type
if (
$columnType == 'string' || //check if column type is string or text
$columnType == 'text'
) {
$query = "update " . $table . " set \"" . $column . "\"=NULL where \"" . $column . "\"='NULL'"; //Build the update query as mentioned in comments above
$r = DB::update($query); //perform the update query
array_push($columnResults, [
$column => $r
]); //Push the column Results
}
}
array_push($results, [
$table => $columnResults
]); // push the table results
}
dd($results); //Output the results
}
Note I was using Laravel 8 for this.