Linking Reports base on different data type of fields or formula by using following steps in SAP Crystal Report:
1. Main Report ItemCode Field with VARCHAR Data Type
2. Sub-Report ItemCString Field with INTEGER Data Type and Formula ItemCFormula
3. Click Insert Menu > Insert Sub-Report. Now Select Field to link and leave Select data in sub-report based on field option unchecked.
4. Preview Main Report and Double Click Sub-report to open its preview. Then go to Report > Select Expert > Record > Add Formula for manual linking with main report.
5. Use formula to link sub-report field or formula with different Data Type to main report field manually.
6. Save and close and Preview main report.