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When using a uniqie index on a text field in mysql, the column collation setting is very important. The collation settings of a column does not only affect sorting and comparsion, but also unique indexes. So you can not insert "a" and "A" into a table that has a unique index on a column that has a case-insensitive collation. The mysql manual about collations: "A character set is a set of symbols and encodings. A collation is a set of rules for comparing characters in a character set."

Here is an example:
The column text in table text1 has a case-sensitive collation (_cs suffix), the column in text2 has a case-insensitive collation (_ci suffix).

PLAIN TEXT CODE:

1.  
2. CREATE TABLE text1 (
3.   `text` varchar(50) character set latin1 collate latin1_general_cs NOT NULL …