#### 1.7.2.4 '--' as the Start of a Comment

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 Standard SQL uses the C syntax `/* this is a comment */` for comments, and MySQL Server supports this syntax as well. MySQL also support extensions to this syntax that enable MySQL-specific SQL to be embedded in the comment, as described in [Section 9.7, “Comments”](comments.html).

 Standard SQL uses “`--`” as a start-comment sequence. MySQL Server uses `#` as the start comment character. MySQL Server also supports a variant of the `--` comment style. That is, the `--` start-comment sequence must be followed by a space (or by a control character such as a newline). The space is required to prevent problems with automatically generated SQL queries that use constructs such as the following, where we automatically insert the value of the payment for `payment`:

```
UPDATE account SET credit=credit-payment
```

 Consider about what happens if `payment` has a negative value such as `-1`:

```
UPDATE account SET credit=credit--1
```

`credit--1` is a valid expression in SQL, but `--` is interpreted as the start of a comment, part of the expression is discarded. The result is a statement that has a completely different meaning than intended:

```
UPDATE account SET credit=credit
```

 The statement produces no change in value at all. This illustrates that permitting comments to start with `--` can have serious consequences.

 Using our implementation requires a space following the `--` for it to be recognized as a start-comment sequence in MySQL Server. Therefore, `credit--1` is safe to use.

 Another safe feature is that the [**mysql**](mysql.html) command-line client ignores lines that start with `--`.

