MySQL distinction between e and é (e acute) - UNIQUE index

and collation is "utf8_general_ci".

And that's the answer. If you're using utf8_general_ci (actually it applies to all utf_..._[ci|cs]) collation then diacritics are bypassed in comarison, thus:

SELECT "e" = "é" AND "O" = "Ó" AND "ä" = "a"

Results in 1. Indexes also use collation.

If you want to distinguish between ą and a then use utf8_bin collation (keep in mind that it also distinguish between uppercase and lowercase characters).

By the way name and age don't guarantee any uniqueness.

I found that

ALTER TABLE students CHARACTER SET utf8 COLLATE utf8_bin;

did not work for me, as it didn't change the collation of existing columns, as can be seen in the results of this query:

SHOW FULL COLUMNS from students;

However, the following query did the job and converted existing columns to utf8_bin collation:

ALTER TABLE students CONVERT TO CHARACTER SET utf8 COLLATE utf8_bin;

(notice the "CONVERT TO")