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By default, MySQL searches are case-insensitive (although there are some character sets that are never case-insensitive, such as czech). That means that if you search with col_name LIKE 'a%', you will get all column values that start with A or a. If you want to make this search case-sensitive, use something like INSTR(col_name, "A")=1 to check a prefix. Or use STRCMP(col_name, "A") = 0 if the column value must be exactly "A".
Simple comparison operations (>=, >, = , < , <=, sorting, and grouping) are based on each character's "sort value". Characters with the same sort value (like E, e, and é) are treated as the same character!