具有条件的MySQL唯一索引

Question

I need to have a uniqueness check either with an Index or Constraint on a table, but this only has to happen if the value of a boolean field is true. If it's false, then it's fine for there to be a duplicate row. I don't think this is possible with a constraint or index in mysql, so is there any other possible solution for going about this problem?

Answer 1

If you can slightly change the definition of the field to be "1" for "true" and "NULL" for false, then you can use a unique index on the two columns:

create unique index idx_table_col1_col2 on table(col1, col2)

NULL values can be duplicated in a unique index.

Alternatively, you would have to use a trigger to enforce this condition.