INSERT INTO tblusers(login)
SELECT
CONCAT(SUBSTRING(Familenaam, 1 ,4), SUBSTRING(Voornaam,1,4))
FROM tblusers
如果要更新，可以执行以下操作：
UPDATE tblusers t1
INNER JOIN
(
SELECT
userid,
CONCAT(SUBSTRING(Familenaam, 1 ,4), SUBSTRING(Voornaam,1,4)) login
FROM tblusers
) t2 ON t1.userid = t2.userid
SET t1.login = t2.login
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