MySQL Database Operations - mysql

Question

If I want to read the table id from a mysql database in order to add it to a write operation would I need to perform two queries? Or is there a way to perform just one when using mysql? As a rule of thumb, you rarely need two queries but should never exceed two queries for a single operation correct?

Answer 1

Can I use one query?
When you say "table id" I suppose you mean the id column of a table... No need to use two queries. You can use one query and you can insert multiple records at once if you wish (recommended).
An example: Insert two products from a products list as new entries of an order (with 37 as order_id) into an orders table. Each product_id (2, resp. 3) will be read from the products table based on the specified product_code value (6587, resp. 9678).
INSERT INTO orders (
order_id,
product_id
) VALUES (
37,
(SELECT id FROM products WHERE product_code = 6587)
), (
37,
(SELECT id FROM products WHERE product_code = 9678)
)
Where the tables have the following structures:
CREATE TABLE `products` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`product_code` varchar(100) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=5 DEFAULT CHARSET=utf8;
CREATE TABLE `orders` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`order_id` int(11) DEFAULT NULL,
`product_id` int(11) DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
and the table products have the following values:
INSERT INTO `products` (`id`, `product_code`)
VALUES
(1,'1234'),
(2,'6587'),
(3,'9678'),
(4,'5676');
The result in the orders table will look like this:
id order_id product_id
--------------------------
1 37 2
2 37 3
Rules of thumb
I, also, never heard about such a rule regarding the number of queries needed for an operation. Anyway, these are some main rules that I strictly follow:
If you have the chance to achieve specific data access operations using only one query - even if it becomes very complex - then don't hesitate to do it. Put the database engine to work to a maximum whenever you have the chance, even if it seems easier to "chunk" the db operations and benefit from the features of some programming language in order to run them.
Make use of - good designed - indexes. They are very powerful in regard of speed optimization. Use EXPLAIN for checking them.
Design your tables in such a way, that no redundant data is to be found in them. For example, in analogy to my example above, the product_code should be saved only in the products table, even it could make some sense to be saved also in orders table.
"Standardize" your own naming rules across the tables in a/all database(s).
Good luck!

Answer 2

I do not think that there are such rules of thumb.
You should use as many queries as you like to achieve your goal. What you end up with actually will depend on the actions to be done, on the performance needed, on the maintainability needed (think of co-workers which might have to change it) and on other requirements or preferences.
If it really would be possible to do every "single operation" with one query, we probably would not need transactions.
My advice would be to solve your problem with as many queries as you need so that you and your co-workers some years in the future still understand what has been done, and to look into transactions (the MySQL manual and a myriad of tutorials on the net explain them quite well).