SQL Comments

A comment is a note for humans. MySQL ignores it.

Three comment styles

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-- This is a comment. Note the space after the two dashes.
SELECT 1 AS one; -- A comment can also follow a statement.

# MySQL also accepts a hash sign for a one-line comment.
SELECT 2 AS two;

/* This comment can
   span several lines. */
SELECT /* or sit inside a statement */ 3 AS three;

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+-----+
| one |
+-----+
|   1 |
+-----+
1 row in set

+-----+
| two |
+-----+
|   2 |
+-----+
1 row in set

+-------+
| three |
+-------+
|     3 |
+-------+
1 row in set

The space after --

In MySQL, -- starts a comment only if it is followed by a space, a tab or a new line. This protects expressions such as 5--3:

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SELECT 5--3 AS result;

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+--------+
| result |
+--------+
|      8 |
+--------+
1 row in set

MySQL read that as 5 - (-3), which is 8. Without the rule, it would have treated --3 as a comment and returned 5. When you write comments, always put a space after --.

Good uses of comments

• Explain why a query is written the way it is, not what each word does.
• Mark a section of a long script.
• Temporarily switch off a line while you debug.

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-- Only active customers; inactive ones are archived monthly.
SELECT 'ok' AS note;

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+------+
| note |
+------+
| ok   |
+------+
1 row in set

Executable comments (MySQL-specific)

MySQL gives a special meaning to a block comment that starts with /*!. The server runs what is inside it, while other databases treat it as a normal comment:

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SELECT 1 /*!50001 + 10 */ AS runs_on_5_0_1_or_newer, 1 /*!99999 + 10 */ AS needs_version_9_99_99;

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+------------------------+-----------------------+
| runs_on_5_0_1_or_newer | needs_version_9_99_99 |
+------------------------+-----------------------+
|                     11 |                     1 |
+------------------------+-----------------------+
1 row in set

The number is a version: 50001 means 5.0.1, and 80000 means 8.0.0. You will see this in files made by mysqldump, so that a dump can include MySQL-specific commands without breaking other tools. You rarely need to write one yourself.

Optimizer hints

A comment that starts with /+ is an optimizer hint, an instruction to the query planner. For example SELECT /+ MAX_EXECUTION_TIME(1000) */ ... stops a query after one second. A comment-style hint is ignored by other databases. See Optimizer Hints.