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POMySQL INSERT IF (custom if statements)
 

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    <p>First, here's the concise summary of the question:</p> <p>Is it possible to run an <code>INSERT</code> statement conditionally? Something akin to this:</p> <pre><code>IF(expression) INSERT... </code></pre> <p>Now, I know I can do this with a stored procedure. My question is: can I do this in my query?</p> <hr> <p>Now, why would I want to do that?</p> <p>Let's assume we have the following 2 tables:</p> <pre><code>products: id, qty_on_hand orders: id, product_id, qty </code></pre> <p>Now, let's say an order for 20 Voodoo Dolls (product id 2) comes in.<br /> We first check if there's enough Quantity On Hand:</p> <pre><code>SELECT IF( ( SELECT SUM(qty) FROM orders WHERE product_id = 2 ) + 20 &lt;= ( SELECT qty_on_hand FROM products WHERE id = 2) , 'true', 'false'); </code></pre> <p>Then, if it evaluates to true, we run an <code>INSERT</code> query.<br /> So far so good.</p> <hr> <p>However, there's a problem with concurrency.<br /> If 2 orders come in at the <em>exact same time</em>, they might both read the quantity-on-hand before any one of them has entered the order. They'll then both place the order, thus exceeding the <code>qty_on_hand</code>.</p> <hr> <p>So, back to the root of the question:<br /> Is it possible to run an <code>INSERT</code> statement conditionally, so that we can combine both these queries into one?</p> <p>I searched around a lot, and the only type of conditional <code>INSERT</code> statement that I could find was <code>ON DUPLICATE KEY</code>, which obviously does not apply here.</p>
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