Comment Syntax

TiDB supports three comment styles:

  • Use # to comment a line.
  • Use -- to comment a line, and this style requires at least one whitespace after --.
  • Use /* */ to comment a block or multiple lines.

Example:

mysql> SELECT 1+1;     # This comment continues to the end of line
+------+
| 1+1  |
+------+
|    2 |
+------+
1 row in set (0.00 sec)

mysql> SELECT 1+1;     -- This comment continues to the end of line
+------+
| 1+1  |
+------+
|    2 |
+------+
1 row in set (0.00 sec)

mysql> SELECT 1 /* this is an in-line comment */ + 1;
+--------+
| 1  + 1 |
+--------+
|      2 |
+--------+
1 row in set (0.01 sec)

mysql> SELECT 1+
    -> /*
   /*> this is a
   /*> multiple-line comment
   /*> */
    -> 1;
+-------+
| 1+

1 |
+-------+
|     2 |
+-------+
1 row in set (0.00 sec)

mysql> SELECT 1+1--1;
+--------+
| 1+1--1 |
+--------+
|      3 |
+--------+
1 row in set (0.01 sec)

Similar to MySQL, TiDB supports a variant of C comment style:

/*! Specific code */

In this comment style, TiDB runs the statements in the comment. The syntax is used to make these SQL statements ignored in other databases and run only in TiDB.

For example:

SELECT /*! STRAIGHT_JOIN */ col1 FROM table1,table2 WHERE ...

In TiDB, you can also use another version:

SELECT STRAIGHT_JOIN col1 FROM table1,table2 WHERE ...

If the server version number is specified in the comment, for example, /*!50110 KEY_BLOCK_SIZE=1024 */, in MySQL it means that the contents in this comment is processed only when the MySQL version is or higher than 5.1.10. But in TiDB, the MySQL version number does not work and all contents in the comment are processed. TiDB has its own comment syntax for the version number. The format of this syntax is /*T!30100 XXX */.

Another type of comment is specially treated as the Hint optimizer:

SELECT /*+ hint */ FROM ...;

Since Hint is involved in comments like /*+ xxx */, the MySQL client clears the comment by default in versions earlier than 5.7.7. To use Hint in those earlier versions, add the --comments option when you start the client. For example:

mysql -h 127.0.0.1 -P 4000 -uroot --comments

For details about the optimizer hints that TiDB supports, see Optimizer hints.

For more information, see Comment Syntax.

"Comment Syntax" was last updated May 14 2020: add alias (#2567) (e3c8da8)