What does -- mean in mysql

In mysql, "--" is a comment character, which means a single-line comment. A space comment needs to be added after the comment character to take effect. The syntax is "-- comment content"; this comment character is the same as " The #" comment character is used for single-line comments. The difference is that there is no need to add a space after the "#" comment character.

The operating environment of this tutorial: windows10 system, mysql8.0.22 version, Dell G3 computer.

What does -- mean in mysql

MySQL single-line comments

1) Single-line comments can use the #comment character, add directly after the #comment character Annotation content. The format is as follows:

#注释内容

Examples of single-line comments using the comment character # are as follows:

#从结果中删除重复行
SELECT DISTINCT product_id, purchase_price FROM Product;

2) Single-line comments can use the -- comment character. A space must be added after the -- comment character before the comment can Take effect. The format is as follows:

-- 注释内容

An example of using the comment character -- in a single-line comment is as follows: The difference between

-- 从结果中删除重复行
SELECT DISTINCT product_id, purchase_price FROM Product;

# and -- is that the comment content is added directly after #, and the second of -- A dash must be followed by a space to add comment content.

MySQL multi-line comments

Multi-line comments use /* */ comment symbols. /* is used for the beginning of the comment content, */ is used for the end of the comment content. The multi-line comment format is as follows:

/*
  第一行注释内容
  第二行注释内容
*/

The comment content is written between /* and */ and can span multiple lines.

Usage examples of multi-line comments are as follows

/*这条SELECT语句，
  会从结果中删除重复行*/
SELECT DISTINCT product_id, purchase_price FROM Product;

Any comment (single-line comment and multi-line comment) can be inserted in the SQL statement, and the comment can be placed anywhere in the SQL statement.

Insert a single-line comment in the SQL statement, as shown below:

SELECT DISTINCT product_id, purchase_price
-- 从结果中删除重复行。 
FROM Product;
SELECT DISTINCT product_id, purchase_price
#从结果中删除重复行。 
FROM Product;

Insert a multi-line comment in the SQL statement, as shown below:

SELECT DISTINCT product_id, purchase_price
/* 这条SELECT语句，  
    会从结果中删除重复行。*/ 
    FROM Product;

Comments can be written in any SQL statements, and there is no limit on the number of comments in a SQL statement.

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