When are quotes used in MySQL? What is the underlying principle?

In MySQL, there are two main areas of timing for using quotes: string values ​​and expressions.

1. String values: When you insert, update, or query string values ​​in MySQL, you need to enclose the string in quotes. There are two types of quotes to choose from: single quotes (') and double quotes ("). For example:

   SELECT * FROM users WHERE name = 'John';
   

2. Here, 'John' is a string value enclosed in single quotes.

3. Expressions: In some cases, you may need to use quotes to denote strings in expressions. This includes things like using string functions, concatenating strings, and using aliases in queries. For example:

   SELECT CONCAT('Hello', 'World') AS message;
   

4. In this example, the CONCAT function is used to concatenate two strings, and each string is enclosed in single quotes.

5. The underlying principle is that the MySQL parser recognizes content inside quotes as a string value. When parsing an SQL query or command, MySQL checks the input for quotes and treats the content inside the quotes as a string. In this way, MySQL can correctly handle string values ​​and perform corresponding operations, such as inserting, updating, or comparing strings. The use of quotes tells MySQL how to interpret the data in the query.