How do I subtract a number from my quantity stored in my mysql database

Question

I have a website which is used to key in and store stock data. Right now I am creating a component which is used to take out stock. What I want to do is key in a number in the text box and the database will subtract the number from the quantity.

I have tried subtracting with a fixed hard coded number such as

//rest api to update record in mysql database
app.put('/stockdata/subtract', function (req, res) {
  connection.query('UPDATE stockin SET Quantity = Quantity - 5 WHERE id=?', [req.body.Quantity, req.body.id], function (error, results, fields) {
    if (error) throw error;
    res.end(JSON.stringify(results));
  })

It works well. But when i wants to change the -5 to my own input I don't know how to get it working. Here is my input code

// component that contains the logic to update a product
window.UpdateProductComponent = React.createClass({
    getInitialState: function () {
        // Get this product fields from the data attributes we set on the
        // #content div, using jQuery
        return {
            id: 0,
            Quantity: 0,
            successUpdate: null
        };
    },

    // on mount, fetch all categories and one product data to stored them as this component's state
    componentDidMount: function () {
        // read one product data
        var productId = this.props.productId;
        this.serverRequestProd = $.get("http://localhost:4000/stockdata/" + productId,
            function (product) {
                this.setState({ id: product.id });
                this.setState({ Quantity: product.Quantity });
            }.bind(this));

        $('.page-header h1').text('Update product');
    },

    // on unmount, kill categories fetching in case the request is still pending
    componentWillUnmount: function () {
        this.serverRequestCat.abort();
        this.serverRequestProd.abort();
    },
    // handle Quantity change
    onQuantityChange: function (e) {
        this.setState({ Quantity: e.target.value });
    },

    // handle save changes button clicked
    onSave: function (e) {
        var productId = this.props.productId;
        // data in the form
        var form_data = {
            id: this.props.productId,
            Quantity: this.state.Quantity,
        };

        // submit form data to api
        $.ajax({
            url: "http://localhost:4000/stockdata/subtractfiv",
            type: "PUT",
            contentType: 'application/json',
            data: JSON.stringify(form_data),
            success: function (response) {
                this.setState({ successUpdate: response['message'] });
            }.bind(this),
            error: function (xhr, resp, text) {
                // show error to console
                console.log(xhr, resp, text);
            }
        });

        e.preventDefault();
    },

    render: function () {
        return (
            <div>
                {
                    this.state.successUpdate == "Product was updated." ?
                        <div className='alert alert-success'>
                            Product was updated.
                    </div>
                        : null
                }

                {
                    this.state.successUpdate == "Unable to update product." ?
                        <div className='alert alert-danger'>
                            Unable to update product. Please try again.
                    </div>
                        : null
                }

                <a href='#'
                    onClick={() => this.props.changeAppMode('read')}
                    className='btn btn-primary margin-bottom-1em'>
                    Read Products
            </a>

                <form onSubmit={this.onSave}>
                    <table className='table table-bordered table-hover'>
                        <tbody>
                            <tr>
                                <td>Quantity </td>
                                <td>
                                    <input
                                        type='number'
                                        className='form-control'
                                        value={this.props.Quantity}
                                        required
                                        onChange={this.onQuantityChange} />
                                </td>
                                <tr>
                                    <td></td>
                                    <td>
                                        <button
                                            className='btn btn-primary'
                                            onClick={this.onSave}>Save Changes</button>
                                    </td>
                                </tr>
                            </tr>
                        </tbody>
                    </table>
                </form>
            </div>
        );
    }

});

Is there anyway I could subtract the quantity in my database with my own input number?

Answer 1

I suspect it's something like this: (never used mysqljs)

connection.query('UPDATE stockin SET Quantity = Quantity - ? WHERE id=?', [req.body.Quantity, req.body.id] ...

Just make sure your Quantity value in your request is not 0!

For more info see How to pass parameters to mysql query callback in nodejs