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Learn how to effectively manage stock and sales data in your SQLite Android database by subtracting quantities from your stock table based on sales records.
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Managing Stock in SQLite for Android: Subtracting Data Between Two Tables

When building an Android application that involves inventory management, it’s common to find yourself needing to keep track of stock levels in the face of sales transactions. Suppose you have two tables in your SQLite database: stock_table for inventory and sell_table for recorded sales. The challenge arises when you need to update your inventory by subtracting the quantities sold from your total stock.

In this post, we'll explore how to effectively perform this subtraction using SQL queries, ensuring your stock levels remain accurate after each sale.

Understanding the Database Structure

Let’s break down the two tables involved in this operation:

1. Stock Table (stock_table)

This table holds the inventory data and includes the following columns:

code (Identifier for the product)

name (Name of the product)

price (Price per unit)

total_stock (Total quantity available in stock)

codenamepricetotal stock1Books5002002Pen105003Reading Table1000202. Sell Table (sell_table)

This table records sales transactions, comprising:

code (Identifier for the product)

name (Name of the product)

sell_qty (Quantity sold)

date (Timestamp of the transaction)

codenamesell_qtydate1Books101-02-2015 8:00:36 GMT2Pen501-02-2015 8:02:45 GMT3Reading Table51-02-2015 8:33:32 GMT1Books51-02-2015 9:04:15 GMT2Pen151-02-2015 9:23:32 GMT1Books41-02-2015 10:43:18 GMTThe Solution: SQL Update with a Subquery

To keep your stock levels accurate, you can use an SQL update statement combined with a subquery. This will allow you to subtract the total quantities sold from your stock levels.

Basic Update Statement

Here’s the SQL query you need:

[[See Video to Reveal this Text or Code Snippet]]

Explanation:

The UPDATE command updates the total_stock in the stock_table.

The subquery (SELECT SUM(sell_qty) FROM sell_table st WHERE stock_table.code = st.code) calculates the total sold quantity for each product code.

The result from the subquery is then subtracted from total_stock.

Updating for a Specific Product

If there’s a need to update the stock for a specific product only, you can refine the query using the WHERE clause. For example, if you wanted to update only for the product with code 101, the query would look like this:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

Managing inventory in an app can be made simpler using SQLite’s powerful subqueries. By correctly implementing the update logic, you can ensure that the total_stock reflects real-time sales, helping you maintain accurate inventory levels and avoid stock shortages.

Feel free to adjust the queries to fit your database structure and always keep track of your data to make informed decisions for your business.

By following the methods outlined in this post, you are on your way to effectively managing your stock and sales data in your Android application using SQLite.