If you have a simple schema:
CREATE TABLE region_codes (
region_code CHAR(6) NOT NULL
, PRIMARY KEY (region_code)
);
CREATE TABLE phone_numbers (
phone_number VARCHAR(16) NOT NULL
, PRIMARY KEY (phone_number)
);
Try the following:
SELECT region_code
FROM region_codes
WHERE region_code = LEFT(@my_phone_number, LENGTH(region_code))
ORDER BY LENGTH(region_code) DESC
LIMIT 1;
all the best!
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