Update specific records of MySQL table

Question

I am dealing with phone system and have to work with multiple service vendors. For one vendor I have a MySQL table country_codes like this -

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Answer 1

If you need to do the fix the data ONLY once, you can try this approach:

0) Backup your data, or better, run the query on copy of the data.

1) Creates a table that contains non-zero country codes. We need separate table because it says in MySQL manual that:

Currently, you cannot update a table and select from the same table in a subquery.

CREATE TABLE country_codes_list (
    country_code INT NOT NULL PRIMARY KEY
);

INSERT INTO country_codes_list
SELECT country_code
FROM country_codes
WHERE country_code <> 0;

2) Update all rows where country code is 0 by finding the country code that matches the beginning of the area code:

UPDATE country_codes AS country_codes_zero SET country_code = (
    SELECT country_code
    FROM country_codes_list
    WHERE country_codes_list.country_code = SUBSTRING(country_codes_zero.area_code, 1, LENGTH(country_codes_list.country_code))
) WHERE country_code = 0;

This could be a very slow query because it uses a co-related sub-query. But it should fix the data in one go.