code: func builtinNullIf(args []types.Datum, ctx context.Context) (d types.Datum, err error) {
  	// nullif(expr1, expr2)
  	// returns null if expr1 = expr2 is true, otherwise returns expr1
  	v1 := args[0]
  	v2 := args[1]
  
  	if v1.IsNull() || v2.IsNull() {
  		return v1, nil
  	}
  
  	if n, err1 := v1.CompareDatum(ctx.GetSessionVars().StmtCtx, v2); err1 != nil || n == 0 {
  		d := types.Datum{}
  		return d, errors.Trace(err1)
  	}
  
  	return v1, nil
  }
docs: // See https://dev.mysql.com/doc/refman/5.7/en/control-flow-functions.html#function_nullif