An identifying relationship “describes a situation in which the existence of a row in the child table depends on a row in the parent table.”
“if a child identifies its parent, it is an identifying relationship.”
The technical definition of an identifying relationship is that a child’s foreign key is part of its primary key.
CREATE TABLE AuthoredBook (
author_id INT NOT NULL,
book_id INT NOT NULL,
PRIMARY KEY (author_id, book_id),
FOREIGN KEY (author_id) REFERENCES Authors(author_id),
FOREIGN KEY (book_id) REFERENCES Books(book_id)
);
See? book_id is a foreign key, but it’s also one of the columns in the primary key. So this table has an identifying relationship with the referenced table Books. Likewise it has an identifying relationship with Authors.
A comment on a YouTube video has an identifying relationship with the respective video. The video_idshould be part of the primary key of the Comments table.
CREATE TABLE Comments (
video_id INT NOT NULL,
user_id INT NOT NULL,
comment_dt DATETIME NOT NULL,
PRIMARY KEY (video_id, user_id, comment_dt),
FOREIGN KEY (video_id) REFERENCES Videos(video_id),
FOREIGN KEY (user_id) REFERENCES Users(user_id)
);
It may be hard to understand this because it’s such common practice these days to use only a serial surrogate key instead of a compound primary key:
CREATE TABLE Comments (
comment_id SERIAL PRIMARY KEY,
video_id INT NOT NULL,
user_id INT NOT NULL,
comment_dt DATETIME NOT NULL,
FOREIGN KEY (video_id) REFERENCES Videos(video_id),
FOREIGN KEY (user_id) REFERENCES Users(user_id)
);
This can obscure cases where the tables have an identifying relationship.
I would not consider SSN to represent an identifying relationship. Some people exist but do not have an SSN. Other people may file to get a new SSN. So the SSN is really just an attribute, not part of the person’s primary key.
You can take a look at MySQL Manual, explaining how to add Foreign Keys on MySQL Workbench as well.
The vertical toolbar on the left side of an EER Diagram has six foreign key tools:
one-to-one non-identifying relationship
one-to-many non-identifying relationship
one-to-one identifying relationship
one-to-many identifying relationship
many-to-many identifying relationship
Place a Relationship Using Existing Columns
An identifying relationship is one where the child table cannot be uniquely identified without its parent. Typically this occurs where an intermediary table is created to resolve a many-to-many relationship. In such cases, the primary key is usually a composite key made up of the primary keys from the two original tables. An identifying relationship is indicated by a solid line between the tables and a nonidentifying relationship is indicated by a broken line.
Create or drag and drop the tables that you wish to connect. Ensure that there is a primary key in the table that will be on the “one” side of the relationship. Click on the appropriate tool for the type of relationship you wish to create. If you are creating a one-to-many relationship, first click the table that is on the “many” side of the relationship, then on the table containing the referenced key. This creates a column in the table on the many side of the relationship. The default name of this column is table_name_key_name where the table name and the key name both refer to the table containing the referenced key.
When the many-to-many tool is active, double-clicking a table creates an associative table with a many-to-many relationship. For this tool to function there must be a primary key defined in the initial table.
To edit the properties of a foreign key, double-click anywhere on the connection line that joins the two tables. This opens the relationship editor.
Mousing over a relationship connector highlights the connector and the related keys as shown in the following figure. The film and the film_actor tables are related on the film_id field and these fields are highlighted in both tables. Since the film_id field is part of the primary key in the film_actor table, a solid line is used for the connector between the two tables.
The vast majority of Linux distributions have PostgreSQL available in supplied packages. Mac OS X, starting with Lion, has PostgreSQL server as its standard default database in the server edition,[7][8] and PostgreSQL client tools in the desktop edition.
PostgreSQL has bindings for many programming languages such as C, C++, Python, Java, PHP, Ruby… It can be used to power anything from simple web applications to massive databases with millions of records.
Client Installation
If you only wish to connect to a PostgreSQL server, do not install the whole PostgreSQL package, but install the PostgreSQL client instead. To do this, use the following command
sudo apt-get install postgresql-client
you then connect to the server with the following command
psql -h server.domain.org database user
After you inserted the password you access PostgreSQL with line commands. You may for instance insert the following
Install PostgreSQL using the apt-get command in gnome-terminal:
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sudoapt-get installpostgresql libpq-dev
After installation is complete, change user to the PostgreSQL user:
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sudosu- postgres
You are now working as the PostgreSQL user. Now, let’s change your database password to be more robust. In this example, I’m setting the password as “s0meth1ng”:
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~$: psql -d postgres -U postgres
psql (9.1.3) Type "help"forhelp.
postgres=# alter user postgres with password 's0meth1ng'; ALTER ROLE
postgres=# q
Restart the PostgreSQL database to let the changes take effect:
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sudo/etc/init.d/postgresqlrestart
Installing and Setting Up pgAdmin III
Install pgAdmin III using the apt-get command in gnome-terminal:
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sudo apt-getinstall pgadmin3
Once installed, you can launch pgAdmin III by quick-launching it in Alt-F2, then typing pgadmin3.
Now, let’s add a new PostgreSQL database server to the list of servers. Go to File > Add Server, and enter the details as the following screenshot:
Once that is done, you’ll now see your new server at the list of servers on the left pane. Go ahead, and create your database. Have fun!
Administration
pgAdmin III is a handy GUI for PostgreSQL, it is essential to beginners. To install it, type at the command line:
sudo apt-get install pgadmin3
You may also use the Synaptic package manager from the System>Administration menu to install these packages.
Basic Server Setup
To start off, we need to change the PostgreSQL postgres user password; we will not be able to access the server otherwise. As the “postgres” Linux user, we will execute the psql command.
In a terminal, type:
sudo -u postgres psql postgres
Set a password for the “postgres” database role using the command:
password postgres
and give your password when prompted. The password text will be hidden from the console for security purposes.
Type Control+D to exit the posgreSQL prompt.
Create database
To create the first database, which we will call “mydb”, simply type:
sudo -u postgres createdb mydb
Install Server Instrumentation for Postgresql 8.4 or 9.1
To install Server Instrumentation, you must install postgresql-contrib:
sudo apt-get install postgresql-contrib
For “”Postgresql 9.1″”+ install the adminpack “extension”:
sudo -u postgres psql
CREATE EXTENSION adminpack;
Alternative Server Setup
If you don’t intend to connect to the database from other machines, this alternative setup may be simpler.
By default in Ubuntu, Postgresql is configured to use ‘ident sameuser’ authentication for any connections from the same machine. Check out the excellent Postgresql documentation for more information, but essentially this means that if your Ubuntu username is ‘foo’ and you add ‘foo’ as a Postgresql user then you can connect to the database without requiring a password.
Since the only user who can connect to a fresh install is the postgres user, here is how to create yourself a database account (which is in this case also a database superuser) with the same name as your login name and then create a password for the user:
Client programs, by default, connect to the local host using your Ubuntu login name and expect to find a database with that name too. So to make things REALLY easy, use your new superuser privileges granted above to create a database with the same name as your login name:
createdb $USER
Connecting to your own database to try out some SQL should now be as easy as:
psql
To create a database with a user that have full rights on the database, use the following command:
The first command line creates the user with no database creation rights (-D) with no add user rights -A) and will prompt you for entering a password (-P). The second command line create the database ‘mydb with ‘myuser‘ as owner.
This little example will probably suit most of your needs. For more details, please refer to the corresponding man pages or the online documentation.
The idea behind this dump method is to generate a text file with SQL commands that, when fed back to the server, will recreate the database in the same state as it was at the time of the dump. PostgreSQL provides the utility program pg_dump for this purpose. The basic usage of this command is:
pg_dump dbname > outfile
As you see, pg_dump writes its result to the standard output. We will see below how this can be useful.
pg_dump is a regular PostgreSQL client application (albeit a particularly clever one). This means that you can perform this backup procedure from any remote host that has access to the database. But remember that pg_dump does not operate with special permissions. In particular, it must have read access to all tables that you want to back up, so in practice you almost always have to run it as a database superuser.
To specify which database server pg_dump should contact, use the command line options -h host and -p port. The default host is the local host or whatever your PGHOST environment variable specifies. Similarly, the default port is indicated by the PGPORT environment variable or, failing that, by the compiled-in default. (Conveniently, the server will normally have the same compiled-in default.)
Like any other PostgreSQL client application, pg_dump will by default connect with the database user name that is equal to the current operating system user name. To override this, either specify the -U option or set the environment variable PGUSER. Remember that pg_dump connections are subject to the normal client authentication mechanisms (which are described in Chapter 19).
An important advantage of pg_dump over the other backup methods described later is that pg_dump’s output can generally be re-loaded into newer versions of PostgreSQL, whereas file-level backups and continuous archiving are both extremely server-version-specific. pg_dump is also the only method that will work when transferring a database to a different machine architecture, such as going from a 32-bit to a 64-bit server.
Dumps created by pg_dump are internally consistent, meaning, the dump represents a snapshot of the database at the time pg_dump began running. pg_dump does not block other operations on the database while it is working. (Exceptions are those operations that need to operate with an exclusive lock, such as most forms of ALTER TABLE.)
Important: If your database schema relies on OIDs (for instance, as foreign keys) you must instruct pg_dump to dump the OIDs as well. To do this, use the -o command-line option.
24.1.1. Restoring the Dump
The text files created by pg_dump are intended to be read in by the psql program. The general command form to restore a dump is
psql dbname < infile
where infile is the file output by the pg_dump command. The database dbname will not be created by this command, so you must create it yourself from template0 before executing psql (e.g., with createdb -T template0 dbname). psql supports options similar to pg_dump for specifying the database server to connect to and the user name to use. See the psql reference page for more information.
Before restoring an SQL dump, all the users who own objects or were granted permissions on objects in the dumped database must already exist. If they do not, the restore will fail to recreate the objects with the original ownership and/or permissions. (Sometimes this is what you want, but usually it is not.)
By default, the psql script will continue to execute after an SQL error is encountered. You might wish to run psql with the ON_ERROR_STOP variable set to alter that behavior and have psql exit with an exit status of 3 if an SQL error occurs:
psql --set ON_ERROR_STOP=on dbname < infile
Either way, you will only have a partially restored database. Alternatively, you can specify that the whole dump should be restored as a single transaction, so the restore is either fully completed or fully rolled back. This mode can be specified by passing the -1 or --single-transaction command-line options to psql. When using this mode, be aware that even a minor error can rollback a restore that has already run for many hours. However, that might still be preferable to manually cleaning up a complex database after a partially restored dump.
The ability of pg_dump and psql to write to or read from pipes makes it possible to dump a database directly from one server to another, for example:
pg_dump -h host1dbname | psql -h host2dbname
Important: The dumps produced by pg_dump are relative to template0. This means that any languages, procedures, etc. added via template1 will also be dumped by pg_dump. As a result, when restoring, if you are using a customized template1, you must create the empty database from template0, as in the example above.
After restoring a backup, it is wise to run ANALYZE on each database so the query optimizer has useful statistics; see Section 23.1.3 and Section 23.1.5 for more information. For more advice on how to load large amounts of data into PostgreSQL efficiently, refer to Section 14.4.
24.1.2. Using pg_dumpall
pg_dump dumps only a single database at a time, and it does not dump information about roles or tablespaces (because those are cluster-wide rather than per-database). To support convenient dumping of the entire contents of a database cluster, the pg_dumpall program is provided. pg_dumpall backs up each database in a given cluster, and also preserves cluster-wide data such as role and tablespace definitions. The basic usage of this command is:
pg_dumpall > outfile
The resulting dump can be restored with psql:
psql -f infile postgres
(Actually, you can specify any existing database name to start from, but if you are loading into an empty cluster then postgres should usually be used.) It is always necessary to have database superuser access when restoring a pg_dumpall dump, as that is required to restore the role and tablespace information. If you use tablespaces, make sure that the tablespace paths in the dump are appropriate for the new installation.
pg_dumpall works by emitting commands to re-create roles, tablespaces, and empty databases, then invoking pg_dump for each database. This means that while each database will be internally consistent, the snapshots of different databases might not be exactly in-sync.
24.1.3. Handling Large Databases
Some operating systems have maximum file size limits that cause problems when creating large pg_dump output files. Fortunately, pg_dump can write to the standard output, so you can use standard Unix tools to work around this potential problem. There are several possible methods:
Use compressed dumps. You can use your favorite compression program, for example gzip:
pg_dump dbname | gzip > filename.gz
Reload with:
gunzip -c filename.gz | psql dbname
or:
cat filename.gz | gunzip | psql dbname
Use split. The split command allows you to split the output into smaller files that are acceptable in size to the underlying file system. For example, to make chunks of 1 megabyte:
pg_dump dbname | split -b 1m - filename
Reload with:
cat filename* | psql dbname
Use pg_dump’s custom dump format. If PostgreSQL was built on a system with the zlib compression library installed, the custom dump format will compress data as it writes it to the output file. This will produce dump file sizes similar to using gzip, but it has the added advantage that tables can be restored selectively. The following command dumps a database using the custom dump format:
pg_dump -Fc dbname > filename
A custom-format dump is not a script for psql, but instead must be restored with pg_restore, for example:
DbVisualizer is a Java Swing app which can generate relation graphs from any JDBC source (including PostgreSQL). I’ve found the best way to view the generated graph (after you have found a layout you like) is to print it to PDF and use Preview.app to view the result. The built-in view is somewhat lacking.
There are also a few Graphviz options around including AutoDoc (example output). This may be a better option if you are automating the documentation generation. With a bit of work you can style the output quite a bit.
double-click on the DB/schema in the navigator pane and then select the “References” tab. If you select the whole DB it’ll give you all of the system tables too — you can filter out tables that you don’t want by selecting the “Specified Tables” option.
psql is a terminal-based front-end to PostgreSQL. It enables you to type in queries interactively, issue them to PostgreSQL, and see the query results. Alternatively, input can be from a file. In addition, it provides a number of meta-commands and various shell-like features to facilitate writing scripts and automating a wide variety of tasks.
Options
-a --echo-all
Print all input lines to standard output as they are read. This is more useful for script processing than interactive mode. This is equivalent to setting the variable ECHO to all.
-A --no-align
Switches to unaligned output mode. (The default output mode is otherwise aligned.)
-c command --command command
Specifies that psql is to execute one command string, command, and then exit. This is useful in shell scripts. Start-up files (psqlrc and ~/.psqlrc) are ignored with this option.
command must be either a command string that is completely parsable by the server (i.e., it contains no psql-specific features), or a single backslash command. Thus you cannot mix SQL and psql meta-commands with this option. To achieve that, you could pipe the string into psql, like this: echo 'x \ SELECT * FROM foo;' | psql. (\ is the separator meta-command.)
If the command string contains multiple SQL commands, they are processed in a single transaction, unless there are explicit BEGIN/COMMIT commands included in the string to divide it into multiple transactions. This is different from the behavior when the same string is fed to psql’s standard input.
-d dbname --dbname dbname
Specifies the name of the database to connect to. This is equivalent to specifying dbname as the first non-option argument on the command line.
If this parameter contains an = sign, it is treated as a conninfo string. See Section 31.1 for more information.
The JSON format was originally specified by Douglas Crockford, and is described in RFC 4627. The official Internet media type for JSON is application/json. The JSON filename extension is .json.
The JSON format is often used for serializing and transmitting structured data over a network connection. It is used primarily to transmit data between a server and web application, serving as an alternative to XML.
Array (an ordered sequence of values, comma-separated and enclosed in square brackets; the values do not need to be of the same type)
Object (an unordered collection of key:value pairs with the ‘:’ character separating the key and the value, comma-separated and enclosed in curly braces; the keys must be strings and should be distinct from each other)
Non-significant white space may be added freely around the “structural characters” (i.e. brackets “{ } [ ]”, colons “:” and commas “,”).
The following example shows the JSON representation of an object that describes a person. The object has string fields for first name and last name, a number field for age, an object representing the person’s address and an array of phone number objects.
One potential pitfall of the free-form nature of JSON comes from the ability to write numbers as either numeric literals or quoted strings. For example, ZIP Codes in the northeastern U.S. begin with zeroes (for example, 07728 for Freehold, New Jersey). If written with quotes by one programmer but not by another, the leading zero could be dropped when exchanged between systems, when searched for within the same system, or when printed. In addition, postal codes in the U.S. are numbers but other countries use letters as well. This is a type of problem that the use of a JSON Schema (see below) is intended to reduce.
Since JSON is almost a subset of JavaScript, it is possible, but not recommended,[7] to parse most JSON text into an object by invoking JavaScript’s eval() function. For example, if the above JSON data is contained within a JavaScript string variable contact, one could use it to create the JavaScript object p as follows:
var p = eval("(" + contact + ")");
The contact variable must be wrapped in parentheses to avoid an ambiguity in JavaScript’s syntax.[8]
The recommended way, however, is to use a JSON parser. Unless a client absolutely trusts the source of the text, or must parse and accept text that is not strictly JSON-compliant, one should avoid eval(). A correctly implemented JSON parser accepts only valid JSON, preventing potentially malicious code from being executed inadvertently.
var p = JSON.parse(contact);
Browsers, such as Firefox 4 and Internet Explorer 8, include special features for parsing JSON. As native browser support is more efficient and secure than eval(), native JSON support is included in the recently-released Edition 5 of the ECMAScript standard.[9]
jQuery library wrap JSON object in function constructor and execute it immediately if JSON.parse is not present. This avoid using eval in the code.
var p = new Function('return ' + contact ';')();
Despite the widespread belief that JSON is a JavaScript subset, this is not the case. Specifically, JSON allows the Unicode line terminators U+2028 line separator and U+2029 paragraph separator to appear unescaped in quoted strings, while JavaScript does not.[10] This is a consequence of JSON disallowing only “control characters”. This subtlety is important when generating JSONP.
