Oracle 21c

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create table EGP_SYSTEM_ITEMS (
  ORGANIZATION_ID number,
  INVENTORY_ITEM_ID number,
  PRIMARY_UOM_CODE number,
  UNIT_OF_ISSUE number
);
create table INV_ONHAND_QUANTITIES_DETAIL (
  ORGANIZATION_ID number,
  INVENTORY_ITEM_ID number,
  PRIMARY_TRANSACTION_QUANTITY number
);
create table inv_uom_conversions (
  INVENTORY_ITEM_ID number,
  UOM_CODE number,
  CONVERSION_RATE number
);
SELECT CASE WHEN ITM2.UNIT_OF_ISSUE IS NOT NULL 
            THEN 
             ( SUM(IOQD.PRIMARY_TRANSACTION_QUANTITY) /  UOM2.CONVERSION_RATE)

            ELSE SUM(ITM2.PRIMARY_UOM_CODE) 
       END AS ONHAND_QTY

FROM EGP_SYSTEM_ITEMS ITM2, INV_ONHAND_QUANTITIES_DETAIL IOQD, inv_uom_conversions UOM2

WHERE IOQD.INVENTORY_ITEM_ID = ITM2.INVENTORY_ITEM_ID
 AND IOQD.ORGANIZATION_ID = ITM2.ORGANIZATION_ID 
 AND UOM2.INVENTORY_ITEM_ID = ITM2.INVENTORY_ITEM_ID
 AND UOM2.UOM_CODE = ITM2.UNIT_OF_ISSUE 
ORA-00937: not a single-group group function

SELECT CASE WHEN ITM2.UNIT_OF_ISSUE IS NOT NULL 
            THEN 
             ( SUM(IOQD.PRIMARY_TRANSACTION_QUANTITY) /  UOM2.CONVERSION_RATE)

            ELSE SUM(ITM2.PRIMARY_UOM_CODE) 
       END AS ONHAND_QTY

FROM EGP_SYSTEM_ITEMS ITM2, INV_ONHAND_QUANTITIES_DETAIL IOQD, inv_uom_conversions UOM2

WHERE IOQD.INVENTORY_ITEM_ID = ITM2.INVENTORY_ITEM_ID
 AND IOQD.ORGANIZATION_ID = ITM2.ORGANIZATION_ID 
 AND UOM2.INVENTORY_ITEM_ID = ITM2.INVENTORY_ITEM_ID
 AND UOM2.UOM_CODE = ITM2.UNIT_OF_ISSUE 
GROUP BY ITM2.UNIT_OF_ISSUE, UOM2.CONVERSION_RATE
SELECT CASE WHEN ITM2.UNIT_OF_ISSUE IS NOT NULL 
            THEN 
             ( SUM(IOQD.PRIMARY_TRANSACTION_QUANTITY) /  UOM2.CONVERSION_RATE)
            ELSE SUM(ITM2.PRIMARY_UOM_CODE) 
       END AS ONHAND_QTY
FROM EGP_SYSTEM_ITEMS ITM2
JOIN INV_ONHAND_QUANTITIES_DETAIL IOQD
  ON IOQD.INVENTORY_ITEM_ID = ITM2.INVENTORY_ITEM_ID
 AND IOQD.ORGANIZATION_ID = ITM2.ORGANIZATION_ID 
JOIN inv_uom_conversions UOM2
  ON UOM2.INVENTORY_ITEM_ID = ITM2.INVENTORY_ITEM_ID
 AND UOM2.UOM_CODE = ITM2.UNIT_OF_ISSUE 
GROUP BY ITM2.UNIT_OF_ISSUE, UOM2.CONVERSION_RATE