Difference between revisions of "Exploring the database"

Latest revision as of 03:18, 21 August 2012

This guide will help guide you in poking around your Dreamwidth installation's database. This could help you during coding! You might also want to read the Database Table Overview for more database information, as well as locations to certain information in the database.

Signing into MySQL

Sign into your development environment with SSH.

cd $LJHOME
bin/ljdb

Basic Database Navigation

Show all tables

Data in the database is stored in a collection of tables, each of which is like a spreadsheet with rows containing a certain amount of columns. To present a list of all the tables in the database, use this command:

mysql> SHOW TABLES;

Show all columns in a table

This command will show you a list of all of the columns in a table, which is all of the pieces of data a row has.

mysql> SHOW COLUMNS FROM <tablename>;

For example:

mysql> SHOW COLUMNS FROM user;

The list will have a table of information about each column (or field):

• Field -- the name of the column
• Type -- the type of data the column stores. You can read more about it at MySQL's documentation.
• Null -- whether or not NULL (the absence of a value) is allowed for this column
• Key -- A table's key determines how you can look up data in the database. PRI here means that is the tables primary key. UNI means that bit of data will be unique for that row--that is, if one row has "moo", another row cannot have the value "moo" in that table.
• Default -- the default value for the column, if none is specified
• Extra -- if auto_increment is here, that means that every new row to that table automatically gets a higher value than the last row added.

Viewing data

The most basic way of viewing is this command, but only run it if you have a few users on your system:

mysql> SELECT * FROM user;

You can show only the columns you're interested in like this:

mysql> SELECT userid,user,name FROM user;

And even limit the number of rows you get back:

mysql> SELECT userid,user,name FROM user LIMIT 5;

You can even search for a row matching specific requirements:

mysql> SELECT userid,user,name FROM user WHERE user='foxfirefey';

Changing data

See UPDATE.

mysql> SELECT status FROM user WHERE userid=1;
mysql> UPDATE user SET status='A' WHERE userid=1;
mysql> SELECT status FROM user WHERE userid=1;

mysql> SELECT userid,user,email,status FROM user;
mysql> UPDATE user SET status='A' WHERE status='N';
mysql> SELECT userid,user,email,status FROM user;

Adding data

See INSERT.