ALTER IGNORE TABLE mytbl ADD UNIQUE (columnName);
This will make a filed a unique index but the command deletes all rows with an empty value, is there any way I can avoid that happening. The field may have NULL values if that helps.
MySQL - Make an existing field unique without deleting empty entries
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Set the "empty" (?) values to NULL and don't use
IGNORE. The purpose ofALTER IGNOREis to remove duplicate values. Multiple NULL are not duplicates of each other, since the expressionNULL = NULLis not true (and is also not false).