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Table of Contents
1. General Information
2. MySQL Installation
3. Tutorial Introduction
4. Database Administration
5. MySQL Optimisation
6. MySQL Language Reference
7. MySQL Table Types
8. MySQL APIs
9. Extending MySQL

40 MySQL Technical Reference for Version 4.0.3 1.7.4.7 `--' as the Start of a Comment Some other SQL databases use `--' to start comments. MySQL Server has `#' as the start comment character. You can also use the C comment style /* this is a comment */ with MySQL Server. See Section 6.1.6 [Comments], page 385. MySQL Server Version 3.23.3 and above support the `--' comment style, provided the comment is followed by a space. This is because this comment style has caused many problems with automatically generated SQL queries that have used something like the following code, where we automatically insert the value of the payment for !payment!: UPDATE tbl_name SET credit=credit-!payment! Think about what happens if the value of payment is negative. Because 1--1 is legal in SQL, the consequences of allowing comments to start with `--' are terrible. Using our implementation of this method of commenting in MySQL Server Version 3.23.3 and up, 1-- This is a comment is actually safe. Another safe feature is that the mysql command-line client removes all lines that start with `--'. The following information is relevant only if you are running a MySQL version earlier than 3.23.3: If you have a SQL program in a text le that contains `--' comments you should use: shell> replace " --" " #" < text-file-with-funny-comments.sql \ | mysql database instead of the usual: shell> mysql database < text-file-with-funny-comments.sql You can also edit the command le \in place" to change the `--' comments to `#' comments: shell> replace " --" " #" -- text-file-with-funny-comments.sql Change them back with this command: shell> replace " #" " --" -- text-file-with-funny-comments.sql 1.7.5 Known Errors and Design Deciencies in MySQL The following problems are known and have a very high priority to get xed: ANALYZE TABLE on a BDB table may in some case make the table unusable until one has restarted mysqld. When this happens you will see errors like the following in the MySQL error le: 001207 22:07:56 bdb: log_flush: LSN past current end-of-log Don't execute ALTER TABLE on a BDB table on which you are running multi-statement transactions until all those transactions complete. (The transaction will probably be ignored.) ANALYZE TABLE, OPTIMIZE TABLE, and REPAIR TABLE may cause problems on tables for which you are using INSERT DELAYED. Doing a LOCK TABLE ... and FLUSH TABLES ... doesn't guarantee that there isn't a half-nished transaction in progress on the table.

MySQL Reference Manual

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