In MySQL, should I quote numbers or not?

Question

For example - I create database and a table from cli and insert some data:

CREATE DATABASE testdb CHARACTER SET 'utf8' COLLATE 'utf8_general_ci';
USE testdb;
CREATE TABLE test (id INT, str VARCHAR(100)) TYPE=innodb CHARACTER SET 'utf8' COLLATE 'utf8_general_ci';
INSERT INTO test VALUES (9, 'some string');

Now I can do this and these examples do work (so - quotes don't affect anything it seems):

SELECT * FROM test WHERE id = '9';
INSERT INTO test VALUES ('11', 'some string');

So - in these examples I've selected a row by a string that actually stored as INT in mysql and then I inserted a string in a column that is INT.

I don't quite get why this works the way it works here. Why is string allowed to be inserted in an INT column?

Can I insert all MySQL data types as strings?

Is this behavior standard across different RDBMS?

Answer 1

MySQL is a lot like PHP, and will auto-convert data types as best it can. Since you're working with an int field (left-hand side), it'll try to transparently convert the right-hand-side of the argument into an int as well, so '9' just becomes 9.

Strictly speaking, the quotes are unnecessary, and force MySQL to do a typecasting/conversion, so it wastes a bit of CPU time. In practice, unless you're running a Google-sized operation, such conversion overhead is going to be microscopically small.