Import CSV to Update only one column in table

I would load the update data into a seperate table UPDATE_TABLE and perform an update within MySQL using:

UPDATE PRODUCTS P SET P.QUANTITY=(
    SELECT UPDATE_QUANTITY
    FROM UPDATE_TABLE
    WHERE UPDATE_PRODUCT=P.PRODUCT
)

I dont have a MySQL at hand right now, so I can check the syntax perfectly, it might be you need to add a LIMIT 0,1 to the inner SELECT.

Answer from @ike-walker is indeed correct but also remember to double check how your CSV data if formatted. Many times for example CSV files can have string fields enclosed in double quotes ", and lines ending with \r\n if working on Windows.
By default is assumed that no enclosing character is used and line ending is \n. More info and examples here https://mariadb.com/kb/en/importing-data-into-mariadb/

This can be fixed by using additional options for FIELDS and LINES

CREATE TEMPORARY TABLE your_temp_table LIKE your_table;

LOAD DATA INFILE '/tmp/your_file.csv'
INTO TABLE your_temp_table
FIELDS 
   TERMINATED BY ','            
   OPTIONALLY ENCLOSED BY '"'    -- new option
LINES TERMINATED BY '\r\n'       -- new option

(id, product, sku, department, quantity); 

UPDATE your_table
INNER JOIN your_temp_table on your_temp_table.id = your_table.id
SET your_table.quantity = your_temp_table.quantity;

DROP TEMPORARY TABLE your_temp_table;

You can use LOAD DATA INFILE to bulk load the 800,000 rows of data into a temporary table, then use multiple-table UPDATE syntax to join your existing table to the temporary table and update the quantity values.

For example:

CREATE TEMPORARY TABLE your_temp_table LIKE your_table;

LOAD DATA INFILE '/tmp/your_file.csv'
INTO TABLE your_temp_table
FIELDS TERMINATED BY ','
(id, product, sku, department, quantity); 

UPDATE your_table
INNER JOIN your_temp_table on your_temp_table.id = your_table.id
SET your_table.quantity = your_temp_table.quantity;

DROP TEMPORARY TABLE your_temp_table;