SQL & PL/SQL :: Update Statement For OLTP Table

I have a table TEMP_TEST_1 to be loaded with multiple rows which contains the available items for a store with their sequence numbers. I also have another table TEMP_INV_1 which holds a column for inventory count of the items, and this column is to be updated after loading the first table TEMP_TEST_1.

The table TEMP_TEST_1 is updated through all possible channels in OLTP. So while updating the inventory, which is the best way. My update should either add the total number of inserts per item into the TEMP_TEST_1 table's INVENTORY_CNT column i.e. existing value + count of new rows OR a full update by taking the complete row count (Grouped by item id) and update the INVENTORY_CNT column

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-- Initial setup SQL's, TEMP_TEST_1 is updates on OLTP through multiple channels
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CREATE TABLE TEMP_TEST_1 ( ITEMID NUMBER ( 4 ),
INVENTORY_CNT NUMBER ( 2 ) );
CREATE TABLE TEMP_INV_1 ( ITEMID NUMBER ( 4 ),
ITEMSEQ NUMBER ( 5 ) );
INSERT INTO
[code].......

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-- Inserts for new items in the store
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INSERT INTO
TEMP_INV_1
VALUES
( 1,
11 );
INSERT INTO
TEMP_INV_1
[code].......