[SQL] When to use single quotes, double quotes, and backticks in MySQL

I am trying to learn the best way to write queries. I also understand the importance of being consistent. Until now, I have randomly used single quotes, double quotes, and backticks without any real thought.

Example:

$query = 'INSERT INTO table (id, col1, col2) VALUES (NULL, val1, val2)';


Also, in the above example, consider that table, col1, val1, etc. may be variables.

What is the standard for this? What do you do?

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