Unique Index with Check clause

[Thomas Lehmann]

Hi,

i have a tricky problem. I have a table like this:
- ID (primary Key)
- State
- Field_A
- Field_B
- Field_C.

The State-field for each dataset can have 3 different states. I want to build a unique Index On Field_A, Field_B, FIeld_C and State buzt only than, when die value of field State has a special value.
How can a handle that problem on mySQL?

The database must do this check because of race condition of my application server.

Any suggestions?

Greatings
Thomas