How to base on a characteristic value to determine a formula in Query?

hi All,

In Query row, we've got 4 characteristic CH_A, CH_B, CH_C, CH_D.  

CH_B, CH_C, and CH_D are DATS type of InfoObjects or mean they contain different date values.  Now we would like to build up formula in column and the logic is:  If CH_A value is 'ZA001', Then this formula = CH_B - CH_C (two date difference), Otherwise this formula  = CH_B - CH_D (another two date difference).

We know that CH_B, CH_C, and CH_D have to be transferred to formula variable to realize the data difference calculations.   But no idea on how in formula to use such If condition.  Could someone give a step by step procedure?

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Thanks!

Kevin