4.5.1.6. mysql Tips

4.5.1.6.1. Input-Line Editing

mysql supports input-line editing, which enables you to modify the current input line in place or recall previous input lines. For example, the left-arrow and right-arrow keys move horizontally within the current input line, and the up-arror and down-arrow keys move up and down through the set of previously entered lines. Backspace deletes the character before the cursor and typing new characters enters them at the cursor position. To enter the line, press Enter.

On Windows, the editing key sequences are the same as supported for command editing in console windows. On Unix, the key sequences depend on the input library used to build mysql (for example, the libedit or readline library).

Documentation for the libedit and readline libraries is available online. To change the set of key sequences permitted by a given input library, define key bindings in the library startup file. This is a file in your home directory: .editrc for libedit and .inputrc for readline.

For example, in libedit, Control+W deletes everything before the current cursor position and Control+U deletes the entire line. In readline, Control+W deletes the word before the cursor and Control+U deletes everything before the current cursor position. If mysql was built using libedit, a user who prefers the readline behavior for these two keys can put the following lines in the .editrc file (creating the file if necessary):

bind "^W" ed-delete-prev-wordbind "^U" vi-kill-line-prev

To see the current set of key bindings, temporarily put a line that says only bind at the end of .editrc. mysql will show the bindings when it starts.

4.5.1.6.2. Unicode Support on Windows

Windows provides APIs based on UTF-16LE for reading from and writing to the console; the mysql client for Windows is able to use these APIs. The Windows installer creates an item in the MySQL menu named MySQL command line client - Unicode. This item invokes the mysql client with properties set to communicate through the console to the MySQL server using Unicode.

To take advantage of this support manually, run mysql within a console that uses a compatible Unicode font and set the default character set to a Unicode character set that is supported for communication with the server:

Open a console window.
Go to the console window properties, select the font tab, and choose Lucida Console or some other compatible Unicode font. This is necessary because console windows start by default using a DOS raster font that is inadequate for Unicode.
Execute mysql.exe with the --default-character-set=utf8 (or utf8mb4) option. This option is necessary because utf16le is not supported as a connection character set.

With those changes, mysql will use the Windows APIs to communicate with the console using UTF-16LE, and communicate with the server using UTF-8. (The menu item mentioned previously sets the font and character set as just described.)

To avoid those steps each time you run mysql, you can create a shortcut that invokes mysql.exe. The shortcut should set the console font to Lucida Console or some other compatible Unicode font, and pass the --default-character-set=utf8 (or utf8mb4) option to mysql.exe.

Alternatively, create a shortcut that only sets the console font, and set the character set in the [mysql] group of your my.ini file:

[mysql]default-character-set=utf8

4.5.1.6.3. Displaying Query Results Vertically

Some query results are much more readable when displayed vertically, instead of in the usual horizontal table format. Queries can be displayed vertically by terminating the query with \G instead of a semicolon. For example, longer text values that include newlines often are much easier to read with vertical output:

mysql> SELECT * FROM mails WHERE LENGTH(txt) <
            300 LIMIT 300,1\G*************************** 1. row ***************************  msg_nro: 3068     date: 2000-03-01 23:29:50time_zone: +0200mail_from: Monty    reply: monty@no.spam.com  mail_to: "Thimble Smith" <tim@no.spam.com>      sbj: UTF-8      txt: >>>>> "Thimble" == Thimble Smith writes:Thimble> Hi.  I think this is a good idea.  Is anyone familiarThimble> with UTF-8 or Unicode? Otherwise, I'll put this on myThimble> TODO list and see what happens.Yes, please do that.Regards,Monty     file: inbox-jani-1     hash: 1904029441 row in set (0.09 sec)

4.5.1.6.4. Using the `--safe-updates` Option

For beginners, a useful startup option is --safe-updates (or --i-am-a-dummy, which has the same effect). It is helpful for cases when you might have issued a DELETE FROM tbl_name statement but forgotten the WHERE clause. Normally, such a statement deletes all rows from the table. With --safe-updates, you can delete rows only by specifying the key values that identify them. This helps prevent accidents.

When you use the --safe-updates option, mysql issues the following statement when it connects to the MySQL server:

SET sql_safe_updates=1, sql_select_limit=1000, max_join_size=1000000;

See Section 5.1.4, "Server System Variables".

The SET statement has the following effects:

You are not permitted to execute an UPDATE or DELETE statement unless you specify a key constraint in the WHERE clause or provide a LIMIT clause (or both). For example:
```
UPDATE tbl_name SET not_key_column=val WHERE key_column=val;UPDATE tbl_name SET not_key_column=val LIMIT 1;
```
The server limits all large SELECT results to 1,000 rows unless the statement includes a LIMIT clause.
The server aborts multiple-table SELECT statements that probably need to examine more than 1,000,000 row combinations.

To specify limits different from 1,000 and 1,000,000, you can override the defaults by using the --select_limit and --max_join_size options:

shell> mysql --safe-updates --select_limit=500
            --max_join_size=10000

4.5.1.6.5. Disabling mysql Auto-Reconnect

If the mysql client loses its connection to the server while sending a statement, it immediately and automatically tries to reconnect once to the server and send the statement again. However, even if mysql succeeds in reconnecting, your first connection has ended and all your previous session objects and settings are lost: temporary tables, the autocommit mode, and user-defined and session variables. Also, any current transaction rolls back. This behavior may be dangerous for you, as in the following example where the server was shut down and restarted between the first and second statements without you knowing it:

mysql> SET @a=1;Query OK, 0 rows affected (0.05 sec)mysql> INSERT INTO t VALUES(@a);ERROR 2006: MySQL server has gone awayNo connection. Trying to reconnect...Connection id:    1Current database: testQuery OK, 1 row affected (1.30 sec)mysql> SELECT * FROM t;+------+| a    |+------+| NULL |+------+1 row in set (0.05 sec)

The @a user variable has been lost with the connection, and after the reconnection it is undefined. If it is important to have mysql terminate with an error if the connection has been lost, you can start the mysql client with the --skip-reconnect option.